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Interview of Elena Aprile by David Zierler on April 30, 2021,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
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Interview with Elena Aprile, Centennial Professor of Physics at Columbia University. Aprile describes the feeling of just having been elected to the National Academy of Science and she describes how the XENON Dark Matter search has continued despite the pandemic. She explains why so much of physics is devoted to try to understand dark matter and where she sees the interplay of theory and experiment toward that end. Aprile describes why finding dark matter will mean finding new physics beyond the Standard Model, and she recounts her upbringing in Milan and her developing interests in physics during high school. She describes her first visit to CERN when she was a student at the University of Naples and what it was like to meet Carlo Rubbia. Aprile discusses her graduate work at the University of Geneva where she worked on scattering protons to study time violation effects. She explains her interest in the UA1 and UA2 experiments at CERN and her work on noble liquid detectors, her appointment at Harvard, and her first involvement with radiation spectroscopy. Aprile narrates her realization that xenon would be valuable for astrophysics and dark matter specifically, and she describes the origins of the LXeGRIT telescope project. She explains why Gran Sasso was chosen on the site of the XENON experiment and some of the technical and economic challenges in dealing with xenon on the scale required to search for dark matter. Aprile reflects on the difficulties she has faced as a woman in her field, and she describes the competitive value in having the LZ experiment and its search for dark matter. She explains how one goes about searching for dark matter without knowing what dark matter is. At the end of the interview, Aprile imagines what it will mean to find dark matter, what mysteries it will solve, and why she will remain steadfastly cautious before confirming the discovery.
Okay. This is David Zierler, oral historian for the American Institute of Physics. It is April 30, 2021. I am absolutely delighted and honored to be here with Professor Elena Aprile. Elena, it's great to see you. Thank you for joining me today.
My pleasure, David. Thank you.
Elena, just to start, let me get on the record a deep and hearty congratulations for your election to the National Academy of Sciences. This is such wonderful and well-deserved news. How do you feel about this?
Well, I felt really thrilled and humble, but also grateful. Grateful to the Academy and to my colleagues who supported my nomination. I was very happy for all the emails and phone calls I received congratulating me. One comment by one colleague, Nima Arkani-Hamed, who I much admire, made me particularly happy as he wrote that the academy honored itself by electing me as a member or something like that, which is of course quite nice!
Oh, that's great.
It was great news, to make it short. It was wonderful.
Elena, please tell me your title and institutional affiliation.
I am a professor of physics at Columbia University in New York City. Actually, I recently was named “Centennial Professor of Physics” at Columbia, a prestigious chair for which I am also grateful.
And Elena, my most important first question is going to be, in the pandemic, because you used to travel so much, how has the science been affected for you? What have you been able to do better on Zoom and not being jet-setting all over the planet, and what has been lacking because you're not seeing your colleagues in person? You're not getting to the facilities? What has been good and bad about this year-plus in the pandemic for you?
I guess I'll start with the good, because I am a fighter and when this pandemic started all I felt was the urgency to fight it. It really hit me in the worst moment because we were about to install the new dark matter detector at the Gran Sasso underground laboratory in Italy, the XENONnT detector. It had been moved underground and we were ready to install it inside the cryostat, and then Italy locked down around March 15 which was just when we were about to do that. And that had a terrible impact on me, seeing this plan that we had, this vision, this acceleration that I kept up with, come to a halt. I mean, I didn't understand or accept that it would be possible.
But the good news is that, yeah, as always, I recover relatively quickly, and fought to keep our plan thanks to the willingness and the dedication of the few people we had left at the lab in Italy. We discussed a lot and with the support of the laboratory director at that time, Professor Stefano Ragazzi, I started our new plan of action, to avoid keeping the detector exposed to the air for so long, wasting so many months of work. Thanks to our XENON people on-site and the lab allowing us to work underground, being not too many, we continued at some slower pace, the installation process, with a lot of worry in the back of my mind, what if one of them would get sick? Because so much was unclear in the beginning of the pandemic and the news all around were so bad.
But yeah, in a miraculous way we managed. So with the detector in its cryostat we could relax a bit and perform operations which, a year later, which is about this time, seem almost a miracle. I still don't understand how we managed to make so much progress in that terrible 2020 year, but we did. That's the good news. I think the inability to travel had a tremendous impact of course, but we could have all these meetings on Zoom and we have been more efficient than ever before. We have had collaboration meetings, technical meetings and telecons almost every day of the week, all on Zoom, and kept making progress.
The travel constraints were actually quite dramatic for this stage of the experiment, because the few people on-site could not get a break with new people coming in. And so they became the heroes of the collaboration, we are particularly grateful to them and recently recognized them with XENON medals. The few on-site people worked very hard, and we couldn't replace them. And as soon as I could I joined them, thanks to my Italian passport I was able to travel. And so, the good news is that we managed to accomplish a lot despite the pandemic and constraints on all sides.
The bad news, as everybody probably tells you, is really missing to be together, to see each other and meet in person. Just last week we had our most recent collaboration meeting, and again I concluded the meeting by saying that the thing I wished most was that we can meet in person soon. And that's for the entire collaboration, not to mention my own graduate students, postdocs, my own group at Columbia. When Columbia closed down we couldn't meet, and we have had Zoom meetings every week, but it's not the same. It's just the lack of personal interaction... which is something that I'm not used to, because I've been all my life so involved, you know, in what we do in my lab and I am so used to see each member of my group almost every day. Even when I am teaching, the lab is part of my life. And so are my students and my postdocs. So it was completely unprecedented, the fact that I couldn't see them. This was the hardest thing for me.
And the same from a personal point of view as, I couldn't see my own daughters even when they live so close to me. They didn't want to see me, not to put me at risk, being the old mama. [laugh] But I couldn't accept it so easily when Giulia or Susanna would say, “No, we're not coming over, because we don't want you to be at risk.” I mean unbelievable. I had to cope with that. I managed to go through this depressive time-- I mean, it was a terrible moment in my life, like for so many. I coped in my own way. Confined in my apartment, I decided to work out a lot more than I used to, and I ended up buying a Peloton bike. And that became my survival strategy. I would use my bike whenever I was stressed. I couldn't wait for it to be delivered and once I had it I used it every day-- just to get my mind off the craziness of being locked here in this glass castle which is my apartment, with its large windows overlooking the Statue of Liberty, the Manhattan skyline, and with no car or person out there. You know, it was incredible.
Anyway, I should go back to the good and bad things of the pandemic. Yes, it was a terrible year, but at the end, for XENON, and for the experiment, it ended up being quite good. I should be more modest, but it's true. I mean, being on so many panels, I know how much COVID has impacted many experiments, I would hear it over and over. It has been quite dramatic for many collaborations, for many experimental programs. We definitely have been also slowed down of course. No question. But I am really thankful that we managed to keep going and to be where we are today with the program, despite the COVID pandemic.
Elena, I'd like to ask a very broad question at the outset. I talk to physicists every day, and I'm well-positioned to appreciate how remarkable it is, all of the different kinds of physicists, all of the different areas of expertise, that are geared towards this search for dark matter. It's really remarkable, and it's perhaps without precedent in physics that you have such a breadth of expertise all geared toward this one particular search. How do you see your particular area of expertise as contributing to this broader search? What is it that you and your idea for the XENON Dark Matter Experiment, what is this contributing to the overall project of understanding what dark matter is?
Thank you, wonderful question. The problem is so enormous and so important, that it really takes so many ideas and so many strategies to attack it. Of course, we would like to see exploration from all sides, at all masses…of whatever it is, this dark matter. I for sure hope it is a massive new particle. To answer your question specifically, what did I contribute? When I got into this, interested in the question, 15 or more years ago, it was clear that a WIMP, Weakly Interacting Massive Particle, was an amazing idea and the most sought-after candidate. A new particle outside of the Standard Model, which could be produced with an accelerator, or seen, through its annihilation products in dense regions of the sky, via indirect detection. But the direct detection idea, that was proposed in '85 or so, was the most appealing to me.
You know, the idea that although very rarely these particles, if they exist, and make the dark matter in this milky way, can eventually interact with the atoms and molecules of the stuff that we are made of, the standard matter that we know. And for me, the possibility to detect these particles directly with an experiment on Earth was most attractive. Because the first thing I thought is, “Yeah, I have the best material that is both a great detector material, as well as a good target for these particles to interact with.” A good atom, the xenon atom, because I had been working with xenon for radiation detection for quite a while. It was clear that the rare interactions of WIMPs with normal matter demanded very massive detectors. There were already many beautiful, sophisticated, superb detector technologies that had been put forward and used for WIMPs direct detection at the time when I started.
But it was clear that going from their mass scale of a few grams, hundreds of grams, to hundreds of kilograms would have been difficult and challenging. And that's where the legacy of my research with noble liquids helped me in breaking into this new field. Liquid xenon in particular is dense, about three times more than water and thus allows you to realize a dark matter target at the hundreds of kilograms, thousands of kilograms, the tonne scale that you need first and foremost to have some sensitivity to see the rare WIMP interactions in a detector. So it was clear that I had a winning detector idea, which I had used for gamma ray astrophysics, to detect gamma rays from radioactive elements, produced in nucleosynthesis of the stars.
But to detect WIMPs the detector had to work in a very different energy range. There were quite some open questions because nobody knew at the time how liquid xenon responds to the very low energies released by interactions of WIMPs. So, we had actually to do quite some R&D in the lab at Columbia, measurements with low energy particles, and we still do today as we still have a lot to learn. The proposal of the XENON Dark Matter project, submitted to the National Science Foundation in late 2001, was very aggressive in scope. We proposed a phased program leading to a 1000-kilogram liquid xenon experiment, which we have actually done with XENON1T, which has produced the best constraints on many WIMP interactions, with leading sensitivity still today.
So my contribution was my expertise with liquid xenon detectors and my drive to push performance to make it the leading technology for WIMPs direct detection, at a very aggressive timescale. I was new to the National Science Foundation. All my funding had come from NASA and early in my career from DARPA. I convinced DARPA to let me do an experiment with liquid xenon to detect neutrinos from Russian submarines at that time. The reason why NSF responded very favorably and quickly to my XENON proposal, was their confidence that what I proposed was likely to succeed, based on my prior work with liquid xenon detectors. And that's how the XENON collaboration started, with the goal to realize a 1,000 kilogram WIMPs detector.
That was aggressive as in my gamma ray astrophysics project, we were dealing with a 25 kilogram liquid xenon detector. Going to a 1,000 kilogram dark matter detector was a big step. How are we going to do it, I wondered, somewhat worried yet excited? And this is an interesting thing that has been forgotten. The NSF proposal for the XENON Dark Matter Project contemplated a series of steps leading to a100 kilogram detector. Because you see I am a, I guess, practical person? At that time, I could see going to 100 kilogram in a fast pace. I had done a 20-30 kilogram detector. It was clear to me that that was feasible. And the 1,000 kg version came not in a single detector, because at that time there were so many technical challenges that we had barely tackled... we didn't know how to fully solve them yet.
And so, the best or the safest was to propose ten detectors of 100 kilogram each, to get to the 1,000 kilogram scale goal, which was important to have. It was very bold but important at that time, because what was on the table was mostly small-scale dark matter detectors, as I said, with very great capability, but at the kilogram scale maybe, with a lot of technological challenges to even get to the tens of kilogram scale. So, I knew that we would win with the scalability of the liquid xenon approach, but I was not bold enough to propose a single 1000 kg detector, because at that time we had no idea how to do it. This is an amazing story because it shows that progress always comes with mastering new challenges, with technological successes. We learnt a great deal by making XENON100 which led the field for many years and we could finally design and realize XENON1T, the first liquid xenon experiment at the ton scale, in a single detector which actually used 3,300 kilograms of liquid xenon.
At the time of the proposal I could not see this step. And so I guess I took the long way, as always, to answer you, but yes, my contribution has been the liquid xenon miracle. And it's true. It's an amazing material. We could talk forever about it. And the idea was not new as well. There were other projects proposed with liquid xenon. I remember there was the UK ZEPLIN group at the time, with the group of my good friend and colleague David Cline at UCLA. The point is that that experiment was not making much progress. So what I brought to the field was my expertise but also my energy and determination to have a liquid xenon detector for WIMPs giving results fast.
That was the most impressive accomplishment of XENON... to give a wakeup call to the dark matter community with leading sensitivity with just a few months of data with a first XENON prototype. The first XENON10 experiment used just a detector built here at Columbia with mostly off-the-shelf components but with many innovative ideas for the detector itself. You know, money was not that much, and we reused as much hardware as I had around the lab. And after six months operating it underground, XENON10 gave results which wiped out the competition or whatever had been done so far. People realized the potential of this technology.
Elena, I'll ask a slightly reframed question. And that is, as you well know, in particle physics, in astrophysics, in cosmology, advances always happen when theory and experiment work well with each other. Sometimes the theory is leading the experiment and the observation, sometimes the experiment and observation is leading the theory. For you in the search for dark matter, what is dominant right now? Are the theorists leading and the experimenters are taking their cues from the theorists, or it's very much the other way?
It's very much the other way. It's been the other way since I started, to be honest. The basic idea was there, but you know there are so many versions of this supersymmetric model, if we stick with this particle candidate. Of course, there are many other dark matter candidates, but the XENON project, like many others, are designed to detect these heavy WIMPs. And when you look back in the history of the project, look back at the 2005 time of XENON10 starting, our friend theorists would tell us, “Oh, you're going to see this. It's right there.” Then as we explored more and excluded the region they had predicted, they would refine the model and tell us we would see WIMPs at the next turn. And then nothing still but they always found a way to explain why the WIMP was not there and how you needed to improve sensitivity which is what we continued to do. Another order of magnitude and another order of magnitude. We're talking almost five orders of magnitude since XENON10. It's a tremendous achievement but is sad that we are still missing a signal.
By pushing the boundary of the technology, we have made the experiment more and more sensitive, and pushed the theoretical boundary, challenging theorists to think anew and find alternative models. So I think the answer is what I said already. I have never felt like there was a clear path in front of me, in terms of theory. But this ambiguity, or better this flexibility in theoretical scenarios, I don't know how to say it now, actually gave me hope to keep going. I always felt “Oh, maybe we just missed it. It was just around the corner.” You know? The detector was great, it was big enough, the sensitivity was there, but then of course you're still looking for a very rare signal. One, two, three events. After you wait a year. And it's very hard to pick up this needle in the haystack among the remaining background which you constantly fight to reduce more and more, but is never zero. Maybe we just missed it. So missing it gives you hope to keep looking, because nobody can stand out today and say the WIMP is dead.
Of course, having found nothing after so many years of effort and in many directions, and at different mass makes us think “Are we on the right path?” Maybe this particle is really a much lighter particle, and we should keep pushing the low mass region with detectors optimized for that and which don't require such a massive target. On the other hand, I believe nobody today can say, “Elena, you're wasting your time in searching for heavy WIMPs.” Because another order of magnitude, which is what we're aiming today, with the new XENONnT experiment, will bring us closer to the point where eventually it's not because there is nothing there, it's because we are not going to be able to see it as neutrinos will become dominant. Neutrinos from the sun, from the atmosphere, from supernovae. And I so much wish for a supernova in the next few years, to see it in XENONnT. These neutrinos will interact in your detector and will confuse or overwhelm a potential WIMP signal. Therefore, at that point you have to change direction or be happy to detect neutrinos.
Elena, as you say, because theory is essentially stuck, from your vantage point, why is theory stuck as it relates to dark matter, and how is the lack of theoretical guidance, how has it made observation and experimentation with regard to dark matter even more difficult?
I wouldn't say the theory is stuck. There are many wonderful ideas, may be just too many. A zoo of dark matter candidates have been proposed, all with their own appeal and motivation, from fuzzy dark matter with tiny, tiny mass to supermassive black holes and in between the WIMP which remains the most studied... for two reasons. One is because it is theoretically well-motivated, as particles with WIMP characteristics are predicted in extensions of the Standard Model. The other reason why WIMPs are attractive candidates for dark matter is because of their detectability in an experiment on Earth. They can scatter with a nucleus or the electrons of atom, releasing a tiny amount of energy which we can hope to detect.
Experimentally is very challenging, but it’s an idea that you can go after, if you can improve the capability of your detector. So there are so many candidates, but there is not one which is really leading and my preference is for WIMPs as I believe I have one of the best technology to detect them. The axion is another wonderful dark matter candidate, but I'm not an expert in the technologies required for axion detection. I would have to learn, and it's a bit too late to play a role. And I want to be a leader, not a follower. So I'm not going into axion research because I don't know how to do it. There are people who are much better than me to do that.
Again, there are so many ideas for explaining dark matter, and some are more attractive than others for a variety of reasons, but I think we should be realistic and go after those which have some realistic chance of being detected. Because physics is an experimental science, and we want to test the theory with experiment. The WIMP idea is testable with sensitive experiments, at least until neutrinos start to dominate.
Elena, this is of course going to be a purely speculative answer for you, but I'm curious when dark matter is discovered-- not “if,” we'll be optimistic -- when it's discovered, when we understand it, is your sense that it will fit within the Standard Model, or by definition, does it have to be new physics?
Oh it has to be new physics. That we know for sure.
Why? Why does it have to be new physics?
Because when you go through the properties that we know dark matter has, from the many and precise observations in cosmology and astrophysics and look at each of the known particles of the Standard Model you conclude that none of them fits the bill, completely. The only one that could fit the dark matter characteristics, is the neutrino. But neutrinos have a mass, and so they cannot make much of the dark matter which must be cold. One has brought forward the idea of a sterile neutrino, a fourth type of neutrino (not the electron, tau and muon neutrinos we know), one that does not interact at all and would be a sign of physics beyond the Standard Model. Sterile neutrinos could explain in part dark matter and search for such neutrinos is ongoing at accelerators like Fermilab. So, it's very clear that if the dark matter is in the form of a particle, it cannot be one of the particles that we are familiar with. It must be a new particle.
Well, I ask-- I asked a question about the future, let's go back to the past. Let's go back to Italy and start with your parents. Tell me about them and where they're from.
We'll start with my youth, you mean?
I was born in Milano a long time ago.
Are your parents from Milano as well?
No, they were from Naples. It just happened that my father was doing some work in Milano, so I was born there and stayed only a few years. So, I grew up in a small town near Naples, in the south of Italy. You know, where the pretty girls are. [both laugh]
My grandmother is from Naples, so I'll absolutely agree with you.
Ah, that's good, okay, wonderful. [laugh] The more south you go, it gets better. So, I come from a small town, with very modest upbringing. My mother is the pillar in my life, I have to say. She's the one who really instilled in me this curiosity for nature and also the resilience which makes me a fighter... yeah, never say no, just keep trying. She didn't have a chance to get a higher education, to go beyond the high school, because this was a time when even if you came from a reasonably wealthy family, as a young woman you were expected to get married and make babies, not to go to university. So, if she had been given the chance to study, I think she would have been a great engineer or physicist. I'm so convinced about it. But so, in her own way, even if she didn't study, she would still always challenge me in asking lots of questions about the physical world.
Elena, what were your parents' experiences during World War II?
Ah, we didn't talk much about this period of their life-- I mean definitely I know my father went to serve in the military, and was in Russia during the war, he said many times how cold it was there, etc., but he did not like to talk about it. During and after the war life was not too dramatic, for my mother at least as she came from a family who owned a lot of land and wealth. I recall more painful stories more from my ex-husband’s parents. He's German and they were in Cologne, and the impact of the war there was much larger.
Did you go to secular schools or religious schools as a girl?
Ah yes, and I did not like it. A few years with the sisters [nuns].
But you got out okay.
[laugh] I got out okay. I mean there were good things, and I guess maybe that's also how my character got formed. I really didn't like all the rules and the church events and the waking up early to pray, etc. I don't like rules. I like to break them.
At what point did you become interested in physics?
I think it was in high school. I enrolled in the Liceo scientifico— I think we still have in Italy these two directions for high school— one with scientific orientation and one with classics orientation with lots of Greek and Latin. And had every-day chemistry, biology, physics, math... The curriculum was quite intense, but I don't recall any experimental physics, and this physics teacher that I had was not too good. I liked physics, especially electricity and magnetism, and I became determined to study it at the University of Naples.
And it was always experimentation that you wanted to pursue?
It was not clear then, although I was always drawn to experiment. I always wanted to find new ways of getting to the same result. I experiment a lot when I cook as well, or I like gardening and there as well I follow my experimental instinct. And so that was always in me. I believe that there is some part of it which is, you know, inside you. Of course, I've seen some of my best graduate students who started as theorists and turned out to be great experimentalists. If not, they're great husbands now, I hope. They can surely fix the plumbing and more in the house, you know? [laugh]
Tell me about the opportunity that got you from the University of Naples to CERN.
Oh, that is one of best stories of my life. I always tell every young man and woman, that one has to want to be someone, but one also has to be lucky. I always dreamt of getting away from the small town and I knew that being the best in school would help me get away one day. Once at the University, in my 2nd year of studies, I applied for a CERN scholarship. The program still exists, and I have supported a couple of American students who applied and given lectures there many years later. I recall one of my physics professors, the one teaching electronics, pointed out the Fellowship opportunity to spend the summer at CERN, the major European center for nuclear research where everything in physics was happening. He suggested to me and a few students who were taking his electronics lab course. We all applied but I don't recall what I wrote in the application. All I remember is that a day in early Spring 1977 I received the acceptance letter in the mail, from Geneva.
That letter changed my life. With the letter there was a train ticket from Naples to Geneva. My ticket to move away as I always dreamt about. I left, and I never went back. I was excited. It was early May and the memory of me at the railways station waiting for the train to take me to Geneva has stuck in my mind. I was leaving. My mother, my father, my brothers, my sister were all there. And then I'm gone. Actually, I returned, of course, about a year later to defend my Laurea in Physics degree. That was the highest degree as there was no PhD program. I ended up staying almost a year at CERN, with an additional contract, to finish the experimental project that I started under the guidance of Prof. Carlo Rubbia. The CERN opportunity itself was tremendous, but it was the opportunity to work with Carlo Rubbia that really changed my life and shaped who I am today. I was lucky to get the Fellowship and even more lucky to meet and learn from one of the best physicists I have ever known.
What was it like when you first met Carlo Rubbia?
Being around Carlo Rubbia was like being always on the alert. It was intimidating. Fear is the word which comes to mind. But also, excitement to learn every day something new.
He was intense.
Even then he was intense, and mind you it was 1977 so he was not yet a Nobel Laureate. He was just very smart and very intimidating. I was intimidated by his vast knowledge and the confidence he showed. He knew the physics, but also the electronics, the detectors, the data analysis, I first learnt about Monte Carlo simulations from Carlo Rubbia. Being around him was a constant challenge as there was always something that I did not know but I am thankful for all the lessons I learnt, including being super demanding of yourself. With Rubbia you had to work hard and constantly learn to have a chance. I am glad I never gave up.
Did you meet Fabiola Gianotti at that point as well?
No, Fabiola is younger than me. Maybe by ten years? I don't know. Much younger. But she told me later—when we met at a conference and she was already ATLAS spokesperson—that she knew of me and praised my work with noble liquids, because she also started to work on liquid argon at CERN, just like I did as a summer student in Rubbia’s group. It was Rubbia that initiated me to the art of noble liquid detectors. In 1977 Rubbia wrote a CERN Yellow Report putting down the idea to use a liquid argon time projection chamber for neutrino detection and rare event searches.
Now was the initial—
We have met a few times. We have met a few times after she became the director.
Was the initial appointment at CERN, was that part of the plan to pursue your PhD at the University of Geneva, or was that separate?
No. The CERN summer student fellowship is really to give research opportunity to undergraduates. It's similar to our research experience for undergraduate in the US, sponsored by the National Science Foundation. It's just a three month, or maybe it's ten weeks? As I said I ended up staying at CERN well beyond the Summer of 1977. Rubbia took me and another student, a guy from East Germany to work with him that Summer. We were assigned to the project to develop the first liquid argon detector, with a 2-dimensional charge readout. The sensitive area was small, about six-by-six centimeters. I had no familiarity with a charge amplifier, I had just learned the basic it in my electronics class in Naples. And when Rubbia asked me and the other young student to optimize the signal-to-noise of this charge-sensitive amplifier, we were both terrified. But we did it and we tested the new detector in a beam test at CERN. The project was not a small thing and we both stayed longer than expected at CERN to finish the test. My first liquid argon detector and my first experience with liquid argon. The project became the topic of my Laurea degree dissertation which I defended at the University of Naples in Summer 1978.
You loved it. You loved CERN.
Yes I loved it. My English, which I studied in high school, became very good and I learned also French just by listening. I never studied French, but I can speak it relatively well. And that helped me later when I applied for the PhD program at the University of Geneva, where you have to teach in French. I loved CERN because it was such a vibrant place at that time. It was a dream come true for a physicist and imagine for a 23-year-old girl who had never been out of Italy. It was international, with so many people from different places around Europe and the world, speaking so many languages. And the science you could breathe it in the air… And then this guy. This guy, this talent that you feared, and you loved at the same time. Every moment of your life. And for many years to come.
This is Carlo, you mean?
This is Carlo, yeah. And there was no weekend, or not often. Because you cannot easily say no to Carlo, at least not then. When he would say, we see tomorrow, and that was a Saturday or even a Sunday, there was no question. I just was not able to say no. And I was not the only one. To see older guys, professors somewhere, not being able to say no to Rubbia was simply daunting. But so it was and for some time, as a young Assistant professor at Columbia, I thought is perfectly fine to ask my students to meet in the lab even if it was Saturday, if the work demanded it.
Now, going to the University of Geneva, is the main motivation there, proximity to CERN?
Not really, but my personal life played a role. After I finished my Laurea degree, Rubbia suggested me to go to Harvard for a PhD—he was a professor at Harvard at that time, commuting regularly between Geneva and Boston. I was intrigued but looked at opportunities closer to home and the PhD program at the University of Geneva within the group of Prof. Hess was also appealing as I met one postdoc working in that group and he had only great things to say about that research and group. I didn't know anything about the United States or Harvard, to be honest, so I decided for Geneva at that time. The connection with CERN, of course was important, because I made so many friends at CERN, but more importantly I was in love with the man who later became my husband and he was a graduate student from the University of Aachen, Germany working full time at CERN, in Rubbia's group. I spent quite a few days at CERN during my PhD period and the rest at the university, when not on shift at the other accelerator center in Switzerland, known today as PSI (Paul Scherrer Institute), which near Zurich, where my experiment was.
What was your research for your PhD? What did you focus on?
I worked on a nuclear physics project. The experiment involved scattering of 500 MeV protons on a polarized proton target to study time violation effects. It involved cryogenic techniques that were new to me and also quite some nuclear physics which I had to learn from scratch. It was a good group, it was a nice period of my life. I took many classes, I participated in the experiment and its data analysis, and I also enjoyed the teaching which came with the position. The challenge was that I had to teach statistics and other topics in French which I never studied, but as I said earlier, I became quite good at speaking it. Being Italian helped.
What was exciting to you at this point? What was going on in physics more generally that you thought you might pursue for a career?
At that time? Well, all was new and exciting for me especially what was going on at CERN. This was the time of the UA1 and UA2 experiments at the highest proton energies achieved at an accelerator. Big experiments, just like we have today—ATLAS and CMS at the LHC. And Rubbia was leading the UA1 experiment, and my husband and many others I knew were working on UA1 and I would constantly hear stories about the pressure to get results, the hard work on the data analysis that my husband himself was working on for his PhD, etc…The competition with UA2, etc. A few years later, in 1984, while I was at Harvard working with him again, Rubbia got the Nobel Prize for the UA1 discovery of the W and Z bosons. He shared it with Simon van der Meer, an accelerator physicist because the discovery was made possible by the advancement in the technology. This is always the case, right? Yes, it was exciting time. You could breathe physics in the air at CERN, the excitement to discover new particles.
But I didn't feel that I missed to be part of the team of students analyzing UA1 data like my husband did. It was more the experimental progress that excited me. I didn't have a clear idea of what to do after, my PhD, to be honest. I went back to Italy for a few months, but then there was the good coincidence again where Rubbia proposed both to myself and to my husband to go to Harvard as postdocs to work with him, and not on the CERN experiment, but on a new experiment. That was the Harvard-Purdue-Wisconsin (HPW) proton decay experiment, located in a salt mine in Utah. At that time several experiments were searching for proton decay. We accepted the position and so I came to the US and stayed!
And you would continue, Elena, with the noble liquid detectors for this?
Yes. You see Carlo’s vision was to go after proton decay with the liquid argon technology. So part of my time was spent in the basement of the Harvard Physics Department where we set up an R7D dedicated to studies of liquid argon, with small scale detectors. Our studies became the seed for the ICARUS experiment, which Carlo proposed while we were at Harvard for proton decay and neutrino physics...it took a long time and many turns but now ICARUS is at Fermilab and starts its search for sterile neutrinos as part of the Fermilab short baseline neutrino program. Finally, the liquid argon technology that Carlo envisioned when I started as a student at CERN is now taking over and is the technology of choice for the DUNE major neutrino program in the US
It took almost all my scientific life to see this happening, but I am happy for Rubbia and for the technology choice because I share Rubbia's view about the power of the noble liquid imaging detector. After we left Harvard I continued to work on liquid argon first at Columbia and also studied liquid krypton before turning my attention fully to liquid xenon, which was to liquid xenon which was more appealing for some of the gamma ray astrophysics that I wanted to do.
Elena, who were some of the other people at Harvard you were interacting with?
Oh, I mean, there was Steven Weinberg and Shelley Glashow and many other giants. I met Sam Ting as he would join some of the events at the faculty house, and I met John Bahcall and many other famous physicists visiting Carlo when he was at Harvard. You know, I was there when he got the Nobel Prize in 1984 and that year was simply crazy in terms of both dealing with Carlo’s energy and his increased demands fueled by the Nobel euphoria. In the physics department, you know, we were simple postdocs and directly under Carlo’s control even when he was far. Our time was spent mostly in the lab and really, we felt a bit like slaves of this powerful man. He would fly in from Geneva, say on a Thursday, and he wanted to see results. Nothing else mattered....
But it was a nice time working on our experiments, almost nonstop. We were really given support. We had access to the electronic shop, the machine shop, to build our little detectors which we filled with liquid argon to study the drift of electrons. But when Carlo would come almost every week from Geneva, or every two weeks, I mean it was like a bombardment of questions and he was never happy about our progress, at least he would not tell us but to others he would. He would tell the world about our results but rarely he made us feel good. But that was his style.
At this point, did you think that you would be making a life for yourself in the United States, or was your intention to go back to Europe?
No, to be honest, I never thought I would go back. It was clear to me despite the very challenging times that we had working with Carlo. Well, with the ICRAUS experiment proposed for the Gran Sasso underground laboratory, in Italy, when that lab was just being with the great vision of another great physicist and friend, Antonino Zichicchi, it was clear that if we continued with Carlo on ICARUS we had a door opened to Europe, even Italy. But when my husband just couldn't take it anymore and one day told Carlo “I quit.” We decided we would stay in America and looked for new positions. My husband first got an offer from IBM, TJ Watson Research Center in NY, but later accepted an offer from Schlumberger Research Center in Ridgefield, Connecticut, and I was lucky to get the position as Assistant Professor at Columbia. We bought our first house in Ossining, NY so we would have about equal commute to work. And so, we got free from Rubbia, left Harvard, happy to start a new life and career.
When did you get involved in radiation spectroscopy?
It was mostly at Columbia. When I started to build my lab, for the project first funded by DARPA, later by NASA, it was all about gamma ray spectroscopy and imaging simultaneously. So I built detectors filled with noble liquids to study which one would give you the best resolution in energy, but I always wanted to realize the beautiful concept of a liquid time projection chamber, which I learned from Rubbia. A detector with imaging capability in addition to spectroscopy capability. I studied for years the energy resolution response of noble liquids, and what limits it in practical detectors, something which we haven't completely understood even today.
What were some of your earliest realizations about how important xenon would be for astrophysics?
Well, it was all about this energy resolution that I had been studying with MeV gamma-rays. I was fascinated at the prospect to detect the radioactive elements produced in the explosion of stars, to detect the dust from which we are made. I mean, to locate actually the sources of this radioactivity, to image the aluminum 26, or the iron 60, which is produced in a supernova explosion. I started to get interested in the imaging and how you can actually identify where these gamma rays are coming from in the sky. And since these lines are in the MeV regime, the interaction in a detector is dominated by Compton scattering. So, you need a Compton telescope to image these lines. And so, I proposed to NASA to use a liquid xenon time projection chamber as a Compton telescope to be tested in near space with high altitude balloons. I was of course inspired by what I learned from Rubbia about the power of such detectors for combined imaging and spectroscopy.
Many of my years at Columbia were spent to develop this Liquid Xenon Gamma-Ray Imaging Telescope (LXeGRIT), which was tested in three balloon flights, from Palestine, Texas, and Fort Sumner, New Mexico, where NASA has the balloon space program.
Now you didn't just propose this, you were spokesperson for this.
Yes I proposed the LXeGRIT project and was the spokesperson. And after these 15 or more years of break I've taken from the field of MeV gamma-ray astrophysics, I see that not much has happened to advance it. There is not one instrument which has successfully replaced my liquid xenon gamma ray imaging telescope. And now that I've learned so much about liquid xenon and related technologies, I am tempted to propose the concept again, to achieve better performance that LXeGRIT. What scares me with NASA, is that the timescale is typically too long.
Elena, what were some of the most important things that were learned from the data collected from the telescope?
The goal was to prove the concept with an image of the 511 keV gamma-ray line from the Crab nebula. It proved to be difficult with the data from the short flights. The demonstration of the liquid xenon detector technology in near space was the greatest achievement, because nobody thought that one would succeed in deploying a relatively complex and cryogenic liquid detector, launched on a balloon, with all the challenges associated with limited power and space to carry coolant. Not to mention the challenge of handling high voltage and many channels of low-noise charge amplifiers and digitizers with associated large data rate, not typical for a balloon payload. The first engineering flight was a disaster, because we could not raise the voltage on the detector’s cathode after the launch. We had to abort the flight and later discovered the trivial, human mistake, responsible for the failure. You know, I have so many stories that are so worth writing a book.
But finally, we got our first, even if not-so-good image of the Crab, we got a PhD thesis written on the project, but most of all we got a few intense years of exciting research and development done. I proposed to NASA an upgrade to improve the telescope for which a much larger level of support was needed, I also proposed a satellite mission but the time was not right, and NASA was not too responsive to novel ideas with too much risk, so I gave up and started to think of a new research direction, intrigued by the dark matter question. I went to the Snowmass meeting in the Summer of 2001 to learn about the status of the field and as I listened to talks on direct dark matter detection with various types of detectors I told myself “I can do better with my liquid xenon detector” and that was the first idea for the XENON project which I proposed merely a couple of months later.
Well, obviously, at that point the field was wide open. Nobody knew anything about dark matter.
Not only, but also experimentally, there were few ideas.
And Elena, you alluded to this earlier, but what was your specific earliest realization that liquid xenon would be so useful for detecting dark matter? What was that intellectual connection for you?
Well, I guess I said it before. It's because it was apparent at the time that the field was dominated by small scale cryogenic bolometers, such as those used in the leading experiment then called CDMS for Cryogenic Dark Matter Search. A very sophisticated technology with powerful discrimination of signal from noise thanks to the simultaneous detection of both ionization electrons and phonons produced by radiation hitting the small and cold crystal of germanium. Great results were shown, I remember at that time.
But the detectors were small and difficult as well as expensive to scale up in size. It was clear that the path forward was a detector technology which would be first and foremost scalable. I liked the idea of the bolometers, where you read two signals. And a noble liquid would give you also two signals. Not the phonon, but the photon signal from the scintillation of the liquid. The detection of scintillation light from noble liquid is challenging as the photons are in the very deep UV. You cannot use your standard photomultiplier tubes. Technologically, it was a challenge, but I had already started within the LXEGRIT project to develop special phototubes to be used in the liquid at low temperature to detect its scintillation light. Developing these photosensors was one of the technological breakthroughs which enabled the family of XENON detectors to be so powerful. Another major breakthrough was the development of efficient cryocoolers for liquid xenon temperature, close to 160K.
Why Gran Sasso? When you were picking sites?
Ah, we had several trips to visit underground laboratories here and there. From the Snolab mine in Canada, to the Waste Isolation Power Plant (WIPP) to the Soudan mine where, the CDMS experiment was. Yeah. Why Gran Sasso? I guess first of all, I was connected to the Gran Sasso Lab, remember, through Rubbia and the ICARUS proposal. Meanwhile I knew that the lab had become the largest and most advanced underground facility around the world but had never visited it. The Italian connection also played a role. I mean, I didn't know much of the Abruzzo region where the Lab is, but I knew it was beautiful.
Most important the lab was not in a mine, but under the Gran Sasso mountains with direct car access through a long tunnel... This which makes a huge difference in mounting and operating an experiment underground. So I wrote a letter to the Gran Sasso director of that time, asking for some space to operate the first XENON detector and I got it... So XENON10 was shipped from Columbia to Rome and installed at Gran Sasso in Spring 2006 and started taking dark matter data that Fall. The first physics result was published early in 2007 a very fast turnaround for a dark matter search. And the result took all by surprise… Wow, it was so much better than what the fancier and more expensive CDMS did and after a lot more time. It was like an awaking for the field of direct detection.
I wonder if you might explain the science, why this needs to be done underground because of cosmic rays.
Oh I understand.
Cosmic rays are a problem.
We haven't told that to the reader. Yes, we are looking for a very rare signal and any detector in a lab on the surface would be overwhelmed by a huge rate of gamma rays and beta particles and alpha particles from the radioactivity which is in the cement in the floor, in the detector itself. The rate from all these particles is so high that you have zero chance to see anything else. And even if you can make your xenon or argon or germanium detector completely pure -- pure means free of radio isotopes -- you still have to fight the ultimate enemy—that's the neutrons. Neutrons do exactly what a WIMP does, give exactly the same signature in a liquid xenon or any other detector. But there is a difference—neutrons lose their energy through scattering at multiple sites. And so, when you have a large detector with imaging capability- meaning that is able to measure the position of the interaction, you can distinguish an event which has multiple site interactions, from an event which has only one interaction. Because the first and foremost requirement for a particle such as a WIMP scattering in your detector is that it just scatters once and goes out. Your event is a single site interaction, we say.
So back to the neutrons. The neutrons are the ultimate enemy of any dark matter search, because they give you a signal that is the same as that of a WIMP, with the exception of this multiplicity, which not every detector can distinguish. And the reason detectors are operated in a deep mine or under thousands of meters of rock is to reduce neutrons, because neutrons are produced by cosmic rays. The cosmic ray muons, which are bombarding us here on the surface, will create the neutrons. The rate of those muons goes down with the depth. So, the deeper you are the less is the flux of these cosmic ray muons, and therefore the less is the production of the induced neutrons, which can ultimately kill your dark matter search. The deeper the better, there is no question. But you have to consider also other factors when choosing an underground laboratory.
One factor which was very clear to me was the laboratory’s location and attractiveness especially to young students and researchers, because an experiment is made of people. I mean an experiment is best when its people are happy to work on it even for long hours, every day and so the location of the experiment is important. If your experiment is in a location that is appealing it's easier to convince your people to travel to and spend weeks, maybe months at a time, as you're mounting the experiment. And I never had to, you know, go through big extremes to convince my graduate students, undergraduates, or my postdocs, to spend weeks of time at the lab in Italy. Despite that it's quite isolated -- it's not in a big city – there are mountains and sheeps and [laugh] lot of good cheese. It is a nice environment—beautiful mountains, and nice people. Great food, great wine. Cheaper than Coke. I remember my students saying they were surprised that it was cheaper to buy a bottle of wine than a Coke. You have to put that into consideration. The happiness of your people.
That's right. Elena, what were some of the technical or even administrative challenges in procuring the amount of xenon you recognized was necessary for this experiment?
There is not much xenon in the air so the first challenge is its cost. For the past 20 or more years working with xenon in my lab I've seen this cycle of price changes for this material. And being someone who cares about money and likes to always get a good deal -- I learned that you have to jump on the market when the price is low. The problem is that the price fluctuates with the worldwide economy. Xenon is a rare gas which we extract from the atmosphere. And the fraction of xenon in the atmosphere is a minuscule amount… It is much rarer than argon for instance. But who cares about xenon? Who wants to extract xenon from the atmosphere? I mean the use of xenon in industry is limited…you need it in light bulbs, the rocket industry uses xenon and the semiconductor industry uses it. But you know, still, a very small fraction in terms of economics.
So, what happens is that the extraction of rare gases from the air is driven by the need for oxygen, actually. All these plants around the world, distillation plants, distill the air to extract primarily oxygen, because oxygen drives the steel market and so the economy. You need oxygen to produce steel. You need steel to build bridges and buildings. And so, whenever there is a boom in the economy because you have a lot of construction going on, a lot of demand for steel, there is more extraction or distillation of air. There is more byproduct of that oxygen, which are the other rare gases, such as argon and xenon. In a good market economy, because you have more supply, the price of the xenon goes down. I've seen prices which go from even only $3 a liter of gas to today’s price, which is more than $20 a liter. So, you see the factors. And, because of the compressibility of this gas, you need about 500 liters of gas to make one liter of liquid. And one liter of liquid is about 3kg, as the density is three times that of water. So if I want to buy a liter of liquid xenon you have to spend $10,000. So, it's a very expensive champagne... But from $3 to $20 per liter of gas is a big factor. And I've seen it going up and down over these years and tried always to buy it at the lowest price.
To that you need funds at the right time, and I even proposed to the NSF to invest in xenon gas, but unfortunately the government is not set up like that. The only way is to call Elon Musk, that I know from a conference which he helped sponsor, or any other philanthropist and say, “You know, you're going to get rich if you invest in xenon.” But I haven't pursued that road. I have just been lucky so far to have the research funds when the price was low. So, there is no technical challenge. The problem is only how to get at a good price those hundreds of thousands of liters of gas that you need for your experiment. You know, in the XENONnT experiment, we are using close to 9,000 kilograms of xenon, worth more than $30 million at today’s price. And what makes you not sleep is the fear that you can lose it! If something goes wrong with your cooling or if a big earthquake hits the lab, there is the risk that your precious xenon or some of it goes back in the air.
Elena, a political or even a cultural question. As you're managing this enormous and complex enterprise, did you detect as a woman that you were experiencing difficulty that you might not have as a man?
Always, David. And still today. From the beginning, you always have to prove yourself not twice but hundred times. It used to be like this at Harvard. It used to be, well, at Columbia also for sure. I mean, you know, I have started at Columbia before there was any policy about sexual harassment, and everyone was less careful about how they talked to you and what they would say. These were different times then. Which is also the time of being the only woman and the first one in the department, after Madame Wu, which was quite a while earlier, because when I met Madame Wu at Columbia she was already retired. I was for a long time the only woman in our faculty meetings. It was intimidating at first but then you get used to it. And I had my second daughter, Giulia, while I was assistant professor at Columbia. And there was no maternity leave.
There was no provision for such thing. I went through it and got a term free from teaching only thanks to the chair of the department then, who is a great guy, Frank Sculli who, maybe because of his Italian heritage, had some compassion, or whatever. Probably every chair would have done the same. You still have discrimination today, although I feel it less, probably because people are more scared of me now and they don't easily say what they think to my face, maybe. Discrimination has become much more subtle.
But it's still there, you say?
Yes, absolutely. I see it is there. I mean I see the nuances because I have so many decades of life in physics, and I am so intuitive, there is no question. Of course, the younger generation of male colleagues is different, you see a big difference between the younger colleagues and the ones who were there since I started. The older generation of male physicists has not changed a bit in terms of their view of women and their positive impact on the department.
On the surface they follow the flow and support diversity, but I am skeptical. I think it's something that you have to tell your daughters your female friends, all the women out there: don't expect that it's going to be easy. It's going to be more difficult even if you have exactly the same qualities as your male colleagues -- in terms of smarts, ambition, everything. But just because you're a man and I'm a woman, there are so many ways and many subtle ways in which I'm going to be affected a lot more than you are along the way. Often it is just small remarks which people don't even realize they are actually aggressive and inappropriate.
Elena, do you see your successes as a result of despite these difficulties, or have you used them as a positive motivator?
Probably more the latter. I've used difficult moments to get stronger. I'm a fighter. The more you challenge me, the more you put me into a stronger state that will help me fight back.
The more you try to put me down, the more I'm going to show you what I can do. You know, that kind of attitude, and the more fighting gets triggered. I mean, you have no idea. And so, in a sense, it's good. But you know, you have to be strong to constantly fight and you have to warn young women to not expect an easy path to success. If you want to get there, you have to have this nice cocktail of features, you know? And it's better for both young men and young women. You know, there are certain things you must have if you really want to get somewhere. It's not going to be easy for either a man or a woman. But to the woman, I would say to be prepared, because your path will be filled with more obstacles. And sometimes it's not even clear where they're coming from, the obstacles. You have to be aware and you have to be ready to fight back.
Elena, back to the science. Where have you seen the LZ dark matter experiment as a competitor, and where have you seen it as a collaborator?
We've always been competitors but always respected each other as colleagues. And as we move forward, we know we want to combine our knowledge and collaborate towards the next generation dark matter experiment with liquid xenon. We have to come together because you cannot have too many of these large-scale experiments, beyond LZ, beyond XENONnT. But this competition has been healthy and very positive for the field, moving us forward at an incredible pace. It has helped everybody at the end. And so today, we are really at the highest level of competition, since we both have realized the largest-scale dark matter detectors which you're going to have for the next several years. And each collaboration wants naturally to be the first in finding a signal. And there is nobody else out there at that sensitivity scale, at least not for a while.
So it's a two-way race, you're saying?
Well, it is, but again the race has never been more positive. You know, the amazing thing is that after all these years when people thought it made no sense to have two collaborations develop the best liquid xenon dark matter experiment, now even the funding agencies realize that having LZ and XENONnT take data at about the same time is going to be essential, since again there is no other technology with similar capability and at least we can cross check each other and even combine our data if we like. The effort to get us together after we “separated” failed. It didn't work. I wanted to do it my way, I wanted to be first. There was no way I was going to be delayed. I went my way and I won. XENON100 was first and for years dominated the field. And they followed with a larger experiment, LUX, but years later and we were already making XENON1T. I always wanted to be the pioneer. I won my way, and I don't regret a bit what I did. I managed to realize the XENON100 and then XENON1T and now XENONnT, of similar size, as LZ with a fraction of the support they have from DOE, thanks to the continued NSF support but also thanks to the many collaborators abroad.
You see, when you put me into a corner, I fight so much back. So, the way that I managed to realize the XENON program was to find other partners and funding, all over the world. The XENON is a much more international collaboration than LZ. We have so many strong groups in Europe, Switzerland, Israel, UAE, and Asia with Japan and recently China.
The family is much more varied and colorful and international, and this means the funding for realizing XENONnT has come not just from the US, like is the case for LZ, which is largely funded by DOE. We have been able to realize a similar-scale experiment but at a lower cost. The bottom line, now that these two experiments are about to start, is that everyone realizes that it is good to have them both. Because if you are alone in front and nobody behind you, it is going to be bad for the science. If we are going to see a signal or see nothing again, LZ must see the same or we have a big problem. And vice-versa. They could still be first but then XENONnT will check them. The two experiments use the same liquid xenon target and type of detector, but they are still quite distinct. The devil is in the detail.
We have chosen very different approaches in terms of solution to problems, especially background controls and purification. So the two experiments are going to have quite a different set of systematics, of problems, right? Nobody actually can bet now which of the two will win or get a result first. It is premature and we will have to see how well our solutions and choices will work. So the two collaborations for sure realize how important actually it is for both to have the two experiments more or less simultaneously taking data or challenging each other. Of course, we all want to be first, but at the same time if you see a signal you want someone else to confirm it. And ideally, you would like the signal confirmed also with another target. It is one of the reasons for having recently joined the DarkSide Collaboration which is developing a large experiment with liquid argon, of similar sensitivity as LZ and XENONnT, but that will take a few more years. I mean, dark matter is such an important question that nobody is going to take a signal for granted, so more experiments and with different materials are essential to make a discovery indisputable.
I wonder how you might compare the competition with LZ to ATLAS and CMS and their work toward discovering the Higgs?
Yeah, it's comparable but played at a much smaller scale. You're talking about collaborations of, in our case, a few hundred people now, like 180 for XENON and 200 for LZ. CMS and ATLAS are collaborations of thousands of people, and I did not envy Fabiola when she was the spokesperson of ATLAS, before becoming the Director-General of CERN. We met sometime at a conference here and there and I would say, “How do you sleep at night with this job you have -- I mean, when I can barely cope with my 150 people? And I like to know each member of my XENON family.” So I don't think I could deal with such large collaborations.
I don't know how strong the competition between the two CERN experiments is, but I suppose it's similar. I mean for sure it's the same feeling that has fueled XENON and the other liquid xenon competitors. One also has to take into account the huge difference in terms of cost of these experiments. When you compare the scale, the cost and the time scale, versus the excitement and results produced I feel very proud of what we have done with XENON. And now with an LZ at similar scale as XENONnT, competition will be more fierce than in the past when you had a XENON100 followed by a larger LUX which would take over for a bit but then we made XENON1T and were again in the frontline and alone for a while. So yes, you will have two liquid xenon experiments with similar reach, and that's going to be positive for everybody. Because what we are fighting now that we made them massive is really the background and how well the two experiments with their different choices will be able to overcome it.
Obviously one big difference with the discovery of the Higgs was that there was a tremendous amount of theoretical guidance for those experiments.
Oh yeah. Well, that goes without saying. At least they knew where to look, what to look for.
So Elena, on that point I guess, I wonder if you can explain how you define a successful experiment if you don't even know what it is you're looking for?
You know, the answer is in what we have shown with XENON1T. You build an experiment to detect this WIMP particle in a certain mass region, and you optimize it for that. But once you have built it, once you have this powerful machine, this new tool, you discover that it's good for much more—for example we have been able to search for dark matter particles in different channels, if you like. Different interactions. We have been able to put the best constraints, even in the low mass region where other technologies are better and yet, XENON1T has done even better. And we have searched for other rare events.
For instance, we have detected the longest decay of the xenon 124 isotope. There is a beautiful Nature paper and our result made it on the front cover. A very important result which was a direct consequence of XENON1T being the first detector with such a big mass of xenon, but also with the lowest background ever realized. And when you start to have so low background, you can see a lot more. It's optimized for something, but you can push the analysis techniques and open the boundaries of what you can do with the data that you have taken for one year with XENON1T.
There are so many papers that XENON1T data have enabled- and the last is one that my own group has worked on, last year. Speaking of COVID’s isolation or whatever. Since we were all locked down and isolated, I gave free reigns to my students and postdocs to actually push for a search for the coherent scattering, elastic scattering of solar neutrinos using XENON1T data. Something we hadn't done yet and a signal we expected to show up, although minimally in XENON1T. I told you before, we expect some background events from neutrinos, solar neutrinos, atmospheric neutrinos, in our detector as it gets more and more sensitive. As your sensitivity improves, so you can look at your data and say, “Oh, am I finding any sign of coherent scattering of neutrinos with my xenon nuclei?’ And so you do a specific analysis just searching for those events. And it was also a test of how to improve our tools for the XENONnT, the next phase of the experiment. Which for sure will have much larger sensitivity.
We did not see any solar neutrinos events but we placed the best limit on WIMPs below 3 GEV. The result was quickly published in PRL and made it as a highlight for the journal. So the conclusion is that the experiment we designed for WIMPs is successful not only because we have helped theoretical models to be fine-tuned based on our null result, but also because we are sensitive to other rare events and thus unexpected discoveries.
[laugh] Elena, I'll ask you for my last question, looking to the future. Best case scenario, xenon succeeds in the very best of your dreams. It succeeds in its mission. It detects dark matter. What exactly will that look like? Will that moment look like? In other words, if you think about LIGO and the detection of gravitational waves. Or supernovae and the understanding of the accelerating universe. What will that moment look like for you and your team? And as a result, what new questions can physics ask that we currently cannot?
Okay, I start with the last question. What else can we do? For dark matter, it's not going to be such an easy thing, at least not with the direct detection approach. Gravitational waves …one is happy to see one event... here we need lots of statistics. We’re not going to define this WIMP, with a few events. At the sensitivity level that we reached we know we expect a handful of events, if we are lucky. And we're not going to make much out of a few events to say what is really the mass. To pin down some of the characteristics of this particle, you will need first and foremost to keep accumulating statistics -- so that's the answer. If I were to be so lucky with my team to have a few events, the first thing I would do is to push for the next generation experiment to see more events.
So, I think a first detection will really be a trigger to go ahead and make a larger scale liquid xenon detector, and the next step has to be very fast, because you want to confirm it and you don't want to spend another 10 or 20 years, right? The other thing that I would do, if I were so lucky, is to see the signal in another material. We have always known that this complementarity of target materials is essential. If it is truly the dark matter particle, you have to see it in the right proportion in xenon, in argon, in germanium, and so on. This is also the reason why lately I decided to join the Dark Side collaboration. First, because I'm excited to go back to the liquid argon technology with which I started my physics career. You know, with age you go back to your first love... but, most importantly I know that we need another experiment with a different target, such as liquid argon, and DarkSide is the only other massive experiment that is coming up, with similar sensitivity as our XENONnT and LZ, and even better sensitivity at higher WIMP mass. We are not going to go to Stockholm with a few events, like in the case of the gravitational waves discovery, or even the Higgs’s discovery. Here it's going to take a little bit more time, because dark matter is such a big thing, with so much impact on other fields.
Everything has to fit well, and one has to be extremely sure and rule out all other possible explanations before claiming a discovery. If XENONnT or LZ find a small signal, you're not going to be able to tell your friends of ATLAS and CMS where to look in the sea of particles produced in the highest energy collisions at the LHC. Because you still have to narrow down the particle’s mass, especially. For that you need more statistics, you need to confirm that it’s a WIMP signal. You know, they don't come with flags, these WIMPs, unfortunately. I wish they all showed up in black dresses. [both laugh] I'm imagining them as little “diavoletti”. Little devils. But they won’t tell me their true nature. They could be neutrons in disguise.
So unfortunately, you always have that doubt in your mind. No matter how clever your students, your postdocs, your analysis, or your statistical methods are, one is not going to obviously stop at a few events… the best would be a chance to see more of them, by running for longer time if one had close to zero background. It would be ideal to see a signal early on and then you keep your machine nicely oiled and keep taking data because you want to see more, or you wish that LZ [laugh] confirms your signal because they also see some events they cannot explain with something else. But ultimately, I think you have to see it in more than one target. I'm quite convinced about it. So guys, just get your act together and let’s go after the WIMP with more than liquid xenon.
It's a good thing you're a fighter. We need a fighter to do this. It's not going to happen otherwise.
Yeah, guys, get your act together and give me another detector which is as sensitive as mine.
I mean, we need it. As you said it yourself, the Higgs was known to be there. Here we don't know. And gravitational waves were also expected from Albert Einstein’s General Relativity. Here we are in the dark. So... it's very frustrating, the fear of the mistake, the fear that you're looking for something which might not even be there has been always in the back of my mind. That's why gamma ray astrophysics becomes appealing to me again when I'm depressed. Because nucleosynthesis is happening in the stars, and we know it from so many ways, right? And if I were to launch a telescope, with a great xenon detector, the best of the best that I can make, I know I would find those gamma-ray lines in my data. But here I can do the best job with my detector, but no-one can guarantee that I will find any sign of the dark matter particle.
Well, we'll see.
So if you find something, you want to be extremely cautious.
Excited but very cautious before declaring victory, that is what I'm saying.
Elena, it has been an absolute pleasure spending this time with you. Thank you so much for doing this, for carving out time in your busy schedule. Congratulations once again on the Academy recognition, and good luck!
Ahh. I think that's great.
Good luck finding the dark matter. It's so exciting, the whole physics community is watching so closely. It's going to be so exciting when it happens.
It will be an awesome conclusion of a long career.