Francis Halzen

Zierler:

OK, this is David Zierler, oral historian for the American Institute of Physics. It is July 20th, 2020. I am so happy to be here with Professor Francis Halzen. Francis, thank you so much for joining me today.

Halzen:

It's a pleasure.

Zierler:

All right, so to start. Would you tell me your title and institutional affiliation?

Halzen:

I am a professor of physics at the University of Wisconsin in Madison. My main research is with IceCube. And I'm the principal investigator for the IceCube Project.

Zierler:

And how long have you been affiliated with the IceCube project?

Halzen:

Forever, actually. [Laughter] The IceCube Projected started with the submittal of the original proposal in 1999. But before that there was a decade of work preparing IceCube. It was always our idea to build a kilometer cube neutrino detector. But we did R&D by building a smaller detector, which was called AMANDA. That project slowly started in the early nineties.

Zierler:

And you were one of the founders of the project or you were brought on later?

Halzen:

As far as anyone can tell, including myself, I came up with the concept. But, you know, the ideas were kind of in the air. I gave talks on it. The actual project started in 1990 with some kind of a collaboration between Berkeley and Wisconsin.

Zierler:

Well, let's take it all the way back to the beginning. Let's head back to Belgium. Let's start first with your parents. Tell me a little bit about your parents and where they are from.

Halzen:

Well, my parents. We're from a place that called Tienen, which was home to about twenty thousand people, which in Belgium is a city. It's a rather isolated place and my father was in business. My father had a construction company, and so did his father, my grandfather, and I guess that even generations before that were all in in the business of building roads and bridges.

Zierler:

What was their experience in the years before you were born during World War Two? Were they affected by the war very much?

Halzen:

Of course, their experience was not very good. My father had a construction business. So you either worked for the Germans at the risk of being killed later or you didn't work for the Germans so you were killed immediately. So he escaped to the Pyrenees for a while in the south of France. He lived in the mountains in hiding. Eventually he made it back to Belgium and survived the war. There was not much of the company left. So he had to start over.

Zierler:

Now, of course, you don't remember this. But I'm curious, in stories that were told to you, what was the situation like when you were born in March 1944? Was it stable?

Halzen:

Well, the only kind of flashes I remember were from later conversations, and I must say we didn't talk too much about the war. There were German attacks, even as late as the time when I was born. I was born when the war was actually still going on. But my parents tell the story that they still had to hide in a ditch on the way of bringing my mother to the hospital when I was born because German planes were flying over the road. There were really not any bad memories because my mother came from a small village outside Tienen, that means they lived among farmers, even though they were not farmers. But it meant they had reasonable access to food. I must say, I never experienced the feeling that they went hungry and by the time I have any conscience of my existence, the war was a long memory.

Zierler:

Right. What language did your family speak at home?

Halzen:

Well, we spoke Flemish, which is some dialect. Flemish is a collection of dialects. And, in fact, the Flemish we speak locally, spoke locally, is really disappearing now. It was never written. It was much closer to German than to Dutch actually. It was a Germanic dialect.

Zierler:

So you can understand German?

Halzen:

Yes. I never, of course, learned German because I grew up after the war, I learned lots of languages, but not German.

Zierler:

Right. [Laughs]

Halzen:

I'm now surrounded by Germans, half of the IceCube collaboration is German. I always have to remind them that I can understand them. I never actually went to the formal process of learning grammar. You know, I can speak German in a restaurant, but, yeah, I don't really speak German.

Zierler:

And you stayed in Tienen your whole childhood? Through high school?

Halzen:

Yes. Through high school. And then I went to the university closest to my home. If you went to university, which was already a very unusual experience in our family, nobody had ever been to university, you went to the university closest to your home. So I went by train to Louvain, and that was my university experience. And I also did my PhD there.

Zierler:

Was your father able to reestablish his business after the war?

Halzen:

Yes. But when it was clear that I was lost to the business, he didn't retire but started working as a consultant for other companies, and he did well afterwards.

Zierler:

What was your exposure to physics when you were growing up in Belgium?

Halzen:

None whatsoever.

Zierler:

None?

Halzen:

Oh, yeah! I had a physics course in high school. It was not very inspiring. But, yes, that was my only exposure.

Zierler:

Were you interested in science intrinsically? Were you interested in how the world works before you had a formal education in math and science?

Halzen:

No, it was a real problem because when the possibility came up of going to the university my father wanted me to study engineering. In fact, he really, I think, wanted me to go into business and thought I was wasting my time. But engineering would be a preparation for doing his business again. I don't know.

But I was interested in languages. I actually studied Latin and Greek. And I was also specializing in mathematics. In Belgium, you see, if you are a good student and you think you're bright and clever -- which I really didn't, I didn't have that confidence—you did mathematics. It's the French tradition. You know, our whole Belgian school system was created by Napoleon. It's very good, by the way. Or, it was. I don't know now. So I went off to do mathematics.

Zierler:

And did you know you were good at math?

Halzen:

I never in my life had any self-confidence at any stage. And, so, no, I didn't know. And I am not sure I am, actually. Because I did math and physics at the same time. And later when I did my undergraduate thesis, there was no doubt it was going to be physics, not math.

Zierler:

And how did you make that transition? How did you know? How did you settle on physics?

Halzen:

You know, most of the things that happened in my life have happened by accident. I even did study engineering for six months and then realized that it was not for me. So, I was totally turned off by anything to do with projective geometry and making technical drawings in China ink. And that did it for me. I was also turned off by physics, mostly because I didn't like the physics labs. So that's why I became a theorist. I managed to avoid doing the physics labs. So then suddenly I was a theoretical physicist, and that was that.

Zierler:

So you mentioned, you did a senior thesis, undergraduate.

Halzen:

Yes. And that's an interesting story. I mean, just looking back, there's no doubt that I was more lucky than clever throughout my life. And, then there was the first paper I ever read. I went to the man who was my undergraduate advisor and the first paper I have ever held in my hands was the paper by George Zweig on quarks. You know, he called them aces. And I actually worked with George later, many years later. George didn't know mathematics and so he did group theory calculations in terms of little diagrams. But I had studied mathematics, I knew group theory. And so this was a whole sudden revelation for me. I saw I understood, read the paper and understood it better than the author. I wrote my very first papers on quark theory, which was not that much of career move because if you worked on that topic at that time, you were a crackpot. But later it eventually paid off. It took a long time…

Zierler:

When it was time for you to think about graduate school, where did you want to go and what did you want to work on?

Halzen:

Well, that's a story. It is. It's interesting. When I graduated, I went and got a job as a high school teacher. It was summer vacation and I got a job. I was going to start in a school nearby in September. And one day I came home and my mother told me that there was a professor who rang the bell, a man who was a professor in Nijmegen and he wanted to talk to me. She said he'd said something about a PhD. I mean, she had collected a few words and I knew what it was about.

And then, to make a long story short, then, of course, they realized in Louvain that they didn't want to see me go to Nijmegen. By the way, they were competing Catholic Universities at the time. And so, suddenly, I was approached to do a Ph.D. And, actually, as a graduate student, I was paid better than as a high school teacher. Big shame? Well, I don't know, who knows. I liked the idea. It's not that it was really something that I wanted to do. The way the Belgian system worked at the time was that you never change university. You're an undergraduate. Then a graduate. As a graduate student, you are already a first assistant. And that was a permanent job. So you basically landed a permanent job as a graduate student. And then eventually you become a professor. Just, you know, that's how it was. But if you were any good, this was a straight line to become a professor.

Zierler:

And so where did you end up enrolling for graduate school?

Halzen:

Well, in Louvain University. I went back to the same professor who a year before gave me Zweig’s paper. And he was very supportive but I had no other choice, there was no way to move to a different university. That has changed, by the way.

Zierler:

Right.

Halzen:

Frans Cerulus thus became my supervisor.

Zierler:

Was it known to be a strong physics department?

Halzen:

That's an interesting question. The theoretical physics department was relatively new and so was this man I was talking about, his name is Frans Cerulus. He did a few nice pieces of work. He was a member of the CERN theory group before he became a professor in Belgium. But there was a French and Flemish division, or maybe better a Walloon and Flemish division. They worked together amazingly well because the other professor was Swiss, immune to Belgian politics. His name is David Speiser.

He came back from the US. He had been a professor at Stanford and Princeton, I think, and had then come to Belgium. And he was fantastic. I mean, he had been exposed to American science and came with all this experience and wisdom. I owe him a lot, actually. The two of them led this group, which actually became pretty good. And then, sadly, while I was at CERN, afterwards, the group was split when they divided the university and moved the French speaking part twenty-two kilometers, somewhere in the fields. And when I went back, some of my collaborators were suddenly separated and at a different place.

Zierler:

What was the process leading to your dissertation topic? Did you get a problem from your advisor or did you work on something that you came up with yourself?

Halzen:

No, he wanted me to work on the three body problem using Fadeev equations, something very mathematical. And I was off on quarks. I just told you the story. And I actually wrote my first paper by myself on nonrelativistic quarks bound in mesons, which people are still working on. You know, I was very proud of that paper. And so I kept working on quarks and flavor symmetries. Because of Cerulus and Speiser we had a lot of foreign visitors passing through; they had a big influence on me. And they also did something that our Catholic university didn't appreciate very much. Every Wednesday, we met with two people whose name you recognize, Brout and Englert. [Laughs] These people who were important to my research. In fact, I basically did my thesis with Brout. Of course, he was not my adviser, but he was in the end allowed by Louvain to sit on my thesis committee.

Zierler:

What did you feel were your contributions to the field with your dissertation at that time?

Halzen:

Not much. [Both laugh] Not much. It was, you know, the time of S-matrix theory.

Zierler:

Right.

Halzen:

And it was about combining group theory with S-matrix theory. And, this was the time, actually I just remember, that Brout and Englert wrote their Higgs paper. I remember discussing it with them. Brout came from solid-state physics, and I remember telling him the paper just proved that they were still solid-state physicists. While I was doing S-matrix theory, shows how clever I was. [Laughs]

Zierler:

What were your prospects after you defended? Where did you want to go next?

Halzen:

Well, this is funny. I mean, I had no idea that after you did a PhD you had to find a postdoc abroad. In fact, among the people who I was lucky to have as colleagues at the time, Peter Minkowski, who is a professor in Bern now, and who became more famous than I did. He was he was a postdoc with Speiser and I did a lot of work with him in different areas. With that I was ready to go. I didn't know that you actually have to get a job and because this group was new, there was no tradition or experience. I went to CERN for three months with a Belgian grant, and then eventually I stayed at CERN.

Zierler:

What year did you arrive at CERN?

Halzen:

Sixty-nine.

Zierler:

And what would be the most exciting things that were happening at CERN at that point?

Halzen:

Nothing. [Both laugh] Well, let me qualify that a bit. Yeah, CERN was not a great place at the time. I remember, I mean, and let me first say the good thing. Because the theory group had like a hundred members and visitors, I found a lot of great people to work with. And therefore it didn't affect me. But I remember, at the time I left, which was two or three years later, the theory group had 125 people. And the director told me, there are only three people who can write a computer program, and you are one of them. And he was so excited about that. There was no interaction with experiment. And the experimental groups, you know, were only doing routine experiments. I may get in trouble if this ever gets public, but I don't care. The experimental groups were typically doing the type of experiments you already had to know the answer. Otherwise, no money would be spent on it. And of course, this was at a time, we are now in1971, 1970 to ‘71, where, as you know, physics was exploding in the U.S., particle physics that is. I mostly worked with Americans and people who came back from the States and then eventually I started working with the Caltech group on something that's called duality. And that subject eventually resulted in string theory. That's where it went wrong. [Laughs]

Zierler:

Where it went wrong? Did you do any work yourself on string theory?

Halzen:

I have two papers on string theory, but nobody knows that. [Laughs]

Zierler:

Tell me about them. When did you do this? 1971?

Halzen:

Yeah, I think so. ‘70 or ‘71. And we were trying to build scattering amplitudes, real physics using string theory. And I cannot remember the details, but I remember, when you define nucleon scattering and when you want to include spin of the nucleon, you couldn't do it. And people still cannot do it, but they didn't worry about that anymore.

Zierler:

Sure, sure.

Halzen:

They are off in a different universe, or many of them are.

Zierler:

Yes. I know where string theory is now, but, at the time, what was so exciting about it? What was its promise?

Halzen:

Oh, I think it was. I should say, this was before it was associated with supersymmetry. Wess and Zumino, that came later. But there was something called the Veneziano model. And that was a string theory, but it was applied not to gravity, but to normal particle like pions and nucleons. And I was just in the transition.

Zierler:

At what point did you stop being interested in string theory?

Halzen:

Oh, I wrote two papers and then, I went off doing something else. Something, you know, is that I am really only interested in learning things. And the only thing in my whole life I stuck with for a long time was AMANDA and IceCube. When I worked on something, by the time I understand it, I lose interest. [Both laugh]

Zierler:

That's to suggest that you still don't understand IceCube.

Halzen:

That's correct. [Laughs] We're working on it, though.

Zierler:

How many years were you at CERN?

Halzen:

A bit more than two, I think. it depends how you count. I returned for many visits later.

Zierler:

And, besides your work on string theory, who were some of your collaborators at CERN? Who did you work with there?

Halzen:

Well, I worked with a man named Paul Auvil, who was a professor at Northwestern and visiting. And I worked with a group of young people, who had been postdocs in the US. One I should mention Jacques Weyers who had been a postdoc at Caltech got me in contact with the Caltech crew. So I think the most interesting papers I wrote were on duality between resonances and particle exchanges. It was kind of a generalization of this Veneziano model. I think that was actually respectable work. And I did it with the Caltech people, George Zweig among them, who was then a professor at Caltech, and a man named Jeff Mandula, who you probably know. He was for many years, an NSF program officer for theory.

Zierler:

Right.

Halzen:

Yeah, I hope I didn't forget anyone because I worked with lots of other people. You know, CERN was great for me, despite what I said, and there were so many visitors as well. I mean, you wrote papers over lunch, talking to people. And I never could really specialize. I was not good at that.

Zierler:

And Francis, in those early years at CERN, there was no sense that it would overtake the United States of high energy physics?

Halzen:

Oh, no. Oh no. No. Absolutely not.

Zierler:

No one was thinking about the LHC? Nothing like that. That was way too early?

Halzen:

No, no. I mean, this is, at that time, no. There was no feeling that CERN was heading for greatness. Actually, in my opinion, it didn't happen until the W discovery in 1986. That's a long time later.

Zierler:

How closely were you following what was going on at places like Brookhaven or Fermilab or SLAC?

Halzen:

Oh, pretty closely.

Zierler:

Yeah.

Halzen:

Because, you know, what happened is I was then supposed to go back to Belgium.

Zierler:

Right.

Halzen:

But in Belgium, I found this situation where half of my friends and collaborators were now at a different university. And both groups became for a while subcritical. They recovered later. I was invited to come to Madison, to Wisconsin, for six months. And that's when I really discovered, science was different in the US. I bring this up because I would spend immediately all my summers in Brookhaven. That was fantastic. You know, Brookhaven, so I had close connections to the people planning the ISABELLE accelerator.

Zierler:

And what was some of the work you were doing at Brookhaven? What was going on at that time?

Halzen:

I knew how to write a program. I mean, let's face it, this was the time we were creating the phenomenology group in Madison. And so I worked in a group there that was motivating the construction of a new accelerator led by a man named Trueman. And we would do calculations to justify the construction of an accelerator that was called ISABELLE. It was very unfortunately never built.

Zierler:

Yeah.

Halzen:

Which was a big mistake.

Zierler:

What were the big goals and objectives of ISABELLE in its planning stages?

Halzen:

Seeing the W.

Zierler:

And can you explain why is that significant, to see the W?

Halzen:

Oh. I mean, the W, that was the Holy Grail at the time. We are at the time that S-matrix theory was waning. With the first steps in the Standard Model and gauge theory and, obviously, the electroweak unification, the goal was to find the two weak intermediate bosons, the W and the Z. And ISABELLE was supposed to do this, and would have done it. But then the idea appeared that it was better to build a proton-antiproton collider because the cross section for producing the W and Z are bigger. I think the US panicked, and thought CERN was going to beat them by doing just that. ISABELLE was proton-proton collider. And they panicked and didn't build it. There's no other explanation for it.

Zierler:

Now, had ISABELLE gone on, would you have stayed at Brookhaven?

Halzen:

[Laughs] I don't know. I don't know. I like Long Island. Not anymore, but at the time, it was a wonderful place. The last time I went there to give a talk last November, it took us four and a half hours to get from LaGuardia to Brookhaven.

Zierler:

Too much.

Halzen:

Too much.

Zierler:

Who was the catalyst for getting you out to Madison?

Halzen:

Well, the person I first had contact with was Vernon Barger. David Cline, who was in Madison at the time and later went to UCLA, always claimed credit for getting me there. But I actually was working with Vernon Barger when I first went there.

Zierler:

And so this was for what, just a visiting position?

Halzen:

A six month visit.

Zierler:

And this would have been what, 1973?

Halzen:

By now, no, no. By now we are ‘72.

Zierler:

‘72.

Halzen:

I don't know the story, actually. I am not big on history. But they didn't have money for a postdoc, but they had money for a six-month visitor. So I went there and this went extremely well. We worked together. Cline told me later he actually paid me as a postdoc in the experimental group. I don't know if people realize this, but ‘71, ‘72 was one of the worst times for particle physics funding at universities. I remember that the first year I was there, we received 400 postdoc applications and had no position. And, so, what they did then, as they couldn't pay me from soft money, they made me a professor. And that's the way I managed to stay another six months.

Zierler:

Did you have any idea that you were going to make a career for yourself in the United States?

Halzen:

No. And if you had asked me, I would have told you that you were crazy.

Zierler:

Yeah. Yeah.

Halzen:

It was my dream my whole life to live in a place like New York or Tokyo. And I ended up in Madison, Wisconsin. And I must tell you, Madison, Wisconsin, at the time, was a very, very isolated place.

Zierler:

Sure, sure.

Halzen:

And I knew what an isolated place looked like. I grew up in one.

Zierler:

Right.

Halzen:

I actually was on leave from Belgium for 13 years. It tells you something about my enthusiasm about the city, but the research was great. I knew after three or four years that I was lost but taking leave of absence was an escape from reality. I had a very hard time getting used to Madison.

Zierler:

Francis, can you talk a little bit about the physics department in Madison? What were its strong points and what were its weak points when you had arrived?

Halzen:

That's the easiest question yet. Madison then had hired the largest high energy physics group in the US. They had the largest DOE budget. It was called AEC at the time instead of DOE. And they did everything. I don't know the number, but they must have had on the order of 10, 12 professors. And any interesting topic was covered, it was just a Disneyland of particle physics.

Zierler:

Did you appreciate at the time that you would have access to national laboratories using Madison as a home base?

Halzen:

Oh, yeah, as I said, I went to Brookhaven my first summer. And I remember, visiting Fermilab when they only had a theory group. David Jackson was the head of the theory group. He put together a fantastic group of postdocs. And I used to go there and give talks, and visit them. I went to SLAC. Especially, you know, because then it's not like now. The number of phenomenologists, people who could actually talk to an experimentalist, was not that large at the time. The interaction with theorists was appreciated. Now theorists join experiments. That was unimaginable at the time. As it was, we were a small number of people at the time at SLAC and Brookhaven, and they had good theory groups with people who could talk to experimentalists which, of course, was a success. It was a great time to be a phenomenologist because the results were so inspiring at the time, right?

Zierler:

What were the big questions that were being asked in high energy physics when you got to Madison? What were the most important questions that were designed to move the field forward?

Halzen:

We were in the dark. It was an interesting time because we were in a transition from S-matrix theory, unitarity, analyticity, and all that to QCD, electroweak theory, et cetera. So it were exciting times. I'll give you one example, which I'm very proud of. I wrote with David Cline and a student, the first paper interpreting high transverse momentum pions as the scattering of quarks bound inside the proton. We were using a model that was called the Berman-Bjorken-Kogut model. It so happens that to leading order in the perturbation theory it is the identical to QCD. It has quarks and gluons, it was not a gauge theory, it was phenomenological. But to analyze high transverse momentum it did the job. Early QCD was one of the main things I was doing in Madison at the time. But you see, in the beginning we were analyzing data with models that happened to have the same leading order predictions as QCD. But, you know, then shortly later, shortly after QCD was discovered. So it was this transition period.

Zierler:

Did you take on graduate students right away or did that take some time?

Halzen:

No, I because it was never clear whether I was going to be there the next year I didn't get my first graduate students for quite some time.

Zierler:

And that's because you didn't know that, Francis, that's because you didn't know if you were going to get tenure?

Halzen:

No, I got tenure immediately.

Zierler:

So where are you going? What's the uncertainty?

Halzen:

I now realize that initially graduate students were afraid of me. This has maybe changed, but at the time I was the furthest from a father figure you could imagine. That may have something to do with it, but it never really crossed my mind to look for students. I was too busy and I had so many collaborators all over the world that they didn't naturally fit in. Vernon Barger was the one who supervised the graduate students. It was a great arrangement. I worked with all of them. I wish he were around now to help me take care of my IceCube students.

Zierler:

Did you follow closely the November revolution coming out of SLAC?

Halzen:

Of course.

Zierler:

What was so exciting about that in your memory?

Halzen:

I actually wrote one of the trivial papers that many people wrote afterwards proposing that the particles were associatively produced, like strange particles. So I was early in the camp of the charm proponents as opposed to the camp of colored particle advocates.

Zierler:

What separated these two camps? What were the main differences?

Halzen:

People have forgotten now because we know that the J/Psi is made of charmed quarks and charm had been predicted by Lee, Gaillard and Rosner, which is all true. But that's not how it happened. Actually, at some point, you couldn't publish charm papers in the journal Nuclear Physics B because clearly this was not the right interpretation of the J/psi.

One of the editors made this decision, but it was an exciting time because, as we know now, everybody knew that this thing would totally revolutionize particle physics. And it did. And not because it was charm, it was clear to everyone that quarks where part of the explanation. This has kind of been forgotten. Most physicists, I think, didn't believe in quarks before the discovery of the J/Psi. I did since my undergraduate days, but the J/Psi made it official that matter is made of quarks, and gluons.

Zierler:

It's such an interesting thing to hear scientists talk about whether they believe in something or not. Why was the quark something that some people believed in and some people didn't until this revolution resolved that question?

Halzen:

Well, I can't give you the answer. I always believed in it because I spent a lot of my thesis work thinking about the problem. Quarks was the obvious explanation of the particle spectrum and, later, high transverse momentum pions. Any other explanation was unnatural, and now quarks are an experimental fact. You know, I'm a phenomenologist. I don't believe in ideas, I read data. I still do. [Laughs]

Zierler:

When did you start working or getting a formal collaboration at Fermilab?

Halzen:

Well, I never did, I actually had a formal collaboration with ANL, Argonne National Laboratory, which was an interesting arrangement because we were a few consultants and one of them was Dirac. ANL at that time had an accelerator and a theory group, and it was one of the main laboratories before it started to live in the shadow of Fermilab. I don't know if I ever had any formal relation with Fermilab. But I would go there several times a year and serve on the lab’s Science Advisory Committee at this time.

Zierler:

And what was going on at Fermi Lab at that time? What were some of the big research projects?

Halzen:

I'm trying to remember. In the very beginning, of course, the luminosity was not very high and they were doing hadronic physics. Total cross section measurements and things like that. And then [Lee G.] Pondrom and his friends build a strange particle beam which made the place more exciting. But Fermilab really came into its own doing experiments that helped launch QCD. I mentioned large transverse momentum pion production before and then, of course, came the neutral currents experiments which I lived through because the data were being analyzed in Madison.

Zierler:

So, at that point, did you want to take on graduate students? And I guess my real question is, when did you feel like you were in Madison for real?

Halzen:

Oh, no. I think after four, I would say four plus or minus one year, I was there for real.

Zierler:

Who are some of your early graduate students who may have taught you something about how to become a mentor?

Halzen:

Over the years I had some stellar students. I am proud that I graduated many women before affirmative action became a high priority. I am proud that most went into academic careers and many ended up in important university jobs around the globe. I think that is because I gave them total freedom to work on topics that they mostly picked themselves and were passionate about. Also, we always had excellent postdocs. They were mostly people who were like me who wanted to work close with experimentalists and Madison was the place to come.

Zierler:

Right. How long did Madison retain that reputation? And would you say is it still ongoing today?

Halzen:

No. It is not a criticism of the past, but Madison is a better place now because it is not as highly specialized. At the time it was totally dominated by nuclear physics and particle physics, which were kind of separate departments that totally dominated the physics department. There were some really talented individuals outside those groups, but there was no real department. That's changed. And as a result of that change, high energy physics shrunk, the nuclear group shrunk, also partly because some of its very talented members left. And so the phenomenology institute still exists but is not integrated in a normal more diverse theory group. At some point we had at the top of what was called the Pheno [menology] Institute in Madison, five professors. We had probably fifteen students and five postdocs. And, of course, that didn't last, it would have been different if the SSC had been built.

Zierler:

Were you involved in at all in the planning phases of the SSC?

Halzen:

No. By then I was not lost, but doing double duty as a neutrino physicist.

Zierler:

Right.

Halzen:

By the time I was focused on the AMANDA project.

Zierler:

When did the AMANDA project start?

Halzen:

Well, I always, I like to say in 1987 because it's a funny occasion. I had this idea and I was at a conference in Poland. And it was a major conference I was giving a talk on QCD and how QCD was relevant to cosmic ray physics. But there was a small parallel session about detector techniques, and I gave a talk on the AMANDA concept, in fact the paper is published in the proceedings. Just to see if anyone, you know, would laugh me off the stage. And then, of course, nobody laughed, so I continued to worry about it and think. And we started talking about this more and more seriously. But we really didn't form a collaboration until two or three years later.

Zierler:

What has been some of the principal objectives of the AMANDA project and how well have those objectives been achieved?

Halzen:

No, you see, I'm a theorist. So from the beginning, I decided that to have any chance of doing this and starting neutrino astronomy, you had to build a kilometer cube detector. Though we always had the hope to discover something because even in the R&D phase we were the biggest detector ever built, AMANDA was a dress rehearsal. The first AMANDA proposal, in fact, one of my German colleagues still has a copy, says that we were going to build a kilometer cube detector, which is incredible. You know, when we had not even one photomultiplier in the ice, and how much it would cost, which was about the right amount if you include inflation. But the goal was to show that this technique of building a neutrino detector out of natural ice worked by detecting atmospheric neutrinos.

Zierler:

Tell me a little bit about your work in cosmic ray physics. When did that start?

Halzen:

Ah, though a particle physicist, I was closer to cosmic ray physics long before AMANDA and neutrino astronomy. I met Tom Gaisser at Brookhaven during the ISABELLE days. Oh, yeah, I know why. What got me into cosmic ray physics is one of my interests in the dark era between S-matrix theory and QCD where we were playing actually with this Berman-Bjorken-Kogut model. We were trying to understand the proton-proton total cross section. One of the big discoveries at the time, 1972 I think, was that the total proton-proton cross-section was rising with energy, and we were trying to explain this in QCD, what we call what we later called QCD-inspired. Well, you cannot calculate the total cross-section. But the idea that we had that there are all these soft gluons that make the cross section rise is still the accepted idea. We still don't have the techniques to do it quantitatively. Through this work I met Gaurang Yodh who was then still a professor at Maryland.

And, from him, I learned that there were cosmic ray data on the total cross section. And, in fact, we eventually, years later, working through the techniques of how this data is obtained, we predicted the LHC cross-section on the basis of cosmic ray data, and got it right. This is not the most glamorous school or physics, but I consider it to be a great achievement. I always, you know, I remember, in seventy-two, people like Heisenberg who were still alive then, saying if we understand why the proton-proton cross-section grows, we have solved particle physics. I'd never forget this. In fact, he was not the only one. That was the kind of the enthusiasm at the time. And I've never forgotten that. And I really think that I was, with my collaborators, explaining why the total cross section grows way back in seventy-two, but nobody cares anymore. [Laughs]

Zierler:

What did you see as some of the most fascinating intersections between cosmic ray physics and particle physics?

Halzen:

Well, they have always complimented each other, right?

Zierler:

In what way? In what way were they complimentary?

Halzen:

I mean, in the beginning, until they built Brookhaven and CERN accelerators didn't really compete with cosmic rays. You remember, all the particles up to charm had been discovered in cosmic rays. And even charm, I think, this is controversial, but even charm you could make an argument that it was discovered in cosmic rays. This is another story. So that's actually something I should mention. After then meeting during a summer at Brookhaven, I met Tom Gaisse who had a bigger influence on me than Yodh and really dragged me into cosmic ray physics. At the time, this was not really a respectable thing to do anymore for a particle physicist, but I couldn't resist. And of course, Tom and I adventured together into neutrino physics later. He at some point was the spokesperson for IceCube.

So, you know, when I say they complement each other, for a while, it's fair to say that the accelerators took over. And, of course, it was with proton decay that people realized that there were some interesting physics that couldn't be done with accelerators. And, of course, proton decay led eventually to the Kamiokande experiment in Japan and to AMANDA and IceCube.

Accelerators were really the tool of particle physics. But then cosmic ray physics changed and was relabeled as particle astrophysics. But we could still call it cosmic ray physics, makes no difference.

Zierler:

Did you see at some point, I mean, after SSC, many people in particle physics sort of moved on to astrophysics. But it sounds like for you that transition was a little more natural and it happened earlier on.

Halzen:

Yeah. I always remind them that I was a particle astrophysicist long before they knew the word.

Zierler:

[Laughs]

Halzen:

As I said, I wrote the first paper that used cosmic ray data in 1974. Certainly the one with Yodh and Gaisser was, I think, in seventy-four. And there may have been earlier papers. I don't remember. It's a long time ago. But, yes, when particle astrophysics emerged in the mid-eighties, you would go to a cosmic ray conference and these would suddenly be all these particle physicists.

It was an interesting sociology because cosmic ray physics was an academic, respectable enterprise. And suddenly appear these people with sharp elbows from particle physics. It was a big culture clash. In most of today's collaborations, you will recognize people who came from particle physics and people came from cosmic ray physics.

And now the question you asked has an interesting side story. It was not proton decay that brought most particle physicist into cosmic ray physics. It was something called Cygnus X-3. And Cygnus X-3 is not a mystery, it's a binary system with a black hole. And binary systems, you know, they make x rays. Then some groups claimed to see Cygnus X-3 in cosmic ray muon data. Now, the muon doesn't get here from Cygnus X-3. If I remember, it is 12 kpc away, I mean the size of the galaxy.

And so, I remember writing with Tom Gaisser and his collaborators at Bartol a letter to Nature that if this were real, these results had to be new particle physics. And, so, that got the attention of many particle physics, certainly to do particle physics with cosmic accelerators, with cosmic beams. The new particle physics anticipated in this paper was never found, but by the time that the topic died, particle physicists were building detectors, right? To detect cosmic neutrinos, muons, everything. So Cygnis X-3 really launched the field along with proton decay, even though it was all fake.

Zierler:

When did you first come across the term neutrinos?

Halzen:

As is clear from this discussion, I was really never a weak interaction person. And, so, I learned neutrino physics by helping, you know, being involved in an experiment. Clearly, I never had any interest in weak interactions or neutrinos for that matter beyond textbook physics.

Zierler:

And so when did that change?

Halzen:

Well, you see, it changed. You have to understand, I really never had the intention to get involved in AMANDA and IceCube. I got dragged into it by accident. The first Berkeley people who started the project were two students of Buford Price at Berkeley, and one of my colleagues, Bob Morse and me. So we were four people. And I was the only one who had ever written a proposal even though Bob was older than me, but he had been a scientist in the Madison high energy group.

And, so, I wrote the proposal. And so I got dragged into this and tried to get out of it many times. I always thought it was a bad idea, I still think so, to have a theorist trying to lead an experiment. But actually, in practice, it worked very well. At different stages, I tried to get rid of this job and it never worked.

Zierler:

[Laughs] So they brought you on because you knew how to get money? You wrote a successful proposal?

Halzen:

In the beginning, it was shared. I mean, the NSF would talk to Bob and me and formally, actually, he was the P.I. But it was pretty much a two man show. Of course, once IceCube came along, it had to be a big managed project. I was told that the project wouldn't go if I were not a P.I at the time that they were about to release the money. And I can tell you, I would not have been the P.I. if they hadn't told me.

Zierler:

You had no choice at that point?

Halzen:

No.

Zierler:

Francis, in what ways did were the aims of IceCube designed to improve upon AMANDA? And in what ways was it its own distinct project with separate research questions?

Halzen:

AMANDA was critical. When we started AMANDA, we had no clue what we were doing. You have to realize, we were going to use deep ice as a Cherenkov detector, but we had no clue what deep ice looked like. It that had never been studied, really. Certainly, nobody knew what the absorption length was for the light that can be detected by a photomultiplier.

There will get a lot of open problems. I mean, even my close friend John Learned would say that you could not detect neutrinos at the shallow depths that we planned for. We actually would have built it at one kilometer depth if that had been possible. Learned argued that you could never reject the background at the depth of AMANDA. We actually did. And we were very lucky that everything kind of fell into place, and that we learned from AMANDA. We could never have done this otherwise, you know, IceCube was an expensive project. Without retiring the big open questions and ambiguities with a smaller experiment, we could not have built IceCube. I mean by that that I realized very well that you could not build a kilometer cube detector without detecting atmospheric neutrinos with a small detector first. And if we discover something with AMANDA, so be it. But one has to realize that AMANDA had between one hundred to one twentieth the sensitivity of IceCube.

Zierler:

Right. And what did you learn from AMANDA in terms of how to structure both the proposal and the science behind IceCube?

Halzen:

Well, that's very simple. It was all about the technology. The science, it evolved, but it was clear from the beginning that for seeing the sources of the cosmic rays you needed at least something the size of IceCube. That was kind of the headline of each proposal, right?

We had to figure it out first with AMANDA. Well, with the first strings of photomultipliers we deployed, you had to answer questions like, does a photo multiplier survive? Does it survive re-freezing of the ice? It's a violent process. Oh, does the hot water drilling technique work?

So these very elementary questions had to be answered. Most of all, there was hot water drilling that had only been done on a fairly small scale in glaciers in the mountains, for instance. But nobody knew whether you could build the machine that we eventually designed to deploy IceCube, right? These were the top two questions. What's the optics of the ice? And, can we realistically deploy a kilometer cube experiment?

And of course, we didn't know what a kilometer cube experiment looked like because the number of optical detectors you had to put in the cubic kilometer volume depended on what the optical properties of the ice were. I don't know what went through our heads because you could read papers that the optical the absorption length for blue light was eight meters. You could read this in textbooks. Of course, it was done for, you know, ice in the lab of which they had removed the bubbles. And my suspicion is still that they were measuring the absorption length of the distilled water that the ice was made of, but I don't know.

But we now know that everywhere in the detector, the absorption length is larger than a hundred meters. In some places close to three hundred. You cannot build a solid in the lab that's as transparent. That was our big break through. We had no clue, it was just pure luck.

Zierler:

And what why was that such a big, big breakthrough?

Halzen:

Oh, that means you could fill a kilometer cube of ice with five thousand modules, which we could afford. We first could afford to buy them, and then you could afford the drilling because drilling is not cheap either. In fact, in this experiment, everything is kind of expensive but nothing actually dominates the cost. That was, again, luck, right? It could have been that the cables or the drilling or the modules were prohibitively expensive. The only bad luck we had, although it was only a setback along the way, was that we tried to build this detector first at one kilometer deep. And there were still bubbles in the ice, which no geologist had predicted. I should be careful with this. [Laughs] No U.S. geologist. There was a group in Russia and a French Russian group who actually had warned us of the possibility.

U.S. glaciologists argued that these bubbles formed when you extract the cores. You can see them in the cores. There is no doubt that there are bubbles in cores extracted from a kilometer depth. The question was, are these formed after you release the pressure on the core and after you heat up the core because the core in the refrigerator in Denver is at higher temperature than in the ice. And it is not under pressure. But the French-Russian group was right. And so, with bubbles in the ice you cannot construct a Cherenkov detector. But we actually saw the bubbles disappeared along the length of the string. We then modeled how bubbles disappear, which is a long story that I'm not going to go into, and predicted that they disappear at 1400 meters. In fact, we now know precisely when they disappear: the last bubble disappears at 1350 meters. And so that's why IceCube is deployed below 1450. So it cost us a couple of years, but it was not fatal. I thought it was fatal because I was not sure that NSF was going to give us a second chance. And they did. To their credit.

Zierler:

Francis, I wonder if you could explain some basic science. Why is Antarctica the place to detect neutrinos?

Halzen:

Oh, first of all, we need a kilometer cube of ice below 1500 meter.

Zierler:

And why ice? An even more basic question. What's the connection between neutrinos and ice?

Halzen:

You want to build a kilometer cube Cherenkov detector. That's where you start. You cannot afford to buy it, like at Fermilab. It costs too much. In fact, you could build one at the surface but it would cost billions of dollars. Which, certainly at the time, was not imaginable. So there are only two possibilities. You build it in deep ocean water or you build it in ice.

And the deep ocean water idea is the more obvious one. And that was tried. They tried to do that off the coast of Hawaii and the experiment failed. Now they are building a detector like that in the Mediterranean. And they have built their version of AMANDA. It's called ANTARES and they are now in the stage of building their version of IceCube. Bigger, actually. Their eventual plans are to build like five or six times IceCube. But that's the most obvious idea.

But once it looked like that was not going to work. Well, our idea was that doing this in ice would be easier and cheaper. And I think that's still correct. I mean, that's not a debate, there's no debate anymore. But that was far from obvious before we had demonstrated the drilling technique and before we actually had determined what ice looked like as a particle detector.

You realize that for a particle detector, you have some specialized firms build some highly specialized substance with optimized properties, which you then take a block of and calibrate it in a beam at CERN and can do all kinds of calibration measurements on. We couldn't do any of this. In fact, I listed the top two challenges. I would say the third challenge was to determine what the precise optical properties were, not just the absorption length. So this detector has not just photomultipliers that detect light, we have lasers in the ice. We also have flashers, LEDs, in every module, twelve of them with different colors. The calibration of the detector is a real challenge. And, you know, as I said, we never had a block to put in an accelerator beam to test it.

Zierler:

Did you ever spend time in Antarctica yourself?

Halzen:

No, for a long time I was unique, but now, no. I have never been there. I almost went two times. I came close to going and somehow it didn't work out. I wanted to go.

Zierler:

Beyond AMANDA being a sort of necessary precursor to IceCube, what science came out of AMANDA that was valuable in and of itself?

Halzen:

I have to be careful. But the value was to detect atmospheric neutrinos, neutrinos made in the atmosphere and measure their flux. Then we could compare our measurements with those done by IMB, Kamioka and Super K, mostly Super K later on. It showed that we could detect neutrinos because the reproduce their results for atmospheric neutrinos and even extend them to much higher energies. The challenge of this experiment is that at the depths we are, there were a lot of people telling me you'll never make this word at one kilometer or even at one point five kilometers.

We detect 3000 cosmic ray muons per second. You are one and a half kilometers deep, and by then, the number of muons going through your detector, cosmic ray muons, are small, but in a big detector it's still three thousand per second. And so you have to reject those. And when you reject those, you see neutrinos. That was the big breakthrough. And in IceCube, we see three thousand muons per second. We see a neutrino every few minutes.

But this is our background. We are not interested. We want to see cosmic neutrinos, so you have to reject atmospheric neutrinos as well. And that was, of course, the ultimate challenge to separate these two backgrounds from the signal we were looking for.

Zierler:

And what is the difference between an atmospheric neutrino and a and a cosmic neutrino?

Halzen:

They just come from a different place. Atmospheric neutrinos you cannot do astronomy with. They are uniformly distributed. They are produced in the atmosphere. The atmosphere kind of forms like a totally steady uniform source of these atmospheric neutrinos. And of course, we use them for studying neutrinos. Super K used them to win a Nobel Prize by discovering that neutrino have a small mass. But you cannot do astronomy with them.

And so the idea was that to see neutrinos beyond our atmosphere, you needed these big detectors. And these big detectors like IceCube are not as good as a particle detector as Super K. IceCube cannot observe MeV neutrinos from the sun, which Super K can. Except during a supernova, but that's a different story. But, so, we have a much worse detector than Super K. But it is designed to be big, not to be good. So it's totally complementary goal. And despite the fact that these cosmic neutrino fluxes are fairly low, right, we now detect some four or five hundred cosmic neutrinos per years of all three flavors. But it's only several tens of them that we can really separate from the background. And so this is a totally different experiment.

Zierler:

In what ways has either AMANDA and/or IceCube contributed to the research that neutrinos oscillate?

Halzen:

OK. So, that's not an AMANDA question. IceCube contributes and more so in the future.

Halzen:

OK, before I answer that question, I have to tell you that after we finished AMANDA we took data for, I think, for about a decade. When we transitioned to IceCube, we found a way to build a much better detector. The modules in AMANDA basically would give you two numbers, the time when the light arrived and how many photons it detected. And not very precisely.

Although you usually don't mention the names of collaborators, now I will: a man from Berkeley named David Nygren who invented the TPC, which is the work horse for a lot of particle physics. He designed the system where we not only integrate the light from every module but record the time of every single photon. When the Cherenkov wave sweeps through the module, it will record when every single photon arrives and send the information to the surface. So you see the photons arrive basically photon by photon. And that has a lot more information in it than just a rough idea of their number and when the first one arrives. The module now records this detailed wave form or trace of the light detected by the modules. It achieved two things. We had much more information on the particles that a neutrino produced, much higher quality information.

And, because we recorded these traces digitally in the module using chips that were, actually, designed by him the whole experiment became digital. You have to think of IceCube as modules detecting light and that information being sent digitally to a computer. And so the computer collects all this information and makes neutrino events out of it. And these neutrino events are sent by satellite to the computer where I'm sitting now. So this is the simplest experiment. It's like a satellite experiment. It is just a digital flow of data except for the light sensors that are sitting in the ice and we have no access to anyway. So it's an extremely cheap experiment to run. It's very, very straightforward and simple.

Zierler:

Based on your experience with AMANDA, what did you know at the outset needed to be done differently?

Halzen:

Oh wait, I didn't answer your question whether we had contributed oscillations.

Zierler:

Oh, yes. Yes. You know, I give most of the talks on this experiment. So I would go around saying that we would never, ever measure oscillations. This was not our goal. And in fact, I think the proposal said this because we wanted to make sure that people who were reviewing the proposal and the NSF understood this was not just another oscillation experiment that would compete with Super K. And then what's happened is that, once IceCube was going and we were all busy looking for cosmic neutrinos and even finding them, suddenly this Mexican student Juan Pablo Yanez in Germany does an analysis and out of the blue shows he can measure neutrino oscillations at five sigma. We couldn't compete with Super K or we couldn't compete with the accelerator beams, but it contradicted what I always believed, that you could not measure oscillations.

And then, of course, we did. We had deployed additional sensors inside IceCube. This was a Swedish idea of Per Olof Hulth who unfortunately passed away, to look for low mass dark matter. And if you use that part of the detector, in less than one year, that group did an analysis that matched Super K, and soon we did better. We are doing better than Super K now. And our next analysis is probably going to do as well as NOVA at Fermilab. So we are suddenly in this business. [Laughs] It's not a great detector, but we have ten thousand of events where other people don't. We have events coming out of our ears because of the size of the detector. And then we can just select the very best events and do the analysis with those. And that's how we are competitive. Nobody had anticipated this. Certainly not me.

Zierler:

So the question was: Thinking about AMANDA, what did you know needed to be done differently for IceCube?

Halzen:

I am not an experimentalist, but many of us felt that the safe thing to do was to build a big AMANDA, and it would have worked. It didn't have all of this digital stuff which we put in the ice and don't get your hands on anymore. That was a big risk. And so, the solution is something I'm proud of. There were different ideas. One idea was to continue to build an improved AMANDA and the other was to adapt David Nygren's system. And, so what I did is I had each of them write a report on the conceptual design, in fact, it was more than conceptual. It was a very clear design for the two possibilities.

I put the two proposals together and called a committee, which was chaired by Barry Barish. And we met at O'Hare, the Chicago airport, on Thursday. I said that when we leave here by Saturday afternoon the decision will be taken. But the committee actually came up with reasons to go the digital way that we ourselves had actually not thought of. You know, it's not easy to calibrate five thousand, six hundred sensors, unless you can do it digitally. You don't want to have to measure time offsets on cables to the surface like we did with AMANDA. And so everybody was happy when we left on Saturday at noon. I just want to emphasize that IceCube is a much better experiment too, not just bigger. To do astronomy actually, something I haven't mentioned, it is important how well can you reconstruct a muon track? Although we have for other ways to do astronomy, the basic way is that you have some cosmic accelerator in the sky. It sends you a muon or neutrino. The muon interacts in the ice and makes a muon. The muon goes through your detector and, to point it back to the source, you measure the direction of the muon. In AMANDA we could do this to one and a half, maybe two degrees. Now we are doing this to zero point two degrees for some events, and better than zero point four for all high energy events. Only the high energy events point back to the accelerator, so they are the ones that matter. And that's also where this Nygren system paid off in a big way.

Zierler:

Francis, you talked before, almost comically, of getting pulled into this against your wishes, and yet at some point this became something that you became obviously quite devoted to. How did that happen? How did that transition work?

Halzen:

I told you in the beginning of this interview that by the time I understood something, I was not interested anymore. But you said something like “maybe you haven't understood IceCube yet”. And you're right. We are still refining the modeling of how photons propagate through this ice. We are now at a point where the birefringence of the ice crystals begins to affect the photon propagation. And we actually managed to model this. So maybe that's the answer. Especially, I didn't get into this saying, ahh, I'm going to build a kilometer cube detector. I actually never stopped writing theory papers. I still write theory papers on whatever catches my attention.

But you also get dragged into IceCube because you spend money. By the time that we were struggling to detect atmospheric neutrinos with AMANDA, we had spent some millions of dollars. And you cannot walk away from something after you've spent that much money. Now, from the prospective today, three or four million, that's a small amount, right? But the university, itself, at several stages when NSF wasn't generous enough, gave or lend us money. At one point, UW put one million in AMANDA, and, at some point, when we were preparing IceCube, it put into the preparations for construction and the design, close to five million dollars. They were paid back by NSF, but I didn't know that then. And you had to succeed first. So, I think you're in a situation where you cannot say I'm going back to Belgium. [Laughs]

Zierler:

You're in too deep.

Halzen:

You're in too deep. That's the way to summarize it. Yeah. I couldn't possibly be so irresponsible to walk away from it. Now I could.

Zierler:

And can you give a greater understanding of, what does it mean that IceCube is still teaching us new physics? What does that mean on a daily basis?

Halzen:

Well, it is and it isn't. OK. I am not going review all the science IceCube has already delivered. We discovered cosmic neutrinos and that came already with a big surprise. You're supposed to discover the sources in your own galaxy first, then those beyond our galaxy. That didn't happen. We discovered cosmic neutrinos and we saw them from way outside our galaxy. The first source that we have detected is actually at four billion light years. There are identical sources, speaking as an astronomer, that are 10 times closer, and should have a hundred times larger flux. Why do we see a source that source 10 times farther away?

And so now we are studying that source. We meaning now not just IceCube, but every kind of telescope in the world using every possible wavelength of light. And so we are learning that there are some things special about that source. The radio telescopes are actually imaging that source by interferometry. And so it's an exciting subject. And that is all ongoing, exciting and hopefully coming to a conclusion. By the way, we think we understand now why we don't see galactic sources. They don't include these huge black holes that are the cosmic accelerators that produce the neutrinos that we actually do see. And the only black hole we have in our galaxy is dormant basically, practically. So I think we begin to get a feeling for that. We see hints of sources in our galaxy, but we don't have the statistics yet. And we need a bigger detector for that.

The astrophysics and the astronomy we're doing is incredibly exciting. Also, to our surprise, you know, we discovered neutrinos with only two years of data, cosmic neutrinos. That was not supposed to happen. In fact, we found that the neutrino flux in the universe is as large as the flux in photons. So that was not anticipated either. Now coming back to your question. I answered your question for the astrophysics, but for the neutrino physics we came up totally empty. Everything we measure, even though the energies of our neutrinos, instead of GeV energy at Fermilab or CERN, have energies of a thousand TeV or more, all the neutrino physics is the same.

The Standard Model just holds. So it's a bit like the LHC, everything we look at is consistent with the Standard Model. And I come back to neutrino oscillations. The point of neutrino oscillations with IceCube is not to compete with NOVA or Super K or T2K. We are measuring the same oscillations at ten times higher neutrino energy. And so this is like doing experiments at a ten times higher energy accelerator. But we see the same oscillation parameters with pretty good precision by now, which is again, disappointing. We were hoping that we'd find slightly different oscillation parameters, which means that there's new neutrino physics. But even at a thousand TeV we don't see any new neutrino physics.

It is a high priority for us to do these experiments better and better, with more and more precision. So when we are talking about the next generation detector, we not only worry about astronomy, we also worry about doing high precision measurements of what we refer to as beyond the Standard Model neutrino physics. You know, there is no standard neutrino model. In the Standard Model neutrinos have no mass and they do. So we know that we are working with particles that carry new physics that is not in the Standard Model. But we don't manage to see it, neither at accelerators or not by going to these ridiculous energies that we go to in IceCube. But you have to keep trying, right?

Zierler:

Francis, you mentioned that you still are active in theory, and I'm curious specifically if your work on these massive projects has influenced the kind of theoretical work that you do.

Halzen:

Yes, of course. I mean, most of the theory papers are somehow related to science IceCube does.

Zierler:

So, in that vein, what are some of the most important theoretical issues that result from this work?

Halzen:

Well, I think the top question is what accelerates the cosmic rays? Where do they come from, and how they are accelerated has not been answered. We have one cosmic accelerator which we are studying to date, but we don't know what the other sources are. But we are learning a lot from this one case. And on that I could spend hours, but I won't. [Laughs] Don't worry. So I think that's still the leading question.

But then the other leading question is, how does a rotating black hole accelerate particles? We don't know the answer to that either. It's one of the most common phenomena in the universe. I think IceCube brought us somewhere. I think most people believe now that supermassive black holes are the key to solving the cosmic ray problem. And, active galaxies, they make these particle beams and I can describe to you in detail how a rotating black hole makes a beam. But this is all Disneyland. There is no grand mechanism that's totally understood and that can be worked out. So there is the cosmic ray problem. Black holes dominate the high energy universe, and the astrophysics of black holes is not understood. There are not even good models, they're a collection of ideas, and we don't know which one are right.

Zierler:

So this is a mystery within a mystery it sounds like.

Halzen:

Yeah. It's the total opposite of neutrino physics of IceCube where we understand everything.

Zierler:

In what ways does the theory, I mean, there's a dialectic that's happening here, right? In what ways does. You're inspired to do the theory based on the experimentation. In what ways does the theory in turn move the experimentation forward?

Halzen:

Very little actually. Because as I say most of the results come as a surprise. For instance, the whole concept of building a kilometer cube detector, where did it come from? And I only can answer the question in hindsight. And as I just told you, there's no doubt that, in my opinion, the most important result of IceCube is the following: astronomers and astrophysicists painted themselves in a corner believing that cosmic rays were some boutique science that was irrelevant to the high energy universe. There are some exotic protons. Well, now we know, by measuring the neutrino flux, that the energy content in the high energy universe in neutrinos is actually the same as in gamma rays. And it's the same as in protons. So protons, neutrinos and light play a similar role in the high energy universe. And that, of course, totally resets the picture.

And now, the reason why we anticipated that a cubic kilometer detector would detect cosmic neutrinos. A kilometer cube detector, of course, detects these neutrinos at this level. And the reason we actually discovered this kilometer cube goal was that all the models we used, whether they were right or wrong, they somehow had the assumption built in that for every neutrino you get a gamma ray. And so it didn't matter. Every model that has this assumption built in predicted a kilometer cube detector. Tom Gaisser and Todor Stanev and I wrote a Physics Report in ninety-five that makes this point. Look, whatever science you look at, you always get the same answer. But now, we know, we had to build IceCube to figure out why they all give the same answer. And of course, it has to do with simple particle physics, right. Neutrinos come from pions and for two charged pions you have a pi zero that gives you two gamma rays. It's called isospin.

Zierler:

You mentioned that there's a lot of exciting work done in telescopes now with neutrino physics. How does your work with IceCube relate to those other projects and telescopes?

Halzen:

Well, the most direct relation is that superb digital system we have. We can send to another telescope the direction, the arrival direction of a neutrino within less than a minute.

For the now famous September 22, 2017 event, we sent the coordinates and the energy of the neutrino out after forty-three seconds. And, so, that's our main activity. We send these alerts. And then the telescopes follow up. This is not organized. We had no idea. We started this program at our own initiative because, you know, we did discover cosmic neutrinos in 2013 and we keep accumulating them. And here we are, 2016, two years later, we have no clue where they come from. So we thought, enough of that. So let's try to send coordinates to other telescopes. We had no idea whether anyone followed this up. But for the 17 September 2017 event, at some point, more than 20 telescopes were looking at this source.

So this has become now a little industry. But we also have MOUs with many telescopes where we match all our data, not just alerts. We have been matching our data with LIGO, for instance, even before the time they saw actual mergers of black holes. And certain telescopes actually, systematically, follow up everything we send, and you can also do this after the fact in many cases, you don't have to do it in real time.

Zierler:

And in what ways have you moved, have you contributed to moving LIGO forward? And in what ways has LIGO moved IceCube forward?

Halzen:

I think we pretty much can survive on our own feet, which is amazing. LIGO didn't need us to become famous and the other way around.

Zierler:

[Laughs]

Halzen:

But certainly it would be a big event if you could study, never mind moving forward but just for science, a neutron star merger in neutrinos, gamma rays and gravitational waves. That would be fantastic.

Zierler:

Francis, I want to ask because IceCube is able to observe, you know, particles, neutrinos that have far more energy than what's even available at the LHC, right? I wonder if, long term, this research will be useful if the world ever gets together and reinvigorates high energy particle physics. In other words, if there's ever going to be a successor to SSC or if ILC ever gets off the ground. When we talk about real next generation projects, how might IceCube be relevant to those endeavors?

Halzen:

Well, that these efforts are complementary is the simplest way of answering your question. To do particle physics, it's important to have the highest energies. Those we will always have in cosmic ray physics and in particle astrophysics. But sometimes to answer questions in particle physics, it's important to do very high statistics precision measurements. Like, we will never see a Higgs. I was wrong in my statement that we will never see oscillations. But this one I will not be wrong on. [Laughs] So we will never see a Higgs.

Zierler:

Why not, Francis, let me just ask, why are you so certain?

Halzen:

No, no. You need a certain number of events. You will never gather the statistics with a particle astrophysics experiment to reach the type of precision to see the decays of a Higgs particle. Yeah, I mean, you cannot do the physics you do with the high luminosity LHC. You may have higher energy, but you don't have the event rates that are required.

So I was going to use as my second example that we'll never see supersymmetry. But actually, the first funding I got for AMANDA was from the Dark Matter Center, which doesn't exist anymore, but was at Berkeley. And it was to look for supersymmetric dark matter WIMPs. If supersymmetric WIMPs had been the dark matter in the universe, we would have discovered them with AMANDA. No doubt about it.

It has to do with WIMPs being trapped in the sun and annihilating into neutrinos. And, so, we are still doing this. We have world best limits for WIMPs that interact with protons through their spin. But, yeah, we've never seen anything.

So there are specific examples where we compete with LHC, this is a second way of answering the question. Without having the statistics and the refined detectors that accelerators have, you have windows of opportunity to do particles physics. But you won't do high precision tests on the Standard Model or see tiny deviations from it. But also interesting in this context, we have seen one W. Now, the W was discovered thirty-six or thirty-seven years ago. The date doesn't matter. And in fact, in 1959, when Sheldon Glashow was a postdoc of Niels Bohr in Copenhagen, he wrote a paper saying that if an anti-electron neutrino, an atmospheric or a cosmic one, interacts with an electron in an atom, it can make a real W. And we have actually seen such an event creating a real W, which we now know was created by a cosmic neutrino. You need a neutrino of 6300 TeV energy. And that's exactly what we measured.

Yeah, we would have discovered the W but are almost 40 years too late. But you cannot do W physics with one event, right? This is one event in 10 years of IceCube data.

Zierler:

Right. Right.

Halzen:

I think this example, this is a nice example. It answers your question.

Zierler:

Yes.

Halzen:

Yes, eventually IceCube would have discovered the W instead of UA1 and UA2. But yeah, what now, what's next? Wait for of another event ten years from now? [Laughs]

Zierler:

So, Francis, I want to ask you, you were originally dragged in because you knew how to write a proposal, right? So, look into the future, right. In order to secure ongoing international support for this effort, what are the cases that you have to make? What are the arguments that you have to make that ongoing support of this is still important for physics?

Halzen:

Well, I think that the case is now much easier to make. Everybody knows about IceCube because of something. We are a main topic in dark matter searches for the reason I just explained. We are a major topic in neutrino physics because we do oscillations. We are a major topic in high energy astrophysics because, as you know, the universe has decided it needs multi-messenger astronomy and we are still the only game in town on the neutrino side. And, so, I think that the case really has been made and that's not a problem.

And we have a method to build a 10 times bigger detector for the same price, expanding the original one. And then you may there's no free lunch, what do you give up? What we give up is the neutrino energy threshold. We won't be able to do neutrino oscillations anymore. I may be wrong on that again. I don't know. We will do neutrino oscillations with one thousand TeV cosmic neutrinos. That's possible, and we are doing that already, but not the conventional oscillation measurements that they are doing at Fermilab.

So we are making the case for the next detector and I would say it has been received very well. We are a priority project in Germany and hopefully we will become a priority project here. And we are doing our homework now to get there. By the end of next year, we are going to have a preliminary design. And we are going to deploy strings in 2023 with new technology on the strings that we can try out. And so we hope to start building the next generation experiment between twenty-five and thirty. And I will not be P.I.

Zierler:

You say that now.

Halzen:

Will I be alive? [Laughs]

Zierler:

Oh, I hope so. I hope so.

Halzen:

Look, I never complained, but as a physicist, having to have the discipline to make your way through construction of a large project, it's something you cannot do twice. Unless you are a person like Barish. I am not Barish. I do not have his skills and his discipline. And I know that. And I think the biggest contribution I made to IceCube was to realize that I didn't have the skills needed to run the project. So I hired a very talented project director who built the thing. His name is Jim [James] Yeck. And I didn't make the mistake that most physicists would make in thinking they are smarter than everyone else in the room.

Zierler:

What do we know now about how the universe works that we didn't know prior to AMANDA and IceCube?

Halzen:

OK, first of all, in astronomy, everyone has his own part of the universe, right. Our part of the universe is 150 TeV to infinity. And that part of the universe, you cannot study with gamma rays only. The universe is not transparent to gamma rays. You cannot study it with protons. So this is the neutrino domain. And I think the short answer is that almost all questions are open to debate in the highest energy universe. However, what IceCube contributed are the tools to do astronomy at the highest energies. And, you know, we made the discoveries, we proved you could do neutrino astronomy.

It will take the next generation of detectors to really do astronomy and answer the question that I told you about. We don't even understand the elementary construction of the accelerator powered by a black hole. So actually, when I give a talk, and you can check it, I gave the introductory talk at the Neutrino 2020 meeting recently, my concluding slide is labeled neutrino astronomy 2020. Let me see if I remember what it says about cosmic neutrinos: they exist, we need more and better neutrinos, and we are on our way to solve the cosmic ray problem. I mean, the cosmic ray problem, that's the outstanding question in this universe above a hundred TeV. But we may discover things that are more interesting than answering the question where cosmic rays come from. That would be exciting.

Zierler:

Francis, you said that you're quite certain that you're not going to be involved in the next phase of this project.

Halzen:

I don't know who the P.I. will be, but I am actually paying attention. We have again several approaches to build the next-generation experiment. It is now up to the people I have attracted to IceCube. I am not involved, or very little. If they ask my opinion, I tell them, but they may ignore it if they want. I'm not involved in the design of Gen2. You know, I may read the papers and complain, but there is a new generation which has experience from working on IceCube. It is very different from launching AMANDA. As I told you, there were just a colleague of mine and two students from Berkeley. Their professor didn't know we were starting that project originally.

Zierler:

I hope you do have the opportunity to watch at least from the sidelines.

Halzen:

Oh, I will.

Zierler:

And so my last question for you is one looking forward and it's going to ask you to use your powers of extrapolation to predict. What are you most excited about in terms of where this project goes next? And what will it do to move the field forward?

Halzen:

Well, that's obvious to answer. We want to find new neutrino physics based on the fact that we have higher energies than anyone else. That may not be the key. The key may be to do precisions experiments like Fermilab. And I wish them well. They're my friends. But that's only one thing. The second thing is to do real neutrino astronomy, where, you know, we can show you a picture of the sky of all the sources of neutrinos. But the biggest thing would be, and we have been consistently saying this, to find something really unexpected. My argument for IceCube never really was solving the cosmic ray problem or finding new neutrino physics. I always said, if you build something that unusual, can you imagine that you will not find something unusual. And that hasn't happened yet. So maybe it'll come to this time, maybe to the particle physics. But you know the equivalent of the J/Psi.

Zierler:

Who do you see in terms of the next generation of colleagues? Who's doing some of the most important work in neutrino physics? And are these people that you think are going to be part of this this next generation phase of the experiment?

Halzen:

That's a question that I cannot answer. It’s not that I don't want to name names. It's easy and you go and look at the next generation is and who is rising to the top of the next generation. In AMANDA, everybody inside AMANDA and outside AMANDA knew who the characters were, who did what, and who contributed what. And that was the way particle physics was like in the ‘60s. This was an experiment of 20 people. And unfortunately, we have grown to well above three hundred. And so we begin to look slowly like an LHC experiment. So who are the main characters for the LHC experiments?

Zierler:

Right. Right.

Halzen:

You don't want to answer that question, you cannot probably.

Zierler:

Yeah. So, there isn't somebody who's naturally rising to the top in terms of leadership?

Halzen:

No. Well, you see, at the LHC, the people were recognized who were the P.I.s at the time, right?

Zierler:

Right. Right.

Halzen:

And there were some older characters that people knew, but very few like my colleague Sau Lan Wu who was associated with the Higgs discovery because she spent her life looking for the Higgs. But that type is unusual, it is a business now. If I did this again, I may as well have stayed in my father's business and constructed bridges and I would be making money.

Zierler:

[Laughs] Well, Francis, the on that note, it's been a pleasure speaking with you today. I'm so happy that we were able to connect. And I'm so glad that we have your perspective at this very exciting time in neutrino physics. Thank you very much.

Halzen:

Thank you. It was a great opportunity. I enjoyed it.

Zierler:

Wonderful.