Marcela Carena

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ORAL HISTORIES
Carena credit Fermilab.jpg

Credit: Fermilab

Interviewed by
David Zierler
Interview date
Location
video conference
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Interview of Marcela Carena by David Zierler on March 3, 2021,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
www.aip.org/history-programs/niels-bohr-library/oral-histories/XXXX

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Marcela Carena, Distinguished Scientist and head of the Theory Division at Fermilab, is interviewed by David Zierler. Carena describes her dual position as professor of physics and member of the Fermi and Kavli Institutes at the University of Chicago and she surveys the many areas of Higgs physics in which she is currently working. She recounts her family’s Italian and Spanish heritage and her upbringing in Buenos Aires and the opportunities she pursued as she became interested in science. Carena describes her undergraduate education at Instituto Balseiro where she developed an appreciation for the interplay of theory and experimental high energy physics. She explains her decision to remain for graduate school where she worked with Roberto Peccei and she describes her research at DESY in Germany and her focus on supersymmetry and sphalerons. Marcela describes the importance of meeting Bill Bardeen during her postdoctoral appointment at Purdue and her subsequent research at the Max Planck Institute where she was focused on the LEP collider at CERN. She explains her decision to move to CERN full time and she conveys the impact of the SSC cancellation from the vantage point of CERN. Carena describes the opportunities that led to her staff position at Fermilab where she continued to develop her interests in supersymmetry and Higgs physics. She conveys the impact of the shutdown of the Tevatron and she describes the emotional component of the discovery of the Higgs. Carena explains why her focus on dark matter and electroweak baryogenesis are natural extensions from the Higgs discovery, and she wonders what it will look like if and when we come to understand what dark matter is. She reflects on what has, and has not, been seen at the LHC over the past decade, and she discusses both the scientific and political value in Fermilab supporting an International Relations Directorate. At the end of the interview, Carena describes her recent interests in quantum information and why quantum computers may yield new insights on the early universe, she conveys her pride in Fermilab’s leading efforts to promote diversity and inclusivity in science, and she explains why there is cause for optimism in the quest to understand dark matter. 

 

Transcript

David Zierler:

Okay, this is David Zierler, oral historian for the American Institute of Physics. It is March 3, 2021. I am so happy to be here, finally, with Dr. Marcela Carena. Marcela, it is great to see you. Thank you so much for joining me.

Marcela Carena:

It is my pleasure.

Zierler:

To start, would you please tell me your title and institutional affiliations? And you'll notice that I pluralize because I know you have more than one.

Carena:

I'm a Distinguished Scientist at the Fermi National Accelerator Laboratory and the head of its Theory Division. I'm also professor of physics at the University of Chicago, and I'm a member of the Enrico Fermi Institute and the Kavli Institute for Cosmological Physics, also at the University of Chicago.

Zierler:

Marcela, back when we used to go into the office physically, if you could remember that [laugh] when, where would you spend most of your time? Would you be mostly at Fermilab, mostly at Chicago, or really going back and forth?

Carena:

It depends on the given period. Now that I have been the head of Fermilab Theory for the past five years, I have spent much more time at Fermilab. When I'm teaching, I go three times per week to U. Chicago. I have students who, when we were able to see each other in person, have been coming a lot to Fermilab. So, I would say, I'm definitely more at Fermilab than at U. Chicago, but I try to balance my presence there as well.

Zierler:

Marcela, one other affiliation you have, of course, as you served as chair of the Division of Particles and Fields at APS. I'm curious how much time that takes on any given week for you?

Carena:

Oh, this is from the past, because that is a four-year position, where you are elected, and you become the vice-chair, then the chair-elect, the chair and the past-chair. So, I served in the DPF chair-line from 2015 to 2018. And, when I was the chair in 2017 that was taking a very substantial part of my time. These days, I collaborate with the community in many other ways that also take a lot of my time, but not as DPF chair anymore.

Zierler:

Marcela, before we take it back to the beginning and develop your family background and your personal history all the way from the origins, I'd like to ask a very “in the moment” question. And it's one of course, we're all dealing with. So first, as a theoretical physicist, one might assume that the pandemic and the social isolation might be ideal in terms of you just being left alone to work on equations and to think big ideas. To what extent is that true? And to what extent are the kinds of collaborations that you're involved in really dependent on in person, interpersonal interaction, so that the past 10 or 11 months may not have been as productive as you would have otherwise hoped?

Carena:

That's an excellent question. And I can give you my personal view on it. I think it depends a bit on the style of the scientist. I like very much to have face to face discussions on the blackboard. And that has not been so easy. We all tried to get iPads and write on a beautiful virtual blackboard, but it is not the same, obviously. For me, it has been quite intense this period, because I have my group, that is a big group, and one tries very hard to be sure that everybody feels comfortable and secure and connected. And so that takes extra effort from the part of the senior people. On the other hand, for research it has been great, really, I can't complain. I started projects in two areas that I was not exploring before with amazing postdocs and students, and I discuss with them several hours on each of the different projects, couple of days per week. So, for me, from the scientific side, it has been great! But I also have to say that it's not so easy to jumpstart new collaborations. If you are a new postdoc arriving to a place and you don't know anybody, it becomes even harder. At Fermilab, we tried a virtual office set up, where people are in their virtual offices and then others can go and knock on the virtual door of someone and see if that person is there to have a more casual conversation, without the need of making a Zoom appointment This works only partially. Personally, for me, I really enjoy the face-to-face contact, not the Zoom face-to-face contact. I hope we are going back to our offices soon!

Zierler:

Have you been at Fermilab since last year, it's exclusively remote for you now?

Carena:

It is mostly remote. I have only been there when I had some specific business -experimentalists, engineers need to be there much more. So, I have the opportunity to go there if needed, but we mostly work from home, not at the office.

Zierler:

Marcela, Fermilab is as great a place as any, to demonstrate how important the worlds of experiment and theory are for one each other, because the interplay is so key. What have been some of the major experiments at Fermilab in recent years that have been most compelling and interesting for the theoretical fields of inquiry that you've been involved in?

Carena:

Well, for me, of course, my lifelong interest is related to Higgs physics. And in that sense, colliders have been the main type of experiments searching for it. At Fermilab I have a close connection with all my colleagues at CMS, one of the experiments of LHC. We have something at Fermilab that's called the LHC Physics Center. So that's one of the areas where a big part of my group and myself are very tightly connected with experimentalists. I am specially thinking about, how to search for new scalar bosons beyond the Higgs, how to explore signs of CP violation through the Higgs sector at LHC. Of course, the other interesting topic I'm excited about is the nature of dark matter. I have been involved quite a bit in thinking about searches for dark matter, both at colliders and beyond colliders, especially – most recently - within the framework of the 2020 European Strategy Update, since I was the U.S. representative there and I was working on that area for the update. At Fermilab, we have many ongoing experiments, and some that are being developed now, that are trying to investigate the nature of dark matter. At the LHC one is trying to search for feebly interacting particles and dark sectors – that do not connect with our visible sector, but have the ability to host dark matter. And then there are other types of experiments that are searching for axion-like dark matter particles that can be much lighter than the kind of dark matter particles one can search for at colliders. There are new experiments, that are being developed as we speak to search for dark matter axion-like particles, and there are exciting experiments using quantum sensors like MAGIS-100 at Fermilab, which is based on atomic clocks. So, there is both the big international collaborations and then the smaller and exciting experiments that are being developed at the lab. Of course, as you know, Fermilab is the neutrino lab in the U.S. and many members of Fermilab Theory, including myself, are collaborating with colleagues working in the neutrino experiments: the short-baseline neutrino program which has three different running experiments, and – in a few years - the gigantic DUNE experiment. As we have recently heard in the news, the muon g-2 experiment at Fermilab is probing the quantum mysteries of empty space by measuring the tiny wobbles of muons to get clues about a perhaps unknown world of new particles. This is very exciting since it can challenge the foundations of physics as we know it - and may even tell us something about dark matter and cosmology at the same time.

So, these are, I think, the most exciting experiments at the lab, with scientists in the U.S. and around the world working together to make them possible. Fermilab theorists contribute actively to interpreting the experimental results and providing novel ideas to exploit existing experiments and devise new ones in our quest for understanding the quantum universe.

Zierler:

Marcela, to give the sense of the administrative structure at Fermilab, as head of the Theory Division, do you have a counterpart who is called the head of experimental physics or how does that work at Fermilab?

Carena:

It is very interesting you're asking this question [laugh] at this particular moment things are evolving. The director of Fermilab and the Department of Energy just recently decided to change a bit the role of theory at Fermilab. At CERN, you're right, you have, like, a theory department and an experimental department. At Fermilab it's a bit different, and what at CERN is now called a department we call it a division. Originally, we had just a particle physics division, where all the research was conducted. Because of the relevance of neutrino science at Fermilab, a few years ago, the Neutrino Division was created. For almost fifty years we had a Theory Department sitting in the original Particle Physics Division, but interconnected with many of the other divisions, in particular the Scientific Computing Division and more recently, the Neutrino Division, while in the Particle Physics Division we had experimentalists working on cosmology, colliders, and muons, including the Muon g−2 experiment. Now, also with have a new Quantum Center and our Fermilab Quantum Institute. We theorists collaborate with experimentalists extended over five divisions. I would say, now the idea is that theory will be unleashed as its own Division and be able to grow and define its own portfolio, which is already tightly intertwined with other divisions, to have new opportunities both inside and outside the lab.

Zierler:

Marcela, once the DOE becomes involved in these considerations, of course, it's not only one national lab that, that we might consider.

Carena:

We have strong research connections with the other labs, as well as with universities. But each lab has their own internal organization, we try to support each other from our own position. I do think this new development is a good situation for us, an opportunity to make what we are doing better.

Zierler:

Right. Right. Well, wonderful. Marcela, let's go all the way back to the beginning now, let's go to Argentina. First, let me hear a little bit about your parents. Tell me about them and where they're from.

Carena:

Okay. Well, my parents have, unfortunately, both passed away, but they both came from immigrant families from Europe. I grew up in a family with strong Italian and Spanish roots. My dad was the oldest of six siblings of an Italian family that came to Buenos Aires around 1913. They got a ranch in the Pampas and built their lives around it, with cattle and grain business. My dad was, well, what I remember of him - he unfortunately died when I was 17 years old; my dad was great at playing violin and guitar, and bandoneon - that is a type of accordion. He loved math and math games, and he was a very generous person. Of course, being the oldest son of a big family, he stayed in the ranch running the family business for quite a while. I don't really know how many college courses he took. I know he was interested in agricultural engineering. The one thing I regret is that I didn't have the opportunity to know him as a grown up, myself. My mom grew up in a very conservative Spanish family. My Spanish grandfather came from a well-off family in Spain. And he went to Argentina, also about the same time, early 1910’s, I don't exactly know the year. He was a seminarist, and soon to become a priest, and he wanted to avoid that path. And my grandma, my grandma Carmen, someone very close to me, she came from a very poor farmer family in Galicia, in northern Spain. When she came to Argentina, she was searching for a more prosperous future. She was an amazing woman, kind and caring, and very smart and determined. In fact, she took a ship alone at age 14 to meet her relatives in Argentina, so that gives you an idea. And I think I was so lucky to have her in my life for a significant part of it.

Zierler:

Marcela, where did your parents meet?

Carena:

My parents met in Buenos Aires when my dad went to do the military service. And my mom was still a girl. Their extended Italian and Spanish families were neighbors, and they ended up living close by for a while. Almost 20 years later, my dad went back to my mom’s neighborhood in Buenos Aires and their paths crossed again, they started seeing each other a lot and got married very, very soon after that. It was basically the Italian-Spanish family friendship that brought them together actually. My mom was really determined to study in college, but she started working very soon after high school. Only after a while she went back to school and became a social worker. She worked for quite a while, almost 20 years, until I was born. And then she decided to take care of me. She married quite late for those times, she wanted to be independent.

Zierler:

Marcela, to the extent that mathematical and science abilities have a genetic component to them, maybe they do, maybe they don't. But is there anyone in your family that you might point to who had significant abilities in those areas that might have had an influence on you?

Carena:

Well, I think my dad was super good at math. As I said, what I remember well is playing math games with him. Other than that, I do not recall other family influence. I was good at math in school. In high school, I had a very inspiring math teacher, who really made me feel the pleasure of problem solving. She was someone who made a very strong impression on me at that time. If I think about physics… I believe it got under my skin before I knew what physics meant at all. My father spent lots of time in the ranch, a few hundred kilometers away from the big city, while I attended school in Buenos Aires – living with my grandmother and extended family. My parents were traveling back and forth during the year, but every summer… every summer we’d all go to the ranch, to the countryside. And those three months there, you know, were my preferred time, horseback riding and exploring, and swimming and, and watching the Via Lactea in the night… When I'm asked how I first got interested in physics, I am convinced it was during those nights watching the sky. I still have this vivid image in my mind, we were all little kids, my cousins and my friends, and we were just staring at the grandness of the universe. I mean, in this area deep in the Pampas, far away from the shining glares of the metropolis, the Via Lactea is so overwhelming in the dark night, right? It's like, wow!! So, we used to sit in the garden in the hacienda, and amid very tall pine trees we were silenced in amazement by the stars. We were just kids and, of course, we had no idea of the physics behind it. But it was most breathtaking.

Zierler:

Marcela, what neighborhood in Buenos Aires did you grow up in?

Carena:

I actually grew up in the neighborhood of Villa Urquiza, and besides the summer times in the ranch I lived there most of my life, until I went to college.

Zierler:

And what kind of--

Carena:

We still have a house there.

Zierler:

And what kind of schools did you go to? Public or private?

Carena:

I went to private schools. For elementary school, I went to a bilingual school: Spanish-English. It was a coed school; it was a block and a half from my house. Then, for high school, since unfortunately my elementary school didn't have a high school section at that time, I had to make a difficult decision…I wanted to keep going to a bilingual school, but my mom was a bit concerned about letting me take a bus every day since those were very difficult times in Argentina. You know, we were under military regime and there was a lot of unrest. And my mom was very protective and awfully scared about me going anywhere more than a few blocks away on a daily basis. Because of this, I ended up going to a religious, only girl school. It was a good school to educate young ladies in a safe environment, but was not very academically challenging.

Zierler:

For you.

Carena:

For me.

Zierler:

You might have been better served at a coed school.

Carena:

Yes, not only coed but with more exciting academic opportunities. Many of my friends, the boys, went to the high school run by priests, that was a block away from my high school, run by nuns. That was not so bad, in the sense that I kept close to many of them, but got distanced from most of the girls who were my childhood friends and went to other schools -just recently I got in touch with one of them, great! I believe it would have been good for me to attend a school with a more demanding curriculum, especially in science. Later on, when I decided to go into college, although I was good at math, I had to work pretty hard to make it through the admission examination, which was quite rigorous; I was not really well prepared. But okay, I survived [laugh].

Zierler:

Marcela was the church a big part of your family and upbringing?

Carena:

Uh, no, no. My Spanish grandparents were not happy with the Catholic church, as I mentioned, my grandfather left Spain in order to escape his family’s demands that he should become a priest. My grandmother Carmen, in her short time in Spain didn’t have good memories of the church role in her family’s life. I would say that, although my upbringing was in a Catholic environment, it was very superficial. My Italian grandparents died before I was born, but my father’s family was also only very loosely attached to the catholic faith. Of course, when I went to high school, that changed a little bit for a few years in my life. But I was never educated with strong religious beliefs at home. I would say today in my life I am very respectful of other people's beliefs. I have many friends who are quite religious and are among my closest friends. And so, I have tried to educate my kids as well in a manner such that they respect other peoples' beliefs and try to find what it makes sense for them.

Zierler:

Marcela, a very broad cultural question. As you say, there were conservative leaning people in your own family. And of course, Argentinian society has very many conservative elements to it. As a girl with an interest in science, were you ever made to feel that this was not an appropriate path of study for you? Were you ever discouraged along those lines, either within your family or by teachers, or other elders around you?

Carena:

I would say at early stages in my life, I was neither encouraged nor discouraged. It would have been excellent to have someone actively encouraging me, but it didn’t happen. The inspiring math teacher in high school did help. In college, I started studying engineering, and, later on, I went into physics, and in both cases, there were less than 10% of females in the courses and in those careers.

I sometimes make the joke that my father, unintentionally, prepared me well for the world of physics. I was the only child he had, and during the summers he would bring me to the cattle market, where all his friends would bring their boys to train them in the business from early on. And of course, in those times, that business was mainly done by men, only by men, not sure how that has evolved now…I was there every summer, and I learned to enjoy the fact that it was totally different from my other experiences in the big city. I learned to deal with an environment that was so different from mine, in many ways, and I made good friends among those boys. It was all quite interesting to me. You see, in some way, I believe those summer experiences prepared me to feel more comfortable in the future in the physics world, where obviously women scientists are a minority. I am glad to observe though that, although slower that desirable, things are changing for the better in that sense.

Zierler:

Now, in the Argentinian system, it's more like the European system where you start already committed to a major, there's no general field of study, and then you declare the major later on. So, this means at the beginning, you were committed to engineering and physics?

Carena:

This is one thing that with my kids going to college in the U.S. I have learned to appreciate. I am convinced it is much better here, that students have at least two years to better evaluate what they like. But indeed, in Argentina, it's like in most European systems, you have to make up your mind after high school. That can be challenging. For me it was! I was good at math but my physics education at high school was minimal. It turned out that the best friend of my boyfriend at the time was studying engineering, was a year older than me. Talking to him I kind of made up my mind that it was a good idea to go into engineering. Looking at his entrance exam notes I rapidly realized that I was totally unprepared for such a tough exam. As I mentioned, I had to work quite hard, but luckily, I passed it and I was admitted; I was very happy -and it set my mind away from the fact that my dad had passed away several months earlier, all of a sudden. After that I had the incredible experience of a gorgeous trip to Europe, first time in my life outside Argentina ever! I saw amazing museums and experienced different cultures and customs. It opened my mind to the world in unpredictable ways, also made me look at my dearest Argentina from a totally different perspective.

When I came back a few months later I dived into my studies to become an engineer. That didn't last long! I mean, soon I realized that building bridges was really not my thing. Of course, at the beginning there was a lot of math and physics, that was fun. I was attending a (semi) private university that used to be run by the Navy: ITBA, Instituto Tecnologico de Buenos Aires, I was super happy with the academic life, but then there was just the structural part of engineering that was not for me. During my second year I decided I was going to study philosophy, but just in case I didn't leave my engineering studies and I went for both of them in the second year. Again, I studied hard for the entrance exam, now at the University of Buenos Aires, right, the public university, which is extremely good. I was admitted and I dived into my first year of philosophy. It was very exciting! But again, after a while I thought it was kind of too fluffy for me.

At the same time, at the Engineering College, I met my future husband, Carlos, and well, we were studying together and, and we started reading Borges… I don't know if you're acquainted with his work, but he is my preferred Argentinian writer, my preferred writer really. I had read Borges in high school, but reading it with Carlos, being a bit more mature, we asked deeper questions about the meaning of God and trying to understand the universe with a more inquisitive attitude. Probably Carlos was already quite decided to pursue studies in physics, but I believe that for me was the fact that somewhere in between engineering and philosophy I met physics. There I was, preparing my last college admission exam and I ended up going to the Instituto Balseiro, which is in the northern extreme of the southern Patagonia, a marvelous place by the mountains. I started studying physics in an amazing place surrounded by people who shared my desire to learn and my crave for understanding and questioning the basic laws that govern our universe, our everyday life. This was the same Institute my math teacher in high school had mentioned to me as a great place to go and do math and physics, and there I was!

Zierler:

Marcela, this is a totally random question, but between your ethnic background in Buenos Aires and Instituto Balseiro, did you ever cross paths with Juan Maldacena in Argentina?

Carena:

Not with him at the Institute because he's a bit younger than me. So, I left before he started.

Zierler:

Uh huh, okay.

Carena:

Of course, I know Juan now, and we met outside of Argentina, in the Aspen Center for Physics, many years ago. We have been good colleagues since then. There are quite a few Argentinian physicists around the world, and we mostly stay connected.

Zierler:

Marcela, how did you develop a specialty in physics? First, did you always know that it was going to be theory? Or did you consider experimentation at some point?

Carena:

At the Instituto Balseiro we had amazing courses in quantum mechanics and quantum electrodynamics. And I had an amazing math professor who was the only female professor in the whole Instituto Balseiro at the time. And then we had great professors in more advanced courses in particle physics, condensed matter, statistical mechanics and much more. I was specially taken by the courses of Quantum Field Theory taught by Luis Masperi (Luigi), he was not only an amazingly charismatic figure but also a superb teacher, and that made me decide I wanted to join his group.

On top of that, although we had excellent experimental courses, my heart was not there. At the time I started doing some basic research in the area of particle physics, the arXiv and all these communication tools we enjoy today were non-existent – it is hard to picture that today! So we were a bit isolated in Bariloche with respect to research done in the rest of the world. I remember being very excited about connecting more with forefront research in high energy physics in the boundary between theory and experiment when Guido Altarelli came to Bariloche from CERN to teach at a summer school. It was the time of the Large Electron- Positron Collider (LEP) and he was talking about electroweak precision measurements and Higgs physics. I found that fascinating! but it was not until a bit later that I started working on what we call theoretical particle physics phenomenology, which means the part of theoretical particle physics that directly cares about experimental signatures. Although hands on experiments was not my strength, I realized early on that I was interested to think about the mysteries of particle physics, trying to probe theoretical ideas with experiments.

Zierler:

Marcela, another cultural question at the Instituto Balseiro, an undergraduate at a place like Harvard or Oxford will often say that their education was parochial. In other words, their education in physics, was the physics of Harvard or the physics of Oxford, right? Their world was not much larger than that. Does that resonate with you? Was your, was your education rather parochial in terms of what was happening at the institute? Or was it a more global approach? And you were aware of the exciting things that were happening in the United States and Europe at the time?

Carena:

Half and half, I would say, at least in particle physics we got a lot of visitors, especially from Europe, actually, as I mentioned, Guido Altarelli was one among several who came from Italy, Spain, Portugal, France, etc., to deliver inspiring summer school lectures. The reason I started studying and working on supersymmetry, was thanks to colleagues from Spain, Tony Mendez and Tony Grifols, who came to Balseiro and gave month-long courses on supersymmetry and other trendy ideas of unification and beyond. And so that's why I concentrated my diploma thesis on exploring supersymmetry and its implications at LEP. I believe the dynamics was different in condensed matter where they were thinking about superconductivity. the Bariloche group was bigger and had strong connections with Europe and U.S. and experiments were carried out in Bariloche as well (unlike the big collider experiments that were off reach in Latin America).

Zierler:

What kind of advice did you get as you were ending your, your original education and thinking about where to go to graduate school? Did you consider staying in Argentina? Did you get the specific advice that based on your interest and abilities going abroad would best serve you?

Carena:

This is an excellent question! Our professors were clear about, and they have also been abroad, that it was the best for me, for us, to go and do a PhD abroad. But at that time, the path forward in High Energy Physics was not well defined. In that sense, it is my impression that for Juan and other people that came later, they were given a better picture of which abroad options were available. We didn't have a lot of guidance. So, I remember, Carlos and I, we wrote to US universities to ask for input, for information to apply. But the information we got was: you need to prove you have a way to support yourself to be a student. This is actually not the case for a PhD in physics, but it was not at all clear to us. Especially at that time in Argentina, the dollar had very high value, and it was well beyond our possibilities. I remember these considerations quite well and, yeah, we decided that was not possible for us to ask that level of support from our parents. We tried the DAAD fellowships to do our PhD in Germany, but physics was not at the top of their priority list, and we didn't get it.

We ended our diploma in physics, we call it “licenciatura” - which is really like at the level of a master in the U.S. – this was December 1985, and we also got married a week later. Carlos and I got fellowships to stay and do our PhDs in Bariloche, and so we stayed! And things were going that way until a couple of months later we went to Brazil, to the Swieca Summer school in the outskirts of Sao Pablo. And things changed right there! I met my future advisor Roberto Peccei who was one of the professors lecturing in the school. He was teaching the Standard Model of particle physics, with such a passion that was impossible to remain detached even if you would try very hard. He was explaining physics with his hands, as we say, which is a very Italian and Argentinian style. And I was like, wow!!

Zierler:

Had he done his collaborations with Helen Quinn at that point when you met him, or that was later on?

Carena:

Oh, yes, that was already done. Yes. Because he was at DESY.

Zierler:

So he was a famous man at this point?

Carena:

Yes, yes! He was a famous scientist. Yeah, I knew of him, but I had never met him. So, one thing that happened at the Brazilian school is that, first of all - some people laugh when I say this - but I used to be super shy. And when some of my collaborators find it amusing that I say I used to be shy, I tell them this: although in high school I was very good at math, when the teacher would ask for the answer of a math problem, which I knew very well, it was very hard for me to put my hand up to give it. Indeed, I was very shy.... also, when I met Roberto, but the last day of the school, I was fascinated with all what I was learning there, and specially with Roberto's lectures, and because of that I went to him, and I said: (after chatting a bit about this and that): “Do you think it's possible for me to go and do a postdoc at DESY?” And then he answered: “Well, maybe it's a better idea for you to first do a PhD.” So, next day, we sat, him, Carlos and I, and we discussed possibilities. And two weeks later, we were back in Bariloche, and we had a telefax (usual communication tool in those times!) that said: I have two Ph.D. positions for you if you want to start this September…We accepted immediately!

Zierler:

And this is as we say, you solved the two-body problem right from the beginning.

Carena:

Yes. That was easy, and we didn’t know how lucky we were, but it became more interesting later. At that point, yeah, we left in September for Hamburg and DESY lab. Roberto was the head of the DESY theoretical physics department, and he became my PhD advisor. Carlos went to study under the supervision of Istvan Montvay and worked on lattice gauge theories. The unfortunate part of that for us, was that while we were in Argentina preparing to move to Germany, Roberto got an offer to become chair of the Physics Department at UCLA. He told as about his potential move ahead of time, in case we wanted to reconsider our move, but we were totally decided to go anyways. So we went. And, and yes, it had its ups and downs, but it was a fascinating experience to move to Germany.

Zierler:

Marcela, it's such an interesting parallel between your current dual affiliations with Fermilab and the University of Chicago and the beginning of your graduate career split between the University of Hamburg and DESY. I wonder if you might compare and contrast those, those dual appointments?

Carena:

Well, at DESY the physics department, occupied the second, third and fourth floor of building IIa, while the fifth and the sixth floors were occupied by the University of Hamburg Physics Department. So, we were all in the same big building, just a couple of stairs away. Of course, there are people from Fermilab going to UChicago and vice-versa, but this involves a one-hour drive, even with light traffic. On the other hand, I think German style, at least at DESY back in the time, used to be much more close door offices. As a student, I did not have a lot of interaction with the people at the University of Hamburg. I was mainly connected to all the activities on the floors that had to do with DESY theory. My main interactions with university professors were because I had to pass the equivalent of a level exam, so that my physics diploma from Argentina would be recognized. Besides that, I would go to the University main campus three or four times per week, because one was required to pass a quite demanding German language exam as part of being students in Hamburg University. We had two years to comply with the language requirements, but we were very motivated and passed the exam in three semesters. It was great because I would have not put so much effort in learning German otherwise. At the end because Roberto was moving to the US, we finished our PhD in one year and nine months, so the German level exam would have not been needed after all! When we went back to Germany after a short stay in the U.S., we took postdoc positions at the Max Planck Institute in Munich, so knowing the German language was very handy, and we even enjoyed very much a reinforcement course at the Goethe Institute. Being at a place, it is of course very important to speak the language to better understand the local culture. My relationship with German colleagues and the Alexander von Humboldt Foundation continues very vibrant today. and I often host German based postdocs at Fermilab, through the Humboldt-Feodor Lynen fellowships.

Zierler:

Did you spend any time at CERN over the summer or travel there at all?

Carena:

Well, while being at DESY certainly not, because it was a very intense schedule, between 9 to 12 hours per week of German language plus working and finishing the Ph.D. thesis in a two-year period. I didn't go anywhere really. I did not go professionally to CERN until I got a postdoctoral position there.

Zierler:

And what were the big experiments at DESY while you were there? What was so exciting during that time?

Carena:

At my time at DESY, the electron-positron storage ring DORIS was in full operation and the year I arrived, 1987, experimentalists using the ARGUS detector discovered B meson oscillations - that means B mesons can transform into their antiparticles. This was decisive in understanding that the top quark had to have a huge mass. As we all know, the top quark was discovered only two decays later at the Fermilab Tevatron collider. Also, a larger version of DORIS, PETRA, was in operation and had made possible the discovery of the gluon in the mid-seventies. During my time at DESY, the electron-proton collider HERA was under construction and marked a new era in international cooperation for construction of particle physics facilities. The HERA detectors H1 and ZEUS have been instrumental in making discoveries to understand the composition of protons from quarks and gluons. I would have not been so informed about the experiments at DESY if it would have not been for an Argentinian co-national, Carlos Gabriel (Cali) whom I met at DESY and was a postdoctoral fellow working in the particle physics experimental program there. We became good friends for life. Cali shared with us his passion for physics, and we used to have long conversations related to his unique life-story connected to the tumultuous military regime in Argentina. In sum, I was very immersed in my thesis, and it was only a few years later, at CERN, that high energy physics experiments really became a fundamental part of my research interests.

Zierler:

I'd like to ask before we get to your thesis research itself, a broader question. And that is, if we step back and look at the overall intellectual thrust of your research career, you fit in so well with a broader narrative of particle theorists whose work ultimately has value in astrophysics and even cosmology. I'm curious, were you thinking along those lines as a graduate student at all? Like, were you thinking that possibly, theoretical particle physics might have important things to offer astrophysics and cosmology? Or did that come later on?

Carena:

Well, the stars were my first physics love affair as a child, and I did a brief detour into astrophysics when the supernova 1987 explosion was detected, but it was just a small diversion. At DESY, I was mainly working on what my advisor was proposing… Of course, sphalerons, a central topic in my thesis. are indeed related to early universe physics, but my real interest in astroparticle physics and cosmology came later on. It started somehow, somewhere during my time at CERN, and then it developed further at Fermilab. Nowadays, of course, as you say, it is a very essential part of the science I'm interested in. Also, it matters that the scientific knowledge in the area has advanced so much and we can ask sharper questions.

Zierler:

Marcela, two questions about being a graduate student in the late 1980s; one scientific and one political. During the late 1980s of course, this was a period of major excitement in the United States because it seemed like the SSC was really going to be built. I wonder if all the way at DESY these developments were resonating. People were paying attention to what was in the works in Texas.

Carena:

I'm sure they were but they did not percolate into my graduate student world.

Zierler:

You were too focused.

Carena:

I had a lot to deal with: a new culture, a new language, and a new direction in my physics research. My social and scientific interactions were more in compartments. I have friends that were from the U.S. and were following more those developments; Andreas Kronfeld was an American postdoc who became a good friend and later my colleague at Fermilab, he was following the SSC excitement more closely. Also, my friend David London from Canada was much into it. For me, I was connected with the U.S. only from the distance. When I came for my first postdoc to the U.S., and then I was visiting Fermilab, that's when I met Bill Bardeen for the first time, and we talked more about science in the U.S.; he was about to move to Texas to become the head of the theoretical physics department at the SSC. When in 1993, already at CERN, I heard the very sad news, I felt profoundly saddened for the loss for science and because I knew how much effort and expectations those colleagues I knew in the U.S., where putting into it.

Zierler:

Marcela, I wonder, it's a different question, but perhaps you'll give a very similar answer--just by virtue of being in Germany, West Germany in 1988, 1989. Were you aware of the coming revolution, was the feeling in the air that the Wall was coming down? Did you have any sense or notion of that?

Carena:

Yeah, actually, in 1988, a group of friends and myself went to Berlin, and that group included a couple of German friends. We drove there from Hamburg through the highway that was in East German territory. It was a very exciting experience getting into eastern occupation territory! It was strictly forbidden to get off the main road and to go very fast. In reality, they wanted to collect some hard currency from Western European tourists, even if we they were poor students as ourselves! [laugh] So we went there, and we did a mistake at a fork and drove away from the main road. Feeling a bit scared we back up about hundred meters and we continued. And then, just before entering Berlin, we were stopped by the police. And they said: “oh, 100 kilometers from here, you went backwards in the highway, and that's not allowed.” They gave us a fine; basically, they took almost all the money we had in our wallets and let us continue. And then we went and crossed at Checkpoint Charlie, it was very striking to experience life behind the east curtain. I left Germany just a few months before the wall went down, but prior to that there was so much discussion among the students and young scientists about the implications of such decision. More senior professors were concerned with the economic impact. When I returned to Germany two years later, Carlos and I got postdoctoral fellowships at the Max Planck Institute in Munich, it was really a different society. Our colleagues would engage in long discussions evaluating the pluses and minuses of German reunification, for economy, health, working conditions and science. Okay, not to mention that it was impossible to buy a used car at an affordable price, since the car market had collapsed under the new demands from the east. Again, we were lucky to meet great friends at the Institute and outside it. It was probably the nicest period of our postdoc years in the beautiful and effervescent Munich of the early nineties.

Zierler:

Marcela, back to the research, what was the central question of your thesis research? And looking back, how at the time was it responsive to some of the larger questions in particle theory at that time?

Carena:

Well, I was coming from Bariloche where I had started to work for my PhD thesis on supersymmetry, because I was interested in the particle physics problems such an extension of the Standard Model could solve. At that time, it was clear that you could not do a straightforward unification of the forces of the Standard Model, because this would predict that protons decay at a rate that would have been already seen in experiments. But we knew that adding the ingredient of supersymmetry you will get a larger value for the unification scale and make the proton live longer. It was an exciting direction to think about. I mean, supersymmetry has a lot of good properties, but the idea of unification of forces was particularly intriguing. Supersymmetry was the topic of “my first PhD attempt’ in Argentina. I was very excited about understanding more about it, where were those SUSY particles that could make the idea of unification of forces work. But then when I went to Germany, Roberto was less excited about supersymmetry and more excited about many other things…

My time in Hamburg was intense and challenging, I was trying to investigate the generation of fermion number in the background of bosonic topological configurations, which, when considering scalar and gauge fields involve “sphalerons”. Roberto was intrigue by them. I believe in the back of his mind the idea was to generate baryon and lepton number from these special field configurations. I got deep into exploring an interesting paper from Goldstone and Wilczek on the so-called adiabatic method and it appeared that some unexpected results were coming from it. At the end of the story, I realized that the adiabatic method was not applicable in my particular case of interest since the adiabatic current became ill defined at points where the scalar field vanished. I learned a lot about sphalerons, but most importantly I learned a lot about the scientific method and how exploration of the unknown can become your obsession until you find an answer that satisfies you.

Although at the beginning I was just thinking about mathematical solutions; later on, the idea of sphalerons became exciting from a different perspective. I remember, Rubakov and Shaposhnikov were visiting the DESY theory group from Russia, and they were thinking about baryogenesis, and how to make the sphalerons at high temperatures play a role in generating baryon number. I recall thinking: “I need at some point to explore this ideas”… and I am doing it still today! We had so many stimulating discussions, I had great opportunities open to me just by being Roberto’s student.

My future good friend, Andreas Ringwald was also thinking about sphalerons in Heidelberg, and he came to DESY. His idea was, if we have enough energy, we can generate these sphalerons at colliders. That didn't work well but was an enticing possibility. At DESY I studied in great detail the thesis work of Richard Mackenzie, a former student of Frank Wilczek, following up on adiabatic methods and generation of fermion number in the background of bosonic topological configurations. Then, by serendipity I had the opportunity to visit Montreal and meet him in person. It was a pleasure to learn that someone in the whole world could appreciate my PhD work so deeply! My work connected to the most deeper questions of the baryon asymmetry of the universe but it was hard to sharpen those connections early on. The mysterious sphalerons didn’t fulfill my expectations at the time. However, their possibly deciding role in explaining the generation of the observed matter-antimatter asymmetry of the universe, at the moment of electroweak symmetry breaking, would remain a topic of intense research until today.

Based more on my mathematical thesis results, I got a postdoc offer at the University of Bern. Roberto helped us once more with our two-body situation and Carlos and I got two offers from Purdue University in the US. Although it was a small particle physics group, it was a superb experience. Thanks to that postdoc at Purdue, I met Bill Bardeen, while visiting Fermilab. He became a great mentor, and that, in time, had a big influence in me being at Fermilab today.

Zierler:

Marcela, were you specifically motivated for postdoc opportunities in the United States? Did you, did you know that a national lab environment in the United States was something that you were specifically motivated to be a part of?

Carena:

Not really, my main interest was to get a postdoc job. I wanted to stay in Europe, honestly. I had never been in the U.S. before, until then.

Zierler:

Did you give thought about going back to Argentina at all?

Carena:

I did, but only much later, after my CERN experience, before coming here for my position at Fermilab. In between CERN and Fermilab was the time for big, life changing decisions.

Zierler:

What was Bill working on at that time, when you first connected with him?

Carena:

He was working on ideas of dynamical symmetry breaking. I enjoyed so much my discussions with Bill! His relaxed attitude and humbleness in dealing with difficult physics questions made a big impact on me. Bill’s deep thinking and understanding of fundamental, challenging physics problems inspired me and gave me the strength to pursue new ideas. I learned so much from Bill! At the time he was thinking of strong dynamics and we didn't know the Higgs was there. There was this alternative dynamical electroweak symmetry breaking mechanism that did not need a Higgs, but instead used an infrared fixed-point behavior for the top quark Yukawa coupling, meaning that it needed a very heavy top quark. That would explain the fact that the top quark had not been seen yet at CERN or at Fermilab, because it was quite heavy. With Bill and collaborators, we explored implication of dynamical symmetry breaking and supersymmetry and predicted that the top quark mass – not yet discovered! - had to be between 140 and 195 GeV (this was 1991). Bill stayed as a powerful figure in my scientific career for many years to come.

Zierler:

How did your next opportunity to become a postdoc in Munich come about?

Carena:

My mentor at Purdue, Tom Clark, was Wolfhart Zimmermann’s only former student. Zimmermann was a Max Planck Institute director who visited Purdue and got positively impressed by our work. After the Purdue experience we got offers from a few places, in four different countries, so that was great. One was in Munich, the others in the U.S., Canada, and Spain. I really wanted to go back to Germany and Munich appeared to be a great city, and indeed it is! I believe today it was my lucky star to go back to Munich, because there I met Stefan Pokorski. He is not only a superb scientist, but also an amazing, generous mentor and now, a dearest friend. He taught me a lot about asking tough phenomenological questions and thinking deeply about data. He strongly marked my research style.

Zierler:

In what ways?

Carena:

He taught me to ask the most provocative and deeper questions, and not to be shy in challenging other’s opinions even if they were presented in a very categorical manner. We used to have these long conversations about physics in the Student’s Kantine, long dinners full of physics thoughts and questions, and then we will come back in the morning fresh and do the detailed calculations in the blackboard at the Max Planck Institute. He generated a vibrant and positive social atmosphere, where it was so easy to pick on each other’s ideas and potentiate the outcome of our research. In those days Stefan, Carlos and I worked a lot on ideas of unification of forces and supersymmetry. We were fortunate to have Stefan there, he was traveling between Poland and Munich, but when at Munich he was on his own and was working like a postdoc. It was lots of fun. And I know today that who I am as a researcher has been deeply influenced by Stefan’s inquisitive approach to physics.

Zierler:

Marcela, what were your overall impressions of the Max Planck Institute?

Carena:

It felt good to be there! There were a lot of students and postdocs in the group, and we used to go out together quite a lot. The staff was super friendly and welcoming, and the scientists liked to take nice walks from the MPI into the Englischer Garten. There were basically no women among the scientists, so that was a downside, the one I remember is Doreen Wackeroth, now a professor at the University of Buffalo, who was a student at the time. When at the institute people were reserved, besides Leo Stodolsky who would always come to ask quirky questions; I don't know how things would have been without Stefan… he made MPI very special. Altogether, it was a fun time of my life there, both scientifically and at the personal level.

Zierler:

And what were the new research projects that you were a part of during your time at the Institute?

Carena:

At the time, the LEP collider was running, and we were all very excited about the higher level of refinement at which the Standard Model was being tested. There were issues at the fundamental level that needed exploration: mass generation, the properties of matter with the triplication of fermion masses and assignment of quantum numbers, and the mystery of forces -the possible unification of couplings, as expected in Grand Unified Models. From measurements at LEP and neutrino scattering experiments, it was clear that the simplest grand unified models were not viable. Supersymmetry appeared as a promising alternative that provided a larger value of the unification scale, thereby preventing too fast proton decay and avoiding conflict with experimental data. I was already thinking much about supersymmetry and ideas of dynamical symmetry breaking, while at Purdue, with Bill Bardeen and others. After meeting Stefan, we dove into computing the requirements on the supersymmetric particle spectrum to achieve gauge coupling unification, based on the interplay among the Weinberg angle, the strong gauge coupling and the yet unknown value of the top quark mass. This also gave predictions for the Higgs mass, depending on the supersymmetric model, and if one would consider equal bottom and tau Yukawa couplings at the unification scale it would yield a prediction of a rather heavy top quark mass, in the 140 to 190 GeV mass range! In retrospective this was a great prediction! And it was essentially the same prediction we obtained in the work on dynamical symmetry breaking. The reason behind it, is that in both cases the top Yukawa had to be close to the same infrared fixed point!

These were very interesting times to be thinking about data and new physics predictions: the top was not there yet and the Higgs as a solution to electroweak symmetry breaking was under much scrutiny. The paradigm of unification of couplings, including the onset of strong dynamics or with perturbative unification was providing a guiding principle with many measurements from LEP and the Tevatron to come.

I also put quite some effort on thinking more in detail about Higgs physics, the type of higher order calculations I was already doing during my first postdoc. But then also with Bill and other collaborators, we start thinking about radiative corrections to the Higgs mass in supersymmetry. And how all could be related… we had a prediction for the top mass, and in supersymmetry we had a prediction for the Higgs mass to be below 130 GeV. I would keep working on Higgs physics much more once at CERN, because it was one of the fundamental reasons behind trying to extend LEP2, to find the Higgs!

Finding the top, finding the Higgs boson and understanding supersymmetry were at the core of it all. Today we know that supersymmetric particles don’t appear to be the lowest hanging fruit...but at the time we didn’t. And so, my thoughts were revolving around these three interrelated topics, trying to get predictions and imagine how they will be tested in the coming years.

Zierler:

And then, Marcela, in 1993, finally you arrive at CERN.

Carena:

Yes.

Zierler:

Because, because of your interest in the Higgs, was that part of the motivation to go to CERN?

Carena:

Well--

Zierler:

Or is that still too early?

Carena:

No, no. At that time, I was indeed very interested in Higgs physics. But I wouldn't say I went to CERN because I thought LEP would discover the Higgs. I would say I went to CERN because being in Europe, going to CERN in the middle of LEP running was just amazing! It was being part of the best I could think of in particle physics at the time. The Max Planck was offering an extension for an additional postdoc year for Carlos and me, at MPI we had a great scientific environment, besides our collaboration with Stefan, and also Bill who went to visit, we had inspiring scientific discussions with MPI directors Stodolsky and Zimmermann, and with Andrezj Buras and Wolfgang Hollik, was a vibrant period, very enjoyable. We only applied to a few great places that fall, CERN included. And one day, John Ellis called me and said: “Well, I would like to offer you a postdoctoral position here (CERN)”. I had only applied with my Argentinian credentials, so John was offering me the World Lab fellowship. I was not so sure what it meant but it was just super to get a CERN offer! We talked a little bit about physics, I do not remember all the details, because I was a bit tense, and by the end of the conversation, I asked him, so what about Carlos Wagner? And he told me, oh, he's on the waiting list. Then I, I did the most silly thing I could think of and said: “Okay, that's great I will wait to see how things develop with Carlos and then I will make my decision – thank you!” Well, that was an irresponsible move, but it came from my heart … A week later, Carlos got an offer. So, that was also a dream come true, going to CERN at the time of LEP when big discoveries were awaited!!

At CERN, for the first time in my life, Ι got immersed deeply into what was going on in the experimental world. Every single day I would discuss with experimental colleagues, postdocs like me, such as Fabiola Gianotti, Patrick Janot, Eilam Gross, Michael Schmitt, Gustavo Wolf, Marta Felcini, … all obsessed with finding the Higgs and searching for Supersymmetry at LEP. At that time, we heard of the sad news about the SSC, it was a big shock for the whole particle physics community, especially in the US, I knew of many colleagues there who had re-arranged their lives around it…

A big effort towards the Higgs discovery began at CERN, and although I was just a postdoc, I had the great opportunity to be deeply involved in writing the physics motivation for LEP2 and how important was it to crank up the energy. And so, Guido Altarelli, a person I admired since I had met him in Bariloche many years before, was the one in charge for making the case for the upgrade, and he asked me to co-lead the Higgs part of it with Peter Zerwas, with the help of many other great colleagues. For me it was mind blowing! And I put all my energy into making the arguments in favor of LEP2 upgrades as strong as possible: discovering the Higgs boson was the main objective and supersymmetry was behind all of it, to advocate for a light Higgs boson - most likely with a mass below 130 GeV or so.

So that was life at CERN… 1995 was a great year for me in physics. I also started to work with two of my long-life collaborators, Howard Haber and Mariano Quiros. With them, Carlos and other excellent colleagues I started a program on Higgs mass precision calculations in Supersymmetric theories and its related rich phenomenology at colliders. And that year the top quark was discovered at the Tevatron!!! This were revitalizing news after the sad fate of the SSC a couple of years earlier.

Zierler:

Yeah.

Carena:

Personally 1995 was also such a unique year for me! I was expecting my first child. I tried to keep it secret because I didn’t want anyone to believe that I was less committed to science because of the baby. In retrospective that was so silly, but that's the way I felt back then. CERN was a great place for physics, but also was huge and there were not many pregnant lady physicists around…

Zierler:

Marcela, another SSC question, of course, by the time you got to CERN, the SSC had been cancelled, or was in the process of being canceled. From your vantage point, you know, before you said that the issue didn't percolate up to you, I'm sure it did when you were at CERN. I'm curious how that news resonated at CERN. In other words, was there, you know, in terms of the competition between European high energy physics and American high energy physics? Did it feel like European was really assuming a mantle of leadership at that point? And, and inversely did, was there additional pressure that was felt at CERN as essentially the only game on the planet at that point that would be able to conduct experiments at these energies? I'm curious how SSC, the developments there, sort of influenced the overall research environment at CERN at the time?

Carena:

And well, again, I'm, I'm probably going to deceive you with my answer… I'm sure that, John and Guido and, and many of the more senior phenomenologists at CERN were having very elaborated thoughts about this, but this was not shared with me, or with other postdocs, as far as I know. So, my, my vision from the cancellation of SSC was mostly based on the people I knew in the US who were betting their careers on it. You know, Chris Quigg, and Bill Bardeen were changing their career paths in a very decisive way to contribute to the big SSC program.

Zierler:

Right.

Carena:

I was mostly interested with the coming phase two of LEP, that was going to scrutinize the Standard Model weak force and explore what was lying beyond it. In late 1995 I got absorbed with building models to explain the matter antimatter-asymmetry of the universe through its generation at the moment of the electroweak phase transition, using as main actors the Higgs boson and supersymmetric particles that we expected to be interacting significantly with the Higgs – the top quark superpartners. The beauty of electroweak baryogenesis is that it most likely will leave imprints that can be tested at terrestrial and extraterrestrial experiments, and in many cases can also accommodate explanations for dark matter within the same new physics model. The simplest supersymmetric extension of the Standard Model I looked at the time, has been by now excluded through data collected by LEP, the Tevatron, and the LHC. But nowadays we are coming with novel ideas to implement baryogenesis at the moment of the electroweak symmetry breaking that are quite intriguing, and they could even leave traces to be seen at future gravitational wave experiments.

The next big step for me was to come to the US where the Tevatron was getting into a new phase and the competition between the Tevatron and LEP was something that was in everybody's mind. Although the Tevatron originally was not built to discover the Higgs, suddenly the opportunity was there! It was tension in the air.... I was caught in between two loyalties, my colleagues at CERN and my new job at Fermilab. It was a real race for the Higgs boson: If we don't push LEP now, then there is an opportunity for the Tevatron to get the Higgs before the LHC. At the end what matters is the discovery, but still…. [laugh]. The Tevatron experiments fought till the end, they did amazingly well, even if they did not win the first prize. My experimental colleagues at the CDF and DO experiments did a heroic job! I was proud to be a small part of it.

Zierler:

Now your affiliation with CERN was as a postdoc. You were not assigned a staff position there?

Carena:

Yes, I was, later in time. I was a postdoc at CERN for about three years. And then I came to Fermilab with an Associate Scientist job. Well in the middle, life was quite eventful… My son Sebastian was born and, Carlos got a staff position at CERN the day after his birth. Two great things in my life, but my next worry was, I had to get a job for myself, somewhere. And yes, I had postdoctoral offers at Heidelberg and then I got a Marie Sklodowska-Curie fellowship to do research at DESY and my plan was to go back to my starting place. I recall sitting at my desk at CERN with four month old Seb on my lap and reading my e-mails, when I got the Fermilab offer, I cannot express in words how happy I felt! The next three years were quite intense, Seb spent his first years of life flying from Chicago to Geneva on a monthly basis, and the CERN Theory Division corridors and cafeteria were his second playground. Fermilab daycare was of huge help for me. Two years later, I actually got a staff position offer at CERN, which I took for a little while, to make our family life better. People at Fermilab and at CERN were very understanding, and my husband very patient, a gem; but the one who made the big difference was my mom. She spent long periods in the US with me, and was a great second mom for my son. She departed almost two years ago, but - wherever she is today, certainly deep in my heart: thank you mom, so much!

Zierler:

Not easy. When you went back to CERN, did you take leave from Fermilab, or you left the position?

Carena:

No, I took leave, I'm trying to remember. I was promoted very fast at Fermilab, like, I became a Scientist (tenured) equivalent position, less than two years after I started to work here, and at the same time, I got offered the staff position at CERN. As you can imagine, we were trying very hard to solve our, two body opportunity, as we call it now. And so, we were applying for jobs and interviewing and we got other job offers that prompted the events. I took leave of absence for a year and then in the meantime, we kind of solidified my husband's situation in the US.

Zierler:

Marcela, it's probably right around this time, when you can finally start thinking about settling down in a long-term career in a particular place.

Carena:

Yes, it was hard to project our lives long-term. Fermilab was very different from CERN, but I fell in love with Fermilab from the first time I came. But it was also hard to leave CERN, because it was a very attractive magnet for us.

While at CERN I had so many great collaborators, working on Higgs Physics, superymmetry and baryogenesis, mainly. I wrote my first paper to explain the possible impact of light supersymmetric particles in the value of the anomalous magnetic moment of the muon, just before the Brookhaven experiment started to take data. At CERN there was a huge international community of colleagues, many of them who have stayed our close friends throughout the years. Theorists in similar stages in their lives, with whom we built long-lasting friendships, such as Gian Giudice, Georg Weiglein, Sven Heinemeyer, Matthias Neubert, Magda Lola, Apostolos Pilaftsis, Alex Pomarol, Silvia Mollerach, Esteban Roulet, Fabio Zwriner, to mention just a few, c'était genial, awesome!!

At Fermilab, I was more isolated, and I had less time to build friendships right away. I concentrated quite intensively in trying to connect new theoretical ideas in particle physics directly with experiments. From my perspective as a SUSY Higgs expert, the culture was very different at Fermilab. And I came at the very end of '96 when Fermilab experimentalists had just discovered the top quark. They were all super excited about the top and mostly not thinking too much about the Higgs, for many reasons, including that it was not in the menu of possible discoveries at the Tevatron. Also, I was a newcomer, and people in the Midwest were more reserved than what I was used to. I was determined to help building a stronger connection between theory and experiments in my areas of research. As I said, at CERN I had contributed to better define the potential of LEP2 for Higgs boson physics. Now I was at Fermilab and thought, okay, there is an opportunity to see what the Tevatron can do for the Higgs, right? So, I put all my energy in that direction, with supersymmetry also in the back of my mind, since SUSY predicted a relatively light Higgs boson that would be more likely to be discovered at the Tevatron. Around the end of 1997, I jumpstarted a considerable effort, together with experimentalists at CDF and DZero, the two Tevatron experiments, to evaluate their potential for discovering the Higgs boson or some SUSY particles. I recruited my good colleague Joe Lykken, who was a Fermilab string theorist at the time, to help me on the theory side, and also give me more credibility with the other theorists in the US. He was starting to pay attention to Tevatron physics and he knew many people in the community that I didn’t. I was glad he teamed up with me to propose a series of workshops called the Physics at Run II workshops. We started with Higgs and Supersymmetry, and other colleagues did other topics.

It turned out there were many excellent experimentalists interested in the Tevatron Higgs working; John Conway and John Hobbs led the CDF and DZero efforts, Howie Haber and I led the theory part. Another key leader of this whole effort was my colleague from UChicago Henry Frisch, who was always optimistic and encouraging about the potential for discoveries. I would fail to go through all the many names of those who did outstanding contributions, but the result were good projections for the Tevatron Higgs potential, and the LEP hints of a light Higgs were enticing. We told the Fermilab Director John Peoples in 1999 and wrote in our report that with 10 inverse femtobarns of luminosity from the Tevatron there was the possibility of getting three sigma evidence of a Higgs boson with mass around 125 GeV. The Tevatron had plenty of time ahead before the LHC to make a game changing discovery! And, you know, when finally, the Tevatron shut down in 2011 the two collaborations combined had 2.9 sigma evidence of a Higgs between 115 and 135 GeV, confirmed of course by the LHC discovery the following year. So that was really quite, quite impressive. I started to enjoy so much my life at Fermilab, working with lots of experimentalists and many theorists on ideas for novel searches for additional Higgs-like particles and other possible hints for new physics. In some way I brought with me all the luggage of great experience I had with the LEP experiments at CERN and tried to develop a similar effort at Fermilab.

Zierler:

Marcela, when, when did your affiliation with Chicago start? The University of Chicago?

Carena:

Actually, I was first approached in the fall of 2000, but I knew I was expecting a baby. I thought that was not the right time for me to start commuting from Fermilab to U. Chicago, it is an hour drive apart. So, I elegantly delayed any answer. Julian was born in mid-2001, and that was when we finally settled down in the Chicago area. I kept only informally connected to U. Chicago until 2008 when I was approached again and that time I said, Yeah, why not? Both kids were already in school, and things were much more under control with them. I loved the idea to interact with students…

Zierler:

Yeah.

Carena:

And teaching.

Zierler:

Marcela, to clarify, without the affiliation at Chicago, your ability to interact with students was rather limited.

Carena:

Yes, it had fluctuated. We had some students at Fermilab., but having students was not the main core of the efforts, which was more focused on training postdocs in a data rich environment, And that's great. But I realized that devoting yourself to teach a course means you have to rethink things to better explain them, and that has lots of benefits. Honestly it took me some time to really commit to students of my own, because of the Fermilab – U. Chicago distance and the fact that students not always have the possibility to commute easily between both places. I'm very happy that the past few years have been maximally rewarding vis a vis supervising students. At the moment, I am working with four female students from U. Chicago. My first student just graduated, and she got a great postdoc job at Caltech, well deserved! Back in 2004, I also created a Latin American student program with support from the Fermilab Directorate, to give fellowships to outstanding students who were doing a PhD at a Latin American university to come and stay at Fermilab. And I think it worked quite well for the ten years that the support lasted. It impacted positively the life of many students in Argentina, Chile, Peru, Brazil, Colombia …. One of those students is a junior professor at Harvard today!

Zierler:

Marcela, from your vantage point, what was the significance of the shutdown of the Tevatron, both in terms of the science and just in terms of the overall mission of Fermilab at that point?

Carena:

From the point of the science, the Higgs discovery and the Higgs phenomenology has been dear to my heart for the last 30 years. Hence, I was excited of what Fermilab could do for the Higgs discovery, and it was an exciting race. Of course, it was clear that the LHC would supersede the Tevatron, and once the LHC was up and running in 2010 it became reasonable to think about shutting the Tevatron off. But there was also the big issue of the identity of the lab, where was Fermilab moving after the Tevatron. I don't want to go into the politics of it, but it was a difficult time for a few years, and a struggle to get to where we are today, where we have, I think, a bright, bright future thinking about neutrino physics, many possibilities for dark matter and cosmology, muon g-2, and quantum information science. We are also building a new superconducting particle accelerator PIP-II, focusing now on producing the best beams of neutrinos. The lab has regained a new identity, which is very strong. And I'm very happy about it.

Zierler:

The way you put it, it's almost as if, in the short term, the shutdown of the Tevatron was difficult. But it also forced Fermilab to reinvent itself, and perhaps it became stronger in the long run?

Carena:

Yes! Back in 2011 it was quite difficult. And if you want to see how painful it was, you can watch the documentary movie called The Atom Smashers. A beautiful movie made by an independent film company that tells you about the ordeal the Tevatron experiments went through in the fierce race for the Higgs, and how we all kept the excitement and the scientific challenge in view.

Zierler:

Right.

Carena:

That I think is, in my mind, the tale of the Higgs at two colliders. I like the movie because of course it's made for public outreach, but it's made with passion and shows all the intense feelings at play.

Zierler:

Marcela, two intellectual questions with regard to your affiliation at Chicago: in what ways has the Fermi Institute, not to be confused, of course, with Fermilab, the Enrico Fermi Institute at Chicago, and then later on the Kavli Institute, in what ways has your affiliation with these, with these organizations been useful in terms of collaborations, in terms of sharing ideas with, with your collaborators and colleagues in the field? How have all of these things been helpful for you?

Carena:

The Enrico Fermi Institute has its own identity separate from the U. Chicago Physics Department. And many times it allows a bit more cross cutting collaboration. That’s important because, because our science these days is moving to a much more interconnected approach between different areas, so you need a place where you can maybe interact more with your colleagues that are doing other types of physics. The Kavli Institute of Cosmological Physics has also opened a new door for me, connecting with astrophysics and cosmology, an area of science that has become very interesting to me. Nowadays we have joint appointments for postdocs that are between Fermilab and, and UChicago physics department and between Fermilab and the KICP. I think the Kadanoff Center at UChicago is also an important connection in the new era of quantum information science, and I want to help that connection with Fermilab grow.

Zierler:

Marcela, on that point, because your work has been central to those developments, more recent developments in astrophysics and cosmology, who have been some of your most important collaborators from those fields, in helping to foster these intellectual connections, and all of the exciting research that's happening as a result?

Carena:

In my work at the boundary between particle physics and astrophysics and collaborate and had many insightful discussions with colleagues at Fermilab’s astrophysics department and the KICP: early on with Mike Tuner, Rocky Kolb, Josh Frieman and Dan Hooper, and also my good friend Katie Freese. Right now, I am working with postdocs on issues related to non-standard Dark Matter solutions

On a different direction, that may ultimately shed light into problems in cosmology, but is too early to say, I'm collaborating with colleagues who have expertise in lattice gauge theories, which has a big overlap with the quantum effort from the high energy physics perspective.

Zierler:

Marcela, when the Higgs was announced, the discovery of the Higgs was announced – did you feel more as an insider or an outsider? In other words, when you have these enormously exciting scientific discoveries, there's so much drama associated with the announcement that there's always an inside and an outside track. Where were you in that? Were you sort of like on the outside and surprised with all of the rest of us, or did you know that this was coming more?

Carena:

I guess I was in the middle, because I was not at CERN anymore, but I knew a lot of the inside stories from experimental colleagues, and we all were gossiping about it! When the discovery was announced on July 4, 2012, it was 2 am Chicago time, and I had about 30 people at my house including colleagues, postdocs, students, non-physicist friends, and my two children. I know Fabiola Gianotti since the time that we were both postdocs, and we are good friends, so I was proud to see her making that announcement. It was a very moving moment for me. But it was not a surprise, because I knew quite a while ahead that it was coming. But still, the actual moment was as breathtaking as when I was looking at the Via Lactea as a kid. It was mind-blowing to see that little bump.

Zierler:

Marcela, I wonder if you could talk about that. Because, as a theorist, you know it's there before it's demonstrated to be there. Right? So how does that work in terms of putting away your diploma, putting away the theory, and just feeling the joy in the experimental verification of the theory? I wonder if you could talk a little bit about that.

Carena:

During the announcement the one person I had my eyes fixed on was Peter Higgs. I think the best answer to your question is in his face, when you see the emotion shaking him so profoundly. There is the person who had first thought of it half a century ago, and you can see his joy beyond measure when confronted with the big discovery. I think most of us who had been working on the Higgs idea for decades shared his sentiment at that instant. Sometimes when you are working very hard on the details of a calculation and you want to make it perfect, you may lose perspective of the deeper meaning. We need moments like those presentations from the two experiments to realize how grandiose can be the accomplishments of humankind. Just to reflect on how long it took to so many people (thousands!) working together with a common goal, and across countries and cultures and languages, is a most powerful and humbling feeling. I mean, my house was full, and we were all silent.

Zierler:

What new questions or research endeavors can only truly get started after the Higgs is officially discovered, both scientifically and administratively. In other words, it's now an opportunity to say this, this achievement has been reached. What can we now do as a result?

Carena:

I'm not sure I totally will answer your question but let me try. Well, the Higgs has been discovered but of course, we know well that the day that the discovery was made marks the moment when new questions are posed. We have so much to learn still - what can the Higgs teach us? What we are exploring now makes me think of Borges: “The garden of forking paths”. What could be our reality if this or that is the truth path? I mean, depending on what the Higgs precisely is and why it is that way, we will have different realities. It can lead to astonishing scientific implications. We need to explain many unknowns of the early universe and ultimately why we exist (there is more matter than anti-matter), why the masses of particles are what they are, does the Higgs know about dark matter, and how? Does the Higgs single out neutrinos? and so on.

And then, there is the question: is the Higgs boson a solo rider, the only one of its kind? Or, are there many other Higgs-like particles that we have not yet observed? In fact, there are good reasons to expect there may be others. You know I am intrigued by the idea of electroweak baryogenesis, that the matter-antimatter asymmetry was generated at the moment when the Higgs field turned on in the early universe. But by now we know that such idea can’t work unless there is something beyond the Higgs itself, and many models are based on the existence of other Higgs particles, yet to be discovered. So that's one possibility for new physics I find quite promising.

Zierler:

Marcela, electroweak baryogenesis is something, of course, that you were thinking about earlier in your career. But I wonder if you can explain what some of the advances both experimentally and theoretically, in more recent years, have yielded so much promising research and excitement in terms of what new discoveries can come as a result of these collaborations?

Carena:

Yeah. The need for dark matter has been established through decades of experimental observations and it is something beyond the Standard Model that we all expect to be there. There are lots of opportunities for new physics, that is, new forces, new symmetries, new particles, and many of them will provide dark matter candidates. Beyond the Supersymmetry paradigm, that can provide, in some incarnations, dark matter candidates that we could see at colliders or in direct detection experiments, there are other type of theories of strong dynamics, that could accommodate symmetries leading to dark matter candidates as well. In the early 2000’s, I explored many fun ideas to understand the mysteries of electroweak symmetry breaking, flavor and dark matter and confront them to experimental data, together with outstanding Fermilab postdocs –in due time professors: Tim Tait, Eduardo Ponton, Nausheen Shah, Jose Santiago, Yang Bai and others.

Again, going back to the idea that physics is something that should be defined by experimental data, the LHC is a paradise, to either prove your wildest expectations or be done with them. As a theorist, we use our tools and mathematical structures in quantum field theory, with symmetries, gauge invariances, conserved or anomalous currents, all to build a framework that may better explain data. Today, particle physics experiments have such high precision, that it makes a theorist’ job more intricate. For example, for baryogenesis you need an additional source of CP violation beyond what is present in the Standard Model, but we are very constrained by data from sophisticated electric dipole moment experiments. So, I came with the idea that maybe the CP violation resides in a dark sector, to which dark matter belongs, and its effects somehow find their way to us. Then, one needs to explain how that happens and how precisely the dark sector contains the right amount of dark matter needed to hold the universe together. So, all this is very intertwined, and makes it challenging to build sensical theories in agreement with observations, while keeping the mathematical rigor. Again, at Fermilab I had the privilege to work with so many brilliant collaborators in the past decades… Discussing and learning with and from students and younger colleagues, has been and remains one of the best experiences in life!

Zierler:

Marcela, on that point, one of the things that is so unique and exciting about the search for dark matter is that there are so many different kinds of physicists coming from different backgrounds and intellectual approaches, who are all converging on this enormous mystery. How is your unique career path and research achievements, how do you see your work contributing to this overall, essentially global goal, that we're in the middle of right now?

Carena:

Yeah, you're right. Dark matter is one of the biggest questions we have and is in the mind of all of us. We do not know what is made of, and there may even be many different kinds of it. As I hinted before, I have approached the quest for dark matter in different ways in many different stages of my life. My colleagues at U. Chicago, Michael Turner and Rocky Kolb thought about the WIMP miracle to explain dark matter, a weakly interacting massive particle. That is the type of dark matter that fits in supersymmetry as a glove. And it may still fit nature, but it was not the first thing that we saw when we switched on the big colliders. In my early years at Fermilab, I was thinking a lot about the complementarity between collider and direct dark matter detection experiments in deciphering the nature of the WIMPs. But Dark matter could be something totally different, for example could be light particles called axions that yield ultralight dark matter candidates. We also have this more general idea of the dark sector, which barely “talks” to us, and maybe there dark photons and milli-charged particles come to existence. We could try to search for those in different ways, and one example could be by looking at effects on polarized light in the cosmic microwave background, which is something I'm exploring right now.

Zierler:

Marcela, I'll ask you to predict the future. So, with that caveat, right? If and when we understand dark matter, do you think it will be dramatic like the Higgs, or like the detection of gravitational waves? Or does the nature of dark matter simply not lend itself to a eureka moment for science, that it will only come about as the result of many different experiments confirming each other over a long period of time?

Carena:

The day we, as a scientific community, say we nailed it, that we know what dark matter is made of, may be of many different pieces, that would be a super huge breakthrough for humanity. What you're asking me is will it come in one shot. And that is much harder to predict. We are looking for dark matter in extraterrestrial experiments, we are looking for dark matter at the LHC, and we're looking for dark matter deep underground in a variety of direct detection experiments. So, I could imagine a lot of different scenarios for discoveries. But I think that we will need more than one proof of the nature of dark matter. And maybe, you know, if there are many different dark matter components, we'll be getting information by pieces, here and there. Then it will take quite a while to put the whole puzzle together. But whenever we do it, and I am confident we will, it will be a Eureka moment!!

Zierler:

Marcela, a similar kind of future prediction question. We're approaching 10 years since the discovery of the Higgs. And of course, the big question is, at the LHC, what's next? And it's still an open question. So, from your vantage point, with this specific question, coming from all of that initial excitement, now that we've discovered the Higgs, what else is there? It's an open question, what do you think the most likely answer will be and what timeframes are you hopeful about?

Carena:

Well, the LHC still has almost two decades ahead to keep running, and we will collect an enormous amount of data. This is probably more my personal taste than a prediction, but as I mentioned before, I think that is likely that there are additional cousins of the Higgs, if you want to call them like that; and I am hopeful that we may find one/some of them. Going back to dark matter, there are so many alternatives with very feebly interacting particles that are harder to detect and may be part of a more complex dark sector, that we may be fortunate enough to get some information there. We have to collect more data and be even more innovative in the search techniques, and in the detector technologies. So, the story of the LHC, in my opinion, is far from written.

Zierler:

Marcela, to what extent, though, are we trapped by the LHC's capabilities? In other words, the questions that you and your colleagues are after? To what extent do we need an ILC? Or to what extent do we need experiments that are not even terrestrially based, that need energies that may only be achievable in a space-based application?

Carena:

David, you asked me before, for dark matter, would it be like one shot and we got it? Well, most likely not, rather we will need information from multiple probes of different nature. I believe that as scientists, maybe it's not the best argument for financial purposes, but as scientists we are obliged to use all our technological power and our physics and math knowledge, to explore the universe we live in. The LHC is, so far, the most ambitious, most complex and also most expensive apparatus ever built by mankind and womankind, And …

Zierler:

I'm glad you said womankind [laugh].

Carena:

Sorry?

Zierler:

I'm glad you said womankind as well.

Carena:

Yeah, I don't like personkind [laugh]. I prefer to say both.

Zierler:

Given that Fabiola Gianotti is heading the whole thing now, it's only appropriate.

Carena:

Indeed! As I was saying, I think that we are obliged as scientists to push the limits of our ingenuity, current technology and computational capabilities, in order to explore our universe. It’s one of the higher purposes of society, so we should be boldly ambitious about it. You can argue, you know, there are a lot of urgent needs in the world, and certainly there are. You can argue that it will be better to put our resources in developing a vaccine, and we do, rightly so! But these things move in parallel. Advances in technology developed to build particle accelerators also ended up being breakthroughs for medicine treatments. As scientists it is our duty to strive for balancing resources, and be conscious about the challenges and opportunities of the world we live in.

Zierler:

Marcela, on that point, just that the existential questions that you raise, I think right now, in general, it's so important for people to understand that, you know, the development of the mRNA vaccine that is going to get us out of this COVID crisis, it came out of basic science, it came out of just research for the sake of understanding. And I think that in physics, those same rules apply. And that we have to, we have to recognize that, while they may be toys in the sense that we're just trying to learn these things, we have to be open minded to the fact that what they will discover can always be of very important value to society, either in the short term or the long term.

Carena:

Yes, I couldn't agree more with you on that point. And I also believe that it is the job of scientists to communicate that to the public, because it's very easy to devalue or be suspicious of those things you don't understand. I think scientists are getting much better at bringing to the attention of the general public the value of basic science and how much it can contribute to improve our quality of life, not only for the richer countries but for humanity as a whole.

Zierler:

Marcela, I'm very interested that Fermilab has an International Relations Directorate. And given your personal background and all of your collaborations and all of the places you work, I can't imagine a person better positioned to do great things with that. I'm curious in your role in, in recognizing that, that physics really is an international endeavor, to what extent you've been sensitive to, including physics research, not just in the well-trodden places of the United States and Europe, but thinking about what's going on in Latin America or Asia or Africa? In what ways have you really tried to emphasize that physics is a, is a global endeavor, and it involves experts from truly all over the world?

Carena:

Yes, bringing smart talented people from all areas and social economic backgrounds together is one of the big powers of science. Whatever someone’s background, religion, culture, language, or gender identity is, we are fortunate to be in a situation where we can communicate and work together to solve challenging science problems that move the world forward. Especially in high energy physics we have an amazing history of collaborating internationally to do great things like the Higgs discovery. I think the way to go forward in international collaborations is opening doors for students and colleagues that may be in remote geographical locations or working under more demanding conditions.

The Fermilab neutrino flagship experiment DUNE involves over 30 countries and is striving to open new possibilities to colleagues around the world. As an example, just today for example I was talking to scientists from a University in Paraguay, who are working with a group at the University of Campinas in Brazil, and also working with universities in Lima, Peru. They are all working together to build the most sophisticated light detection system for the neutrino liquid argon detectors of the future DUNE experiment at Fermilab. They call this light trapping technology Arapuca, which is the Guarani word for traps that they make to catch birds in the Amazon, here we catch light! The idea has already been tested at the DUNE prototype at CERN and is now being used at the DUNE predecessor at Fermilab. It is amazingly successful, and with the help of Fermilab Scientists and technicians they will build a whole system. So, this shows the power of bringing people together, because colleagues in Paraguay or in Peru couldn't have done it independently, but as part of a bigger team they can... I feels me with joy that, as part of Fermilab’s international team, I have been able to contribute in making this possible.

Being myself from Latin American, I strongly believe that what is important is to help build capabilities in the countries that one would want to partner with. It is most important to help scientists in Latin America or Africa to make the case with their own funding agencies and governments to build laboratories and infrastructure, that will, in turn, empower them to train the next generation of scientists, engineers and technicians. In my role supporting Fermilab’s collaboration with Latin America, that’s where a big part of my effort has concentrated.

Zierler:

Marcela, you mentioned earlier, your service work for APS was a significant contribution on your part, it required a lot of time and resources. My question there, you know, it's broad for all of service, is the motivation. Did you feel a sense that APS really needed some help and expertise that you could provide? Or was this an opportunity specifically to do things for APS that would be specifically useful to the research that you and your colleagues were doing at the time?

Carena:

In my role as chair for the Division of Particle and Fields of the American Physical Society, my main motivation was contributing to the U.S. high energy physics community, and also connecting it with European scientists and some component of the Latin America effort. Nowadays DPF is also tightly connected to DAP for astrophysics, DNP for Nuclear Physics and DPB for accelerators and that brings a sense of a stronger, interconnected physics community. I think in due time we should all volunteer part of our time to give back to our communities, and in my case that implies the U.S. but also Europe and Latin America. I believe in general is good for scientists to be conscious about what we can give back to society. And it is especially relevant for students and postdocs to have this feeling that they belong to a community who cares about science as much as about ethical issues beyond geographical frontiers.

Zierler:

Marcela, you touched on it a little bit in talking about your collaborative work with Latin America. But one of the things we hear so much these days is that to the extent that the, the, the central focus of high-energy physics is really at CERN, globally, right now, neutrino physics is really a bright spot for the United States. I wonder if you can talk about what exactly is so exciting about the long baseline neutrino facility or the LBNF, the DUNE experiment? What's so exciting about that, both in terms of the science and more broadly, for the overall idea that there really is large scale and exciting American leadership that continues in physics well into the 21st century. Right.

Carena:

There are many important questions in neutrino physics that need to be answered, including what is the real nature of the neutrinos; are there new types of forces connected to them; how do neutrinos get mass; do they play a decisive role in the generation of the matter-antimatter imbalance? DUNE will tell us something about whether neutrinos have additional sources of charge-parity violation, and that may impact our expectations on electroweak baryogenesis being the mechanism of matter-antimatter asymmetry generation. My theory colleagues at Fermilab are extremely active in investigating all we can learn from neutrino experiments, both from the short baseline neutrino data as well as from from long baseline neutrino experiments data, including DUNE in the longer term. There are many intriguing questions related to light interacting sectors, dark sectors and sterile neutrinos that we are trying to scrutinize as well.

The U.S. has defined a path that is complimentary to that one of Europe. At the same time, the U.S. is a major player in the LHC upgrades, while CERN built a neutrino platform and is helping with DUNE. CERN and Fermilab are collaborating to make each other's experimental program the strongest possible. Big components of the DUNE experiment will be coming from different European institutions too. So, there is this global science program in place, which is excellent because it potentiates the power of each region through international collaboration in a unique manner. Nowadays we have big ambitions, the technology is much more complex, and everything is quite expensive. So, we need to collaborate. I see Fermilab building a strong, bright future. The research in the area of neutrinos is opening the door to other opportunities as well. We are already seeing this with the superconducting technology that we developed for our new particle accelerator – at the same time that we get more powerful neutrino beams we have our new quantum center to use the same technology for better qubits for quantum computers. Fermilab theorists are deeply involved now in investigating what those qubits can bring for new experiments in particle physics as well as tackling challenging quantum field theory problems at the longer term. Accelerators for neutrinos are a path to exploration that becomes many paths – Borges again [laugh]. And Fermilab is well positioned to be a leader lab in the 21st century.

Zierler:

Marcela, to bring our conversation right up to the present. Most recently, you've become more interested in quantum information. How did that intellectual transition happen? And in what ways have you been able to utilize your expertise in other areas? And in what ways is this sort of a brand-new world for you?

Carena:

I got interested in, in quantum information, because new developments in that area are maturing, and as scientists, we have to keep our eyes open towards where advancements may come from. Some of the leading questions may have been there for a long while, but only now some of the relevant technology may be ripe, opening opportunities to actually build quantum computers that could shed light on some of the questions we have. I see it also as a two-way bridge: one can encounter quantum information science challenges in solving high energy physics puzzles, and high energy physics problems can call for developing better tools in quantum information science. So, it's both ways. For my very personal perspective I have been thinking about phase transitions in the early universe. This is a very difficult quantum problem, in a sense may be harder than the problems that lattice gauge theorists are working on to evaluate quantities that demand real time computations. Maybe, quantum computers are what we will use in the end to understand what happened early in our universe’s history.

Right now I am working with Fermilab colleagues and U. Chicago students on toy calculations to learn how to better think about quantum field theory problems with quantum algorithms.. It's a new era, a new research direction for me, we shall see. I believed I mentioned we have a theory consortium supported by DOE and based at the Fermilab Theory Division with powerful University partners; we are building a new effort at Fermilab, and some of our partners have been thinking about this for several years and more.

Zierler:

Well, Marcela, now that we've brought the conversation really right up to the present, I'd like to ask for the last part of our talk, a few broadly retrospective questions about your career. And then we'll end looking to the future. Throughout our conversation, you have indicated almost as a side comment, how important diversity is for STEM, having people from different perspectives, different backgrounds, different cultural beliefs, the idea being there that science really needs that diversity of thought. Well, of course, this past year, science and technology has undergone something of a bit of reflection in the field because it needs to recognize institutionally the importance of diversity and inclusivity. I wonder if you can comment between your major affiliations with the University of Chicago and Fermilab what work has been done in these efforts? And from your own personal perspective as, as a Latin American woman reflecting on your own career? In what ways does the field really still need to move forward? Because where we are now, is not where we ultimately need to be.

Carena:

Of course, this is a very important topic. It's clear to me that, although there is a lot yet to be done, a significant effort has been put into motion in the last year at Fermilab and other places, and for obvious reasons importantly triggered by public events. I have been discussing with my kids, especially my older son who has been also quite involved in thinking about diversity, or the lack of diversity, in physics, since he is pursuing a PhD in the field. I'm very glad to see that things have evolved enormously since the time I was a PhD student or postdoc, and in a very good way. I see so many brilliant women in the field who are making great contributions. Of course, diversity is in many directions. At Fermilab we are trying and need to try even harder to get better representation of Black scientists and Latin scientists, and students. And one needs to work hard to achieve that. Of course, because of my own background I have always been quite sensitized to this.

One thing my group, Fermilab Theory, is doing, is reaching out to local universities that are predominantly Black and Latino serving institutions. And this summer we are running for the first time a quantum internship program/school aimed at minority undergraduates in physics and math. The idea at first is just to show that science can open doors and change the way some students think about their future. In many cases, one of the things I found out, is that many students do not realize they can pursue a PhD in physics and have a salary while they do it, unlike in medicine or law school. It's not that we are going to change the world with our school, but hopefully we can bring some new perspective to our students, and one needs to start somewhere….

When I speak at a public forum, many times I am asked how it is to be a Latin woman in physics? Yeah, it has its nuances, but no doubt that having diversity makes the field much stronger. A society that is only made of people who are alike, becomes less innovative as time passes. So I think diversifying the students and faculty pool is essential, but there's an amazing amount of work ahead.

Zierler:

Marcela, back to the science, if you look at the entirety of your career, it can almost be looked at as a chapter book, at some of the great unsolved mysteries in physics in the modern era, right? Everything from pushing particle theory beyond, beyond the Standard Model, to looking at what might be beyond the Higgs, looking at what really is dark matter? Or more recently, where are we ultimately going to go with neutrino physics? And what are quantum computers going to look like? So, there's mysteries and question marks all over your career. Let me center my question then around the satisfaction of discovery. As you look back over all of your research accomplishments, what sticks out in your memory as something that, at the beginning of your career, really was not well understood, but because of your contributions and those of your collaborators, we really do understand at least some aspect of all of these question marks in physics?

Carena:

I think my biggest contribution to particle physics is related to Higgs physics and all the new approaches that can be related to it in possibly solving pressing problems in the field. The original Higgs idea was there basically since I was born, but I worked very much on it from early on in my career trying to understand the possible implications of a Higgs boson particle, its possible connections to baryogenesis and strategies for experimental searches. I also have been fascinated by the fundamental question: is the Higgs an elementary particle.?, we think most likely it is, but more data is needed. All the work I have done is giving fruits in the sense of what we can scrutinize in experiments. If we think about the electroweak baryogenesis idea, it's very important to see how the Higgs interacts with itself, and that is a big challenge for the LHC. Moreover, the Higgs can also talk to dark matter, that could be yes or no answer, but, but I think the Higgs, once we understand it thoroughly will teach us a good deal about our yet unknown parts of the universe. Again, somehow, I believe the Higgs and the early history of the universe hold important clues yet to be revealed, and we will slowly get some answers from experiments in the future. We need to be patient, after all it took half a century for the Higgs to revel itself…

Zierler:

Marcela, my last question is going to demonstrate, I think, a theme that, that has been true throughout our conversation, and of course, for your career, it's that looking forward, there is so much foundational work to be done in physics, that it's almost like the world that Einstein was looking out at in 1915. There's so much that's going to be done and discovered. And your research demonstrates all of the things that are really poorly understood at this point. And so many of those question marks are really going to occupy physicists generations into the future. And so, my question is, what are you most optimistic about in the near term, however long you define your desire to be active in the field, whatever, whatever that time period is for you, as you imagine these things. And of course, physicists never retire. So, I hope it's a long-time frame for you. What are the things that you're most optimistic about that you think you will really be present and part of the discovery that's really going to make the next major foundational discovery in the field?

Carena:

If you ask me, like one thing, I very much hope we understand what dark matter is made of and how it works before I decouple... That would be my number one.

Zierler:

And it's doable? That's, that's the question. This is something that you can conceive of--

Carena:

Yeah, I think it's doable, I think we are already learning of it from many areas. We are studying galaxies and the large-scale structure of the universe to understand basic properties of dark matter. And even though we have not detected dark matter in the laboratory yet, this tells already a lot about what dark matter is not. We have multiple communities investigating the nature of dark matter, because it's not just high energy physics, but also the astrophysics and QIS communities using their best tools in this endeavor…leaving no stone unturned. But of course, dark matter can be so many things.

Zierler:

Yeah.

Carena:

To understand baryogenesis would be my personal desire, anything in that direction would be so mind-blowing to me! But dark matter is my bet. Mr. Higgs made a bet and it took 50 years, so we should learn to be patient. On the personal side, my son Sebastian just started to do research in theoretical cosmology - those long days at the CERN cafeteria had an impact, by osmosis, [laugh]. Hence, I may be following the fate of dark matter from longer than I planned for (Laugh)

Zierler:

Marcela, I first got in touch with you in August of 2020. And ever since I have been pursuing this discussion with you, I just want to say it has been well worth the wait, your insight, and the profundity of all of your explanations of the science, of your sociological appreciation that research is a collaborative endeavor and it requires all kinds of people, and just the excitement and sense of wonderment that you brought to your research and all of the benefits that's conferred for physics. It's been a great honor spending this time with you. And I want to thank you so much for doing this.

Carena:

David, I really thank you for this amazing conversation, and making me think calmly about things that are part of my everyday life, but I don't externalize them so much. It has been my real pleasure to talk to you.

Zierler:

Excellent.

[End]