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Interview of Kevin Lesko by David Zierler on May 5, 2021,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
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Interview with Kevin Lesko, Senior Physicist at Lawrence Berkeley National Lab and former Spokesperson for LUX-ZEPLIN (LZ), an international collaboration searching for dark matter. Lesko explains why so many different kinds of physicists are involved in dark matter searches and how theorists have provided guidance for experimental and observational work to understand dark matter. He recounts his upbringing in northern California, the scientific influence of his parents and older siblings, and his decision to attend Stanford, where he worked on a tandem Van De Graaff in the nuclear physics lab. Lesko discusses his graduate work at the University of Washington, where he worked under the direction of Bob Vandenbosch on nuclear fission research, and he describes his postdoctoral appointment at Argonne, where he pursued experiments in nuclear fusion and neutrino physics. He explains his decision to join the staff at Berkeley Lab and how his interests centered increasingly on astrophysics with the Sudbury Neutrino Observatory. Lesko discusses his collaborations in Japan and KamLAND’s discovery of the absolute measurement of neutrino oscillations and the origins of the Homestake collaboration. He describes the transition of support for Homestake from the NSF to the DOE and he explains his entrée to the LUX collaboration and the reasons for the merger with ZEPLIN. Lesko explains how LZ needs to be ready to detect dark matter either as a singularity or is comprised of multiple components, and he considers what it might look like for dark matter to be detected. He recounts LZ’s success in ruling out dark matter candidates and he reflects on LBNL serving as a home base while his collaborative research has always been far-flung. At the end of the interview, Lesko considers what we have learned about the universe as a result of LZ, and why mystery and curiosity will continue to drive the field forward.
Okay. This is David Zierler, oral historian for the American Institute of Physics. It is May 5, 2021. I’m so happy to be here with Dr. Kevin Thomas Lesko. Kevin, it’s great to see you. Thank you so much for joining me.
Well, thank you for including me in this. I’m looking forward to it.
Kevin, to start, would you please tell me your title and institutional affiliation?
I am a Senior Physicist at Lawrence Berkeley National Lab in the Physics Division.
Now in your capacity as spokesperson for LUX-ZEPLIN, is that as part of your official duties at the lab, or is that considered something separate, a separate appointment?
LUX-ZEPLIN (or LZ) is an international collaboration searching for signals of dark matter in a deep underground laboratory. There are 34 different institutions and over 250 scientists and engineers in the collaboration. I was elected as spokesperson of the LZ collaboration by a general vote of the membership. Obviously, my home institution (LBL) must approve that, but this sort of leadership position is right in line with our role as the lead laboratory for the experiment. To be clear, my spokespersonship ended in February, so my two years are up at this point.
Oh, okay. So you can focus a little less on the administrative side and a little more on the science side.
I can sit on the outside and offer advice freely to my replacement. [Laughs]
Kevin, as a window more broadly into how LBL works, where are you situated both hierarchically and organizationally at the lab?
LBL is a multidisciplinary research laboratory run for the Department of Energy by the University of California. There are efforts in physics, in chemistry, biology, computer science, energy efficiency, etc. I’m in the Physics Division, and that is one division of the four in the General Sciences Area. General Sciences includes physics, nuclear science, engineering, and accelerators. Within this area, I’m in the Physics Division, leading LBL’s efforts in the Direct Detection of Dark Matter. Previously, my group worked on LUX and is now leading the efforts on LZ, as well as R&D efforts for future experiments.
A question we’re all dealing with right now: How has your science been affected one way or another over this past year-plus in the pandemic?
It has definitely had an impact. We were in the middle of constructing the LZ detector when the pandemic restrictions were put in place. These had a substantial slowdown on what we could accomplish. The inability to get to South Dakota for our collaboration and activities within the different institutions were all slowed down. The pandemic has certainly impacted our ability to finish that construction.
What about just in terms of not seeing your collaborators in person? In other words, what can be accomplished over Zoom, and in what ways is video conferencing no substitute for the real deal?
I certainly agree it’s not the real deal. One of the things I miss the most being there is that I can’t simply walk down the corridor to check in and talk to the group members. In the past, I used to be able to walk down to their offices and say, “Hey, what’s going on? Show me what you are working on.” I can’t do that now. Zoom appointments are so much more formal, more structured. It isn’t the same.
And as you say, spontaneity is important for science.
It is, and if somebody’s got a bright idea or they just did something in the lab that they’re proud of, they can come find you and show you, and that doesn’t happen as much with Zoom.
Kevin, a few broad questions at the outset before we go back and develop your personal narrative. I’d like to ask generally. The search for dark matter is so fascinating because there’s hardly a subfield in physics that’s not represented in this collaboration in one form or another. Almost every kind of physicist from experimentalist to theorist are focused on this all-important question. Where do you see your work specifically, both in terms of what it offers in a singular way that’s not happening anywhere else, and in what ways do you see it both helping other projects that are searching for dark matter and those projects helping the way you’re doing this?
You are right. A lot of the most exciting physics research is focused on some very big questions: dark matter, dark energy, neutrino oscillations. We live in a fortunate time where we have the chance to answer some of these fundamental questions about dark matter with direct observation. We can perform experiments where we can control the detector and the experiments, and not just passively observe the universe. We are able to explore and see how much we can find out about the universe.
Direct detection of dark matter is a good approach to help us begin to understand some of the fundamental aspects of the universe we live in. These are experiments looking for broad answers to the universe we live in. Can we directly detect the particles that make up dark matter? How massive are they? How do they interact with the rest of the particles we currently know about? Is there one or many dark matter particles? Or is dark matter something entirely different?
What I like to do is build the instruments, build the detectors, and figure out how to answer a question by making the apparatus that allows us to look for those answers. That was true even as an undergraduate and as a graduate student. I like developing design, performing the technical construction, and then commissioning the devices – using my hands and understanding the hardware -- these are the things I enjoy the most and where I think my strengths lie.
Given your role as an experimenter, I wonder if you can reflect on where in your career in the search for dark matter theorists have provided some guidance to you and the work that you do, and where the work that you do has provided some guidance to theorists.
I think we were fortunate with the WIMP miracle. The WIMP model of dark matter gives us some ideas as to what to hunt for, what sort of singles we should focus on. The field was fortunate to have such a good consensus from many of the theoreticians as to this starting point.
As the hunt has continued and some of the simple discoveries were not realized, we were able to work with theoreticians to provide additional input and have recently focusing on lighter mass participles and to help tune our models. Kathryn Zurek, when she was at Berkeley, was somebody we dealt with then quite a bit (and still do). More recently, we’ve talked to Wick Haxton regarding effective field theories. Because the universe may not be simply spin-dependent, spin-independent. Things may be more complicated. We need to ensure that we are not focusing on experiments that introduce biases into our searches: we must keep our eyes open.
Recently, the LZ collaboration has also met with Dan Hooper and talked about WIMPS. This sort of general discussion is very helpful and having workshops where people can have open discussion in smaller settings, hear different opinions, is enjoyable and productive for the experiments.
Kevin, to the extent that the search for dark matter is a rollercoaster—it comes with emotions. There are moments of real optimism. There are moments of possibly pessimism: are we ever going to understand it? Just as a snapshot in time circa May 2021, where are you on the spectrum between pessimism and optimism?
I’m really excited to get LZ up and operating. LZ and XENONnT are both an order of magnitude more sensitive than the earlier experiments. Both collaborations have worked very hard at reducing backgrounds, which are now at a very impressive level. There were some hints earlier this past year out of XENONnT about some signals right at the experiment’s energy threshold. I think I’ve seen enough of these sorts of discoveries right at the threshold at the end of an experiment to be cautious, to be pessimistic about the claims. We would like to approach those claims early in the operations of LZ and resolve the anomaly reported by XENONnT. But with their claim, you can see the excitement in the community: there were something like 150 papers that appeared within the next month. So clearly there’s a lot of interest in what we’re going to see, and turning on this next generation is going to be very exciting for us.
Given the fact that we don’t understand what dark matter is, I wonder if you can explain scientifically, or even from the perspective of deductive logic, how to go about building a detector or an experiment without necessarily knowing what the feedback mechanism to understand what it is that you’re looking for.
The WIMP model of dark matter gives us some ideas as to what to hunt for, what sort of signals we should see. The beauty of the xenon detectors is that it gives you multiple handles on the interactions and the ability to discriminate against the dominate backgrounds. Many members of our collaboration have decades of experience with the two-phase liquid xenon detector design. As the xenon detectors have progressed, their sensitivity has greatly increased. We have better understanding of the micro-physics of xenon and much better calibration of the experiment. Perhaps our control of backgrounds, which is critical in rare-search experiments, is the most significant advance. Within LZ, that’s where I focused most of my effort: measuring and reducing the intrinsic backgrounds that are introduced by radioactivity with all the different components of the detector.
Given the technical similarities and the reliance on xenon, I wonder if you can reflect on the ways that you see what Elena Aprile’s work is with regard to collaboration and competition.
Anytime you have a field where there are opportunities for great discovery, there’s competition. In dark matter, that’s especially true. There are multiple collaborations using different detector media, different approaches. I think the choice of xenon is a natural one because of its particularly effective detector characteristics. There are three strong collaborations working in the xenon area, and there have been some different choices scaling up in size. There has been some leapfrogging between the different collaborations as things have progressed, and I think there is a spirit of competition: “Oh, we can do a little better on this aspect of it,” and people think about how they might do a little better on something else.
That competition is healthy, and it hasn’t turned out to be the “mortal combat” between groups. Many postdocs are talking and aware of what the other postdocs are doing in the groups across the world. There is some exchange of people between the different collaborations as students go on and become postdocs and postdocs look for faculty jobs. I find the competition healthy, and it also is good for the collaboration itself. Sometimes we realize that a choice must be made when a particular approach is not working well or when a decision must be made to move ahead without taking more time for further optimization or perfecting of a component area. You can spend a lot of time making an experiment which is already good, just a little bit better. But having somebody breathing down your back can make you think hard about, “No, this really is good enough and we should move on to the next problem.” I think having multiple collaborations is helpful for the field as a whole.
I wonder if you might see ATLAS and CMS in the discovery of the Higgs in similar terms.
Yes, there is similar competition at the LHC. There’s also synergism between the two groups. They have worked closely with each other. They’re sharing some tools and some analyses, and I think that’s another good example. Maybe it’s a little more difficult or different in that they share some infrastructure: they’re using the same accelerator. But I think, that’s a good way to look at it. Different approaches to the detector and any discovery that two groups find certainly has a lot more acceptance in the community.
Well, let’s take it all the way back to the beginning. Let’s start first with your parents. Tell me a little bit about them and where they’re from.
My father was a physician, a surgeon. He grew up in Ohio and came to California after World War II. His medical school at Stanford was paid for with the GI Bill. My mother grew up in Iowa – she was a chemist. Her father was a professor in chemical engineering, and she studied chemistry in college. My mom and dad met in college at Ohio Wesleyan, so I think they were pinned before he went off to his military service in the South Pacific. When he came back from military service, they were both in California. They married here before my father started his medical studies. They raised our family in the Bay Area.
What town did you grow up in?
I grew up near San Rafael in northern California in Marin County.
And you grew up in science, essentially, from your parents.
Certainly, education was very highly stressed in the family. I don’t think I was ever given an opportunity to think that I wasn’t going to college. [Laughs] It was just assumed. I’m the youngest of four. My sister is ten years older, and she became a research chemist, and the two brothers between Pat and me became engineers. So yes, there was a big focus on education, higher education, and an expectation that we would follow that route.
Did you have a strong curriculum in math and science in high school?
Yes. The math department in my high school was phenomenal and the science was very good, more so in the biosciences than in physics, which is curious in that I ended up in physics. But the math department was great, and I think a lot of my success in the career was due to the faculty there, especially Mr. Medigovich. That was Redwood High School in Marin.
Did you ever think about going to school farther from northern California, or Stanford was right there; it was an easy choice for you?
Well, back in those days you applied to just a few schools. You didn’t apply to ten; you applied to three or four. I had a choice between Stanford and UCLA, and Stanford was closer and a school with an excellent reputation. In fact, they were the first school to accept me, and I had a great education there. So, I stayed a little closer to home, but 45 miles from San Rafael to Stanford back then was a long way. That’s a lot of time on the Greyhound, so I didn’t go home on the weekends. [Laughs]
And was it physics from the beginning for you, or that came later after a more general approach to your undergraduate education?
In junior high school, I had an astronomy course and I really enjoyed it. Many of us built reflector telescopes. That was going to be my focus. Even through high school I was quite interested in astronomy. My sister was married to an astronomer, and I got to know him well. His advice was, “Well, if you want to go into astrophysics or astronomy, get your bachelor’s in physics first, and then you can specialize in grad school.” So that was the route I was following. Then when I was at Stanford, and I worked in the nuclear physics group, I got started with that group when I found a summer job on campus running at one of the accelerators. I just never left experimental physics. I learned how much I enjoyed building and conducting experiments as an undergraduate.
You mean at SLAC. You were working on the accelerators at SLAC.
No, on campus. There was a tandem Van de Graaff in the nuclear physics laboratory, in the basement of the Varian building. I found an undergraduate advisor who was willing to take me on, and I spent sophomore, junior, and senior years working on different experiments there, as well as earning a little money running the accelerator at night.
Who was that advisor?
His name was Dave Clark, and he was at Stanford for a couple years, then went to the University of Rochester. A lot of his work involved lasers. He then moved on to Kodak. At this point I think he’s retired; he was an excellent advisor. That was sort of my first real hands-on approach to building instruments, building things that you could measure nuclear properties with at the Van de Graaff facility.
Did you ever toy with the idea of theory, or it was always going to be experimentation for you?
No, I liked using my hands too much [chuckles], so it was always going to be experimental. I realized I was better at that than I would be at theory, so no, I was always an experimentalist.
To the extent that you thought beyond the physics program at Stanford, what was your sense in the mid-late 1970s of some of the most interesting things going on in experimental physics at that point?
Clearly, there was a lot of advancement on understanding quarks at that time. I spent most of my time in the basement of Varian working on my own small experiments and focusing on coursework, but there was a lot of excitement going on about quarks that had matured in the ’70s. All that was going on up at SLAC was exciting and eventually rewarded with multiple Nobel Prizes. Personally, I liked working in smaller groups. I like the approach of a smaller experiment, one that wasn’t tens or hundreds of people, but just maybe two or three or four people. It fit me better. Maybe a little less competitive that way, but that evolved with time. [Chuckles]
What kind of advice did you get when you were starting to think about graduate school—people to work with, programs to apply to, the kinds of things that were most useful for what you were interested in?
At that point, I had done experiments on the Van de Graaff, and I think I used that as guidance: where are the strong nuclear physics laboratories, and what does the faculty look like? What sort of work are they doing? With that, I was focused on Seattle, Princeton, Stony Brook, and Rochester (New York). I looked at those because of their strong nuclear physics programs they had sort of a similar approach to what I had witnessed at Stanford. I ended up wanting to stay on the West Coast, so I went up to Seattle. I benefitted by some great advisors and physics professors at Stanford and Seattle.
Who ended up being your advisor there?
It was Bob Vandenbosch. We worked on experiments which were looking at the fusion and fission properties of large nuclei, something very different than I had done before, and used the accelerators at Berkeley. The SuperHILAC is where we got the different ion beams, and we did these studies. That was my first exposure to not an in-house facility but a user facility, just traveling down from Seattle to use the accelerators at LBL was new to me. My thesis was on the fission and fusion properties of rapidly rotating nuclei. It looked at the balance of the forces, the Coulomb repulsion, angular momentum aspects, and then the nuclear binding forces. With very large nuclei, there was thought to be a critical point where the Coulomb repulsion was enough to make things unstable at a particular point, and we were looking into the liquid drop properties of nuclei, and in particular the impact of angular momentum on fission.
What was Bob’s style like as a mentor? Did you work closely with him?
Let’s see. He was always present for advice, and certainly when we did experiments, he was always there. I think a lot of my actual day-to-day mentoring and development was done by the postdocs in the group, and that’s a model that I’ve also followed, that you find somebody a little closer in age. But you have the experience and wisdom of a professor to help or go to when you run into particular problems. He was very personable – a gentleman. He was a very open individual. He wasn’t strict or formal, but it was just easier to ask a postdoc about some detail of analysis or computer code, and sharing an office with a postdoc, you tended to ask questions that way as well.
What were some of the central conclusions of your thesis?
That more work was needed to really separate the aspects of fission. [Laughing] We were able to reduce the interference from competing mechanisms to cleanly identify true fusion events, but more work was needed to detail the influences of angular momentum on the subsequent fission.
Besides Bob, who else was on your thesis committee?
Let’s see now, Ying Halpern, and Eric Norman.
What post-docs were available and compelling to you at that point?
When I finished Seattle, I interviewed and had my choice among Argonne National Lab, Rochester, Stony Brook, and Princeton. The program at Argonne National Lab was very close to what I had just done, something I was comfortable with, and I was happy to go see what Chicago had to offer after being on the West Coast for my entire life.
And at this point in terms of your professional identity, are you thinking at all about astrophysics or cosmology?
No. I was focused on nuclear physics. We did several experiments which were relevant for astrophysics, but at this point I was focused on heavy ion physics, understanding properties of the nucleus.
What group did you join in Chicago?
The group structure there was different than at Seattle, I was in the Physics Division. They had a tandem Van de Graaff and they had just built a post-accelerator Linac for it called ATLAS. I worked with John Schiffer and in several groups there. As in Seattle, I worked on a variety of the experiments, some of which were related to what I had done as a graduate student, as well as some entirely new fields for me.
For the first year, my experiments were on ATLAS, and we did some measurements of fusion properties with a variety of beams and targets with different neutron numbers. Using nickel beams and tin targets, we were able to fuse nuclei over a very large number of neutrons to understand what impact that had on fusion properties.
The second year I was in Chicago, I was exposed to some work in neutrino physics. There was an experiment there, E-645, which was taking place at Los Alamos, and there was a weak interactions group, Gerry Garvey and Stuart Freedman, who were working on that project. I think at that point I was curious and interested to see something else, and the idea of traveling to the Southwest was interesting, working at a new facility at Los Alamos. So, my work evolved from heavy ion physics to neutrino physics for the second year I was at Argonne.
Would you say that that got you a little closer to astrophysics again?
Yes, my work did get me closer to astrophysics. I was doing very different physics experiments than I did as an undergraduate or graduate student. That’s one aspect of my career that I’ve enjoyed is moving from field to field, not doing the same experiments for 25 or 30 or 40 years. At Stanford, I was doing polarized proton capture and measuring at the half-lives of different nuclear states, and then as a graduate student and for my first postdoc I was working on low-energy heavy ion physics, and then I started working on neutrinos. Then I worked on several neutrino experiments and organized efforts to create an underground lab, and most recently, I’ve focused on dark matter. I’ve worked on a wide variety of very different physics topics. I think that’s a strength in the US and with the institutions where I studied, they all allow individuals to find experiments that they find interesting and help support them and find ways to pursue that. This is particularly true at LBL.
How did the opportunity at Berkeley Lab come up for you?
There was a physicist who was a postdoc up in Seattle and on my thesis committee, Rick Norman, who had a position at LBL. He was a recent hire at LBL and contacted me when I was at Chicago, saying, “Would you be interested in coming back and doing some astrophysical measurements at the cyclotron?” At that time, my father was very ill and the opportunity to come back to California was well-timed personally, and to come back to Berkeley was obviously a very attractive offer, so I came back for my second postdoc to work with Rick at the 88-inch cyclotron and do astrophysics measurements there.
What were some of the questions surrounding the cyclotron’s use at that point?
We were using the cyclotron to understand properties of nuclei—the spin structure, the lifetime of isomers—which would shed light on how elements are created in the universe. Odd-odd nuclei often have complicated nuclear structure and long-lived isomeric states that impact the process of forming nuclei by neutron capture. These nuclei were referred to as hold-point nuclei because they pose difficulties for subsequent s-process nucleosynthesis. Determining the detailed nuclear level schemes enabled determination of synthesis rates and details of the stellar environments where nucleosynthesis takes place.
I looked in detail at the properties of two nuclei, promethium, and lutetium, which were these hold-point nuclei, to understand that structure and help deduce information from astrophysics. Again, you see a little evolution back to astrophysics and astronomy in doing these experiments. These experiments required a lot of my low energy nuclear physics.
Would you say during the early years that astrophysics at LBL was kind of a rather small, niche discipline?
Yes, it was. Rick was hired in as a divisional fellow to develop a nuclear astrophysics program, which wasn’t the main focus of the Nuclear Science Division.
Who do you see administratively at this point as really key in understanding that astrophysics could and should be part of LBL’s long-term research portfolio?
Rick was the leader in the development of the astrophysics measurements at the cyclotron. That was under the tenure of James Symons as the division director. Then a couple years into my postdoc there, we encountered an opportunity to join the Sudbury Neutrino Observatory. That was a major change in the direction of the Nuclear Science Division. That was a big step for the NSD and LBL into the field of neutrino experiments. These experiments offered the potential for major physics discoveries.
Now your transition from post-doc to staff scientist—was it rather low key? Were you looking elsewhere and LBL wanted you to stay? How did that all play out for you?
I wanted to stay in Berkeley. Promotion from within usually isn’t automatic, and I had to pursue that within the division. I hadn’t gone out looking for other positions. I was happy doing what I was doing, and once I got into SNO, it seemed like a natural position for me to advance at LBL. Berkeley needed people at the staff scientist level to build and commission SNO.
Now how far developed was Sudbury at this point when you joined?
There was a central core of groups that had been developing the idea at SNO for several years. When we joined in ’88, the collaboration was rapidly being established. The experiment was just developing roles and responsibilities for the different participating groups. This was before the first agency review, the ‘Temple review.’ Ed Temple used to lead these reviews for major DOE experiments. Ed Temple’s review of SNO that was my first experience with large project review process was the SNO review in Ottawa in ’89. That is where I remember my first in-depth interactions with Art McDonald.
I was going to ask when you met Art.
When we were investigating roles for the different groups – we needed something appropriate for LBL which had strong engineering abilities, so we immediately went to the engineering division. The head of mechanical engineering, Bob Fulton, came to that collaboration meeting. The PMT support structure was a significant engineering and construction subsystem, and it was a good project for LBL with its engineering strengths. I became the group leader for the PMT support structure, and LBL was the laboratory that produced the 60-foot in diameter buckyball (Geodesic sphere). [Laughs]
I wonder if you can explain that a little bit more, how the buckyball was relevant for this.
SNO is a detector that was based on a 1000 kg heavy water target contained inside of an acrylic sphere. The neutrino interactions produce light, and we developed an array of light sensors that could observe the neutrino interactions in the target. We were concerned about radioactive backgrounds in the detector materials. This is the theme for underground experiments. You’re moving experiments deep underground which is a difficult environment for physics. You do this so you can get away from cosmic rays; you also want to get away from radioactive backgrounds. A geodesic sphere is one of the most efficient—that is strength-for-weight constructions – we were able to build an 18 meter-in-diameter structure that held 9,456 phototubes (PMTs). These PMTs covered something like 68% of the surface area in light sensors and did that with a very light 30,000-pound stainless steel structure. The geodesic was ideal for a very strong structure, a rigid structure, that could support a large mass of phototubes in a stable structure. In fact, we had to provide an anchoring system so that the support structure and PMTs would not float when the cavity was filled with water.
How well-defined was the research mission in terms of knowing exactly what it was that you were looking for, and to what extent was it just, “Let’s put this together and see what happens”?
SNO was special in that several experiments, starting with Ray Davis’ Homestake experiment, observed deficit of neutrinos. The observations stretched over nearly 20 years. That deficit of neutrinos could either be due to the solar models incorrectly predicting the neutrino flux, or that the neutrinos were doing something so that the existing experiments were not able to detect them. SNO was one of these special experiments where no matter what we saw, it would be interesting: if we observe neutrino oscillations, particularly interesting, but it could also just have shown that the solar models were wrong. The deficient was significant, only a third of the predicted number of neutrinos were observed by the Davis experiment. SNO promised to be a win-win experiment, so we could either contribute to the astrophysics or contribute to the physics of neutrinos.
How much are you spending onsite and how much are you back at Berkeley and you can do things even way back then remotely?
The PMT support structure was assembled in three campaigns. I was fortunate. I got to see the winter in Sudbury for three different years. [Laughter] During those campaigns when we were building things underground, I was typically on-site for one to two weeks a month, go home for a couple weeks, and then go back again. I was overseeing the installation of the support structure for those campaigns in my role as group leader at LBL. For most of my career, I’ve spent about a week a month away from home.
What do you see as some of the big accomplishments of this research? What did it allow in terms of new questions that could be asked as a result?
Obviously SNO’s discovery of neutrino flavor transformation was instrumental for our understanding of physics. We proved the solar models were right, remarkably accurate, and that neutrinos did something new, something exciting, and they did it in a way that was different than the quark sector. The big discoveries are (and combined with other experiments) neutrinos oscillate between the flavors, and they do so with a mixing angle which is different than the quarks. Why neutrinos do that is still a big question.
SNO added a little to our knowledge of dark matter, neutrinos are a small percentage of dark matter, but neutrinos could not entirely resolve the dark matter problem. But really, I think these experiments—the Kamioka, Super-K, the KamLAND, the SNO—all pointed to the fact that neutrinos are interesting. They’re different from the other particles, and there is likely something very deep about why neutrinos oscillate. That’s going to be left open for the next generation of experiments to answer.
What was your next big project?
As SNO was finishing up, I again worked with Stuart Freedman, on KamLAND. I was working in Japan on the reactor-based experiment there. For the first couple of years, I served as the project manager for that, assembling the DOE scope and monitoring that for the Department of Energy.
Where in Japan was this located?
Kamioka, in the same hall as the original Kamiokande experiment. The closest city is Toyama. It’s on the western side of Japan. Very, very beautiful area, very scenic area. I really enjoyed my time in Japan. Just a beautiful country. I appreciate the food, lifestyle, and I admire my Japanese colleagues a great deal.
What were some of the cultural differences in Japan that needed some getting used to for you?
We were fortunate that we had a Berkeley theorist, Hitoshi Murayama, who came with us for our first visit to Japan. Hitoshi helped explain how we should express our interest in the experiment and helped us understand how to approach the Japanese physics organization.
Kevin, as you explained with SNO, institutionally how did LBL become involved in this research in Japan?
Stuart Freedman heard about KamLAND and led our approach to the Japanese leaders. In 1999, Stuart, Brian Fujikawa, Hitoshi, and I went over to meet with the KamLAND leadership. We began working on a memorandum of understanding with the collaboration with LBL serving as the lead lab for the US effort. There were others in the US also interested in this: Giorgio Gratta at Stanford and some of his earlier collaborators including Jerry Busenitz and Andreas Piepke. Giorgio was very interested in KamLAND and had made some of the first contacts with Suzuki. A US collaboration was formed, Stuart leading the US science side of things, and I helped Stuart with the project management for a couple of years.
Did Japan have its own version of a DOE that supported their side of this project?
Yes, they had their own funding, and their funding aspect approach was very different than the US. US distributes project funding over multiple years. Uncertainties in congressional support can invoke some anxiety for the large projects. In Japan, projects are funded in a lump sum. “Here’s your funding. Go do great things.” It’s hard to go back and get more, as I understand, but their funding was up front. They had it all and the US followed a model of meting this out over some years, and then would go through the review process. The US is much heavier on review process as well with regular checks on your progress.
Scientifically, what were some of the goals, the major goals of this collaboration?
KamLAND was designed to observe the oscillations with a different source of neutrinos. Our goal was to establish that we were observing oscillations and there was not some other mechanism causing the flavor transformation observed by SNO and SuperK. We were able to observe the oscillatory pattern as a function of distance from this different source of neutrinos – nuclear reactors.
What were some of the technical challenges in putting this together?
Backgrounds were a big issue – background radioactivity. Use of liquid scintillator at that volume was a big effort and developing the balloon to hold the scintillator was a challenge. This was a challenge that the Japanese were able to resolve very successfully. Just getting that large volume of liquid scintillator underground was an effort. KamLAND was different than SNO. SNO used an elevator. All the heavy water went down inside of little tanker trucks. In KamLAND, they drove big tanker trucks into the mine and delivered 20 tons at a time.
The logistics of building things underground is always a challenge. Even with horizontal access, you’re limited to the size of detector components, and building any of these detectors underground takes a lot of planning and careful engineering. For both SNO and for KamLAND there were substantial logistical problems assembling a large detector deep underground. I was able to apply many lessons from these experiments as we thought about future underground labs and experiments.
What were some of the most significant findings as a result of this collaboration?
KamLAND’s big discovery was the absolute measurement of neutrino oscillations. KamLAND’s measurements, together with Super-K and with SNO, locked up that we understand neutrinos oscillate, and they mix differently than quarks.
And just to zoom out a little bit, what is the broader value cosmologically, astrophysically to this discovery or understanding?
Ultimately, neutrino oscillations may be responsible for the matter-antimatter asymmetry we observe in the universe today. At the Big Bang, matter and antimatter were perfectly balanced. Something upset that balance so that today we have a universe that is dominated by matter. The asymmetry may have been produced by neutrino oscillations. The long baseline neutrino oscillation experiments hope to pursue this and understand oscillations in greater detail. This may help us understand why we live in a matter-dominated universe.
Now did you travel back and forth as you did with SNO, or you were onsite for longer stretches of time in Japan?
Mostly I commuted for several weeks at a time. One summer I was awarded a JSPS fellowship for four months. My family came over with me for much of that summer.
Yeah. What did you do next?
While SNO was winding down, I was still interested in neutrinos. Many people from SNO were interested in neutrinoless double-beta decay. I was involved for several years with the MAJORANA Demonstrator. LBL has great expertise in germanium detectors, and I sought to apply some of that to double beta decay.
Can you describe a little what the MAJORANA Demonstrator is?
The MAJORANA Demonstrator is an experiment using germanium detectors attempting to characterize the backgrounds that would be present for a major neutrinoless double-beta decay experiment, and to develop plans and techniques to build detectors that would be adequately sensitive to double-beta decay and less sensitive to the backgrounds.
I worked on double-beta decay for several years. At the same time, I was interested in developing an underground laboratory for the US. Most of my career had been spent flying to experiments, and I began thinking that it should be possible to consider an underground laboratory in the US for experiments. In the year 2000, I started to pursue ideas for an underground laboratory. Wick Haxton started the Homestake collaboration. I helped him organize a session at a town meeting in advance of the Nuclear Physics Long Range Plan and served on the collaboration’s executive committee.
When did you first meet Wick? Would it have been up in Seattle?
My first meeting with Wick was probably around 2000 or slightly earlier than that. I certainly knew him from American Physical Society meetings and during the time I was developing SNO. Wick and John Bahcall were regular visitors to our SNO discussions.
So what were some of the earliest discussions about experiments that could detect dark matter? What was going on at that point? What seemed feasible?
I came to investigate dark matter a little later. This came about because of my underground lab efforts and my involvement at Homestake. Initially, Wick led the efforts to create the Homestake laboratory. That effort was stopped in 2003. Then in 2005, the NSF restarted the effort to develop an underground laboratory. I stepped up to serve as the PI for Homestake. Working with the governor and state officials, we were able to develop significant support for the facility, coming from the state of South Dakota, T. Denny Sanford’s donation and ultimately funding from the NSF. With these funds and interest in establishing a domestic underground facility, we developed the initial suite of experiments that would be housed at Homestake. LUX collaboration first appeared as we were creating the initial suite of experiments. The Large Underground Xenon experiment was led by Tom Shutt, Dan Akerib, and Rick Gaitskell. This was my first exposure to dark matter experiments. It was as a result of selecting LUX to be housed at the Homestake facility.
Were you involved at all about siting considerations, as in why Homestake as opposed to other mines?
I was the PI for Homestake, and I was advocating very strongly for the South Dakota facility. I led our efforts to present the site strengths to the NSF’s site selection team and the entire NSF selection process. During the five-year effort to create the underground lab in South Dakota, we prepared multiple reports and studies. The siting committee was very impressed with all we could present during their visit to the Black Hills.
Why Homestake? What was compelling to you about this site?
Homestake has a large amount of existing infrastructure, excellent rock for excavation, and extremely strong local support. The state of South Dakota is phenomenally supportive of this effort and invested tens of millions of dollars. T. Denny Sanford donated an additional $70 million. This gave us expeditious access to a very deep site with very competent rock in which we could make large cavities. The site had everything from the political strengths of having congressional, state, and local leaders who were fully behind our work and coming up with major financial support, to good existing infrastructure without interference from active mining. These all made it a natural choice for a dedicated underground science facility.
Now was the mine active at this point?
It was active in 2000 when we were first looking at sites for a US facility. Homestake shut it down in 2003 and at the point when we took over in the new proposal (2006-7), it was not an active mine. When Homestake left, they donated the facility to the state of South Dakota and the underground was no longer being maintained.
Did you recognize that physics experimentation at Homestake was something that was conceived of even decades before? Were you aware of this history?
We all knew of the solar neutrino experiment there by Ray Davis and his collaborators. His experimental site was one of the first places we visited when we went underground in 2001. Yes, we knew they had been doing neutrino experiments in Homestake for 30 years. Ray’s experiment was another example of excellent local support for science by Homestake. The experiment didn’t lease their space, Homestake enjoyed scientific research in their mine. South Dakota really appreciated having a world-class experiment, a Nobel-prize winning experiment, in their state. The local residents got to know Ray and Ken Lande and their collaborators. That’s still true today. There’s a great appreciation for science in a small little mining town in western South Dakota.
Did you have opportunity to meet with Denny Sanford at all?
I met him multiple times including when we would have our site reviews and annual visits by the NSF and state officials. I also met him at several events sponsored by the Governor of South Dakota. His contributions were, again, instrumental to this government-private collaboration to build the facility. It would not have happened without T. Denny’s support and also that of Governor Mike Rounds.
What were some of the technical challenges to get everything…I want to say up and running, but it’s really down and running.
At Homestake, there’s a major level at the 4850 Level to do experiments, that is where the Davis experiment was. That is about a mile underground, and there are two shafts that access that area, so it’s a vertical elevator ride down a mile. Both of those elevators were built in the late ’30s, early ’40s, and they both needed maintenance. One of my first steps was getting those up and safely operating so that we could start a science program at Homestake.
What were some of the safety protocols that you had to contend with?
Working underground is more difficult and there are hazards you do not encounter when performing experiments on the surface. You don’t find manuals on how to build experiments underground – our history isn’t that long yet. From the mining legacy at Homestake, we benefitted from the local mining experience, a lot of the rules and regulations came out of the mine safety, MSHA. That’s good for mining, but the tolerance for incidents and accidents is higher than in modern science. So MSHA was good for general aspects of gaining access, ventilation, or excavating rock. It was a good place to start, but we had to add to that culture how scientists work as well.
What was your understanding of the NSF’s decision to stop funding in 2010?
That was not a happy time in the Lesko family. [Laughs]
Was it an existential threat at that point also? Was it all going to be over when the NSF pulled out?
There was not a promising path for Homestake after that. I was particularly frustrated when I looked at the science we proposed to do in Homestake, and I’ll just focus on the physics, these experiments all probed fundamental questions about our physical universe. These were all experiments that the NSF indicated they wanted to play major roles in. These experiments all needed to be underground to measure—neutrinos oscillations, neutrinoless double-beta decay, astronomical neutrinos, dark matter, nuclear astrophysics experiments—all required a deep underground laboratory and were, and still are, all incredibly important for our understanding of physics.
So, for the National Science Board and their physics consultant to say that they weren’t going to support our efforts to answer these questions was very hard for me to understand. What the NSB did was hard to understand because the science was so compelling, is so compelling, it seemed natural that they would want to develop those experiments here in the US in a dedicated multidisciplinary facility. Their actions portrayed to me that they really were not capable of the development of major, multidiscipline science facilities of scale. That they felt that the DOE was more able to succeed in these efforts. I think the National Science Board felt that building an underground facility was beyond their ability, despite the exceptionally strong support by the State, by T. Denny Sanford and the scientific community. So, the tension I felt was between such compelling physics opportunities vs. not developing the domestic, dedicated facility to enable these experiments. I felt strongly that the US needed to express leadership in building a facility to advance science across multiple disciplines – in physics, biology, geology, and engineering.
Now as PI, did you have opportunity to go to Washington? Would you meet with policymakers, the stakeholders who were making these decisions to advocate?
On a regular basis. I used to have to cut my hair and wear a tie. [Laughter]
What points did you convey that you felt really landed home, and where did you feel like you were not gaining traction, which ultimately may have foreshadowed the NSF’s decision?
With the congressional delegations of South Dakota and California, we had very strong support. The underground facility was a natural project well-suited to South Dakota and enjoyed fantastic local and state support. It was really the decision of the National Science Board – that they didn’t feel developing and running a forefront physics facility was in their wheelhouse – that remains difficult to understand. Of course, I never directly advocated for the Homestake project to the NSB. A PI was not permitted to present the status, support, or science motivations of the project to the NSB. I found that part of the decision-making particularly frustrating.
How long was the gap between when the NSF pulled out and when the DOE came in? Was there a moment where there were just no funds and everything stopped, or was there a clean transition?
There had been ongoing discussions between the NSF and DOE concerning the experiments to take place in the NSF laboratory, DUSEL. The DOE was heavily invested in many of those experiments. The NSF and DOE had established a joint oversight committee. These experiments included long baseline neutrinos, neutrinoless double-beta decay, and dark matter searches. These experiments were all very high priorities set in the physics long range plans.
My interpretation is that the DOE expected DUSEL to be there to house the experiments, and when the NSF pulled out, that was not a result that the DOE particularly wanted to see. Soon after the NSB decision, I presented to a high-level DOE committee about options for the facility, the status for hosting physics experiments, and provided input for DOE for them to consider their options. This was the Marx-Reichanadter committee. At that point, we were fortunate that the DOE was able to pick up the continued funding so that the science program could continue, and we could exploit the investments from South Dakota in delivering world-leading science. Our funding never went to zero, thanks to the NSF and DOE. There was enough funding from the NSF and for a closeout of their involvement in DUSEL. Because of the very fast reaction by DOE, in fact, we were able to pick up and continue with a modest operations budget managed through LBL.
Did DOE work exclusively through LBL, or were there other National Labs or mechanisms to support DUSEL?
Initially, after the NSF DUSEL effort ceased and we became the Sanford Underground Facility, LBL was the point of contact for the DOE effort. Gil Gilchriese and I oversaw the effort in South Dakota. There was a very strong organization in South Dakota. Mike Headley and his team had a very qualified group that were keeping the facility going, and Gil and I worked to make sure that we could maintain their efforts and to see to fruition the early suite of experiments in SURF.
Did the day-to-day change at all with the DOE coming on board, or was it just a different pot of money?
There was a change in scope. With the NSF, the science program was going to be multidisciplinary with a very large focus on biology, geology, and engineering, as well as physics. Initially with the DOE, the focus was exclusively physics. So that aspect of the science program changed, and there was obviously a narrowing of the underground space development within SURF with a focus to completing the Davis campus and installing the Majorana Demonstrator and the LUX experiments, and getting those commissioned. A few years later, we developed a low background counting facility, and then the CASPAR nuclear astrophysics programs were developed as well, but it was a narrowing down of the science to physics and focusing on a smaller number of campuses. Obviously, the science program was not installed as quickly as we would have had with DUSEL. It was much slower for several of the experiments.
Now as facility head, were you going back and forth between MAJORANA and LUX, or were you more focused on one than the other?
When I was the PI for DUSEL, and these experiments were gaining traction, it was inappropriate for me to be involved in any experiment. Therefore, I withdrew my membership from all experiments. I had been in MAJORANA Demonstrator and LUX, so I left those experiments to uniquely focus on the facility. That was true through the rest of the DUSEL effort. When SURF funding from the DOE went through LBL with a much-reduced scope for the facility, I was looking around and saying, “What can I do with the rest of my career?” which at this point, it’s getting closer to retirement. I realized I could have a larger impact on dark matter, on LUX, and on LZ. I rejoined the LUX experiment for the last year or two of that experiment. These were efforts where I could contribute to science again since I wasn’t conflicted by being in the leadership of the facility.
What was the state of play with LUX at that point when you rejoined?
LUX was collecting data for their longer run. And I joined a little when the first paper came out. I was asked to chair the internal review paper for the first publication.
When is it LUX and then when does it transition to LUX-ZEPLIN?
LUX completed its planned running in 2016. At that point, there were already plans to develop a larger instrument. With direct detection dark matter detectors, the sensitivity of the experiment scales with the detector mass, and at that stage it was going to be the case where LUX’s sensitivity was asymptotically becoming limited. Therefore, it was important to get a larger detector deployed. Plans were already started on the LZ, which was a major collaboration between the LUX collaboration and the ZEPLIN collaboration in England to work on a larger instrument. Gil Gilchriese really advanced those plans and pulled the LZ collaboration together.
So given LUX’s success up until this point, what was understood about what LUX had already done, what it had already detected, and what was understood about next generation, what was needed post-LUX?
LUX was in a world leadership position in excluding parameter space for WIMPs for most of the time we were publishing data. We were the best in not finding dark matter, and we had excluded out a fair amount of parameter space and simply said, “There are no WIMPs in the simple spin-dependent, spin-independent model.” There was a large area between LUX’s limits and where experiments were ultimately limited by neutrino backgrounds, backgrounds you can’t shield for. (These are neutrinos coming through the Earth.) The focus for the next detector was really to cover much of that parameter space between where the 100- or 300-kg experiments stopped being sensitive and then down to the neutrino interference area. That’s where the scale of the 10-ton detector came from, that and figuring out what’s the biggest detector we could put into the Davis cavity. The work designing LZ was started in the final period of LUX running.
What did ZEPLIN add to LUX and what were some of the institutional considerations to make this collaboration a reality?
ZEPLIN had very strong groups in the UK, and expertise in instrumentation of xenon TPCs, and we were happy to grow LUX and to add more of the UK groups. The UK focused their efforts on dark matter into the xenon program. They had a decade’s worth of experience running these experiments underground. It was a boost to add their expertise and that of the Portuguese group to LZ.
When does it become not just Berkeley but other components of the DOE that are interested in LUX and the search for dark matter?
LUX was a strong collaboration from the start. There were many groups involved with significant dark matter experience: Case Western Reserve, SLAC, Berkeley, Maryland, Santa Barbara, Davis were all involved, and others. Many of the institutions that are still members of LZ were part of that. Some of the PIs came from CDMS, so people like Harry Nelson down at Santa Barbara. Rick Gaitskell at Brown. Tom Shutt, who was at Princeton, and then Case Western and finally SLAC.
What were the circumstances behind you being named spokesperson?
It’s a two-year appointment, and within the governance there’s an election. The collaboration was looking for someone who had experience leading large groups, someone with significant underground experience, well-known to the SURF team and building detectors. They elected me at the end of 2018. It was an interesting period, especially with COVID striking. Despite that, we accomplished a lot. [Laughs]
In terms of building detectors, is LUX-ZEPLIN, is LZ going on the basis that dark matter is one thing and not many things, or is it built for either possibility?
It’s looking for any signals, and clearly the interpretation of the signal or lack of signals will depend on if it’s one particle or many particles. But if the particle interacts by the weak interaction, regardless of whether it’s one or many, we have a good chance of seeing it. The signal strength will depend on if there’s a spectrum of masses. If there are a bunch of lighter ones, those will be harder for us to see. If it’s a bunch of heavy ones, then it will be easier for us to see. The collaboration must be ready for all possible outcomes. The interaction of the particle with the target itself will generate signals and interpreting those signals in terms of WIMPs is the responsibility of the collaboration.
And just to bring our conversation right up to the present, now that you’ve stepped down as spokesperson, how has that changed your day-to-day, and what opportunities do you have to do more of the science and less of the administration?
I have been advocating for the importance of there being experienced hands on-site, and I put my hands where my mouth was and have taken the opportunity to spend a lot of time in South Dakota, even during COVID, to see how I can assist the experiment. Clearly, I know people in South Dakota very well from my 20 years of experience there. I know most of the technical crew there very well. I’ve just been fortunate to be able to spend 11 weeks in the past 8 months to assist with the underground construction. Again, underground construction never goes according to schedule. It’s always more complicated than you thought, and just having experience getting through that and finding ways to get around some of those problems is where I think I’ve been able to help the collaboration since my tenure as spokesperson concluded. Also just not having to worry about some of the aspects of collaboration business and focusing a little more on analysis and what the collaboration is doing on those topics has been refreshing: less protocol and more physics.
Kevin, given your optimism even in the near term of what LZ can accomplish, I wonder if you might play out hypothetically what the infrastructure and the moment might look like when dark matter is detected. In other words, I’m so lucky in my work to have heard blow-by-blow accounts of the J/psi at SLAC in 1974 or the Higgs in 2011, 2012 or the detection of the gravitational wave for LIGO, right—these moments of “There’s the signal. We’ve got it.” What might that moment look like for LZ?
The closest similar discovery that I immediately recall and one that was also made with underground detectors was the observation of a supernova through their neutrinos in ’87. There were a small number of events observed in two different detectors. The observations were time-correlated. Dark matter events in our detectors will more likely be a small excess. Small at first. My expectation would be that we would have a great deal of skepticism about such a discovery, and we would conduct an awful lot of work answering questions, “How could we have gotten this wrong? Have we missed a background? Is this just due to some new form of background?” A lot of analysis and calibration will be required. The claim of a significant discovery such as dark matter should only be made after all other plausible explanations such as backgrounds or detector malfunctions are ruled out. Then, after additional calibration of the detector, have additional work on backgrounds if we still have a significant excess – then we will have duty to present our results to the community for critical review.
We are fortunate that there are several detectors coming online with similar sensitivity. We should not have to wait long for independent confirmation of any potential signal. Then, with several observations I think the physics world would take notice. I remember that a former occupant of the Davis cavity, Ray Davis for whom it is named, worked this way. For over thirty years he reported his neutrino deficit. He continued to calibrate his detector and improve it. Through many years of analysis and calibration, his confidence in his measurement grew. I think we’re in a situation where LZ is large enough, that the exclusion from the existing experiments gives us a good opportunity within our data to see a dark matter signal. We’ve been very careful to control our backgrounds. We have a very good assay program and were able to establish expectations as to what will be there, and we must confirm those measurements with in situ analysis, but I have confidence in the LZ collaboration to perform the necessary calibrations and complete the thorough analysis that would let us make such a claim of discovery. We have an excellent instrument, in an ideal location. I am looking forward to LZ finding something that will advance our understanding of the physical universe.
In the way that LIGO has two sites for the specific purpose of redundancy, is that relevant for LZ? Do you need multiple detections from different places in order to confirm signals and results?
I remember back in the days of neutrino oscillations before SNO when oscillations were first discussed: most of the high energy physics community didn’t believe Ray Davis for 30 years because he was reporting a deficit in his measurements of something like half an atom a month. They just didn’t believe it was right. But there were criteria being discussed after some of the atmospheric neutrino detectors were also reporting anomalies. One criterion was to claim a major discovery, such as neutrino oscillations, you needed multiple techniques, multiple experiments before it was really on solid ground. Having additional experiments absolutely will be needed to claim a discovery of dark matter. Multiple independent observations lead to credibility of any claim.
Again, the competition we are witnessing in dark matter is great for the field. If two experiments in different parts of the world see the same signal, have different backgrounds, different analyses and still see the signal, that lends a lot of credibility to the discovery. It is still better to have some other techniques or experiments with different media, something other than xenon. If we have multiple detectors and collaborations observing WIMPs, then I would expect a similar situation as we had with neutrinos where within the span of 2 or 3 years the community flipped from not believing Ray Davis, to one where we are today with everyone understanding we have neutrino oscillations.
[Laughs] Now this hypothetical scenario is playing out in real time right now with the Muon g-2 anomaly at Fermilab where there is a lot of excitement and skepticism about whether this is new physics beyond the Standard Model. Would the successful confirmed detection of dark matter mean, by definition, new physics beyond the Standard Model for cosmology, or is it possible that the detection of dark matter fits within the current Standard Model of cosmology?
Well, we don’t have a WIMP in our Standard Model yet, so whatever that particle or particles that make up dark matter, they must be exotic.
Yeah, but in other words, the way that, say, the Higgs is mostly understood as the capstone to the Standard Model, not new physics, would parameters surrounding the WIMPs be a capstone to the cosmological Standard Model, or by definition would have to be beyond that Standard Model?
I guess I would look at it more in the particle physics side of things, saying, “We’ve got another particle here. We’ve discovered something new that isn’t a natural extension of our known collection of particles. Yes, maybe it’s supersymmetry. Maybe that’s what you’re looking at here.” But this would be the observation of something new in the physics world that we have observed in the laboratory for the first time. That’s a major extension of our understanding of particle physics. At the same time, our understanding of cosmology would have to adjust to include this discovery.
Yeah. You mentioned supersymmetry. Given that currently there are no plans to operate at higher energies beyond what the LHC is currently working on, to what extent is that limitation terrestrially limiting what you can do with dark matter detection? Or is it unrelated?
I have colleagues who, I think if you had asked them 20 years ago when they were getting involved in experiments at CERN what they thought they would be doing today, it would be the whole new zoology of supersymmetry.
They expected to see it and would be spending these years understanding all the supersymmetric particles.
But we still have a real problem with dark matter and our Standard Model. I’m happy to focus my attention on that.
Yeah. It’s an entirely speculative question, but I’m curious. If the SSC went through, would we be a lot farther along than we are on the detection of dark matter, or you see that as unrelated?
Certainly, there would have been earlier efforts earlier to find supersymmetry. We would have been exploring the WIMP space decades earlier. If the SSC had gone through and they didn’t find supersymmetry, that would have stimulated theoretical interest in other dark matter theories as we are beginning to see now. We still know dark matter is there. Something which is of the weak scale is still one of the most attractive answers. But the successes of LUX and Xenon1T have resulted in new efforts to look under different light poles for dark matter.
As you said before, one of LUX’s successes so far has been negatively defined. We know what dark matter is not, so what’s the list? Is there like a top five? How big is this list?
What are the great discoveries?
What do we know is not dark matter?
Just about everything we can deduce about dark matter comes from the astronomical observations. We deduce that it doesn’t interact by strong interactions but it does interact by gravity. That it is stable or at least very long-lived. That there is five times as much of it as normal baryonic matter. That it is exotic material and not the particles we are familiar with. LUX and the other direct detection experiments are putting limits on the mass of dark matter particles and its interaction strength with normal matter.
You touched on this earlier. I’m sure you’ve heard the metaphor that when you lose your keys in the street, you tend to look under the streetlamps because that’s where you can see. How do you know that we’re not just looking under the streetlamps right now for dark matter?
I don’t think we exclusively look under the WIMP streetlamp. In fact, there are the axion searches and many newer experiments extending down to much different energy ranges. It’s appropriate to have the ability to look over a wider range, but right now this is an area where we have the instrumentation to search effectively, and we should exploit those techniques. We also should be pursuing avenues to expand what that search is, and you see that in the dark matter community now with the great interest in lighter-mass dark matter, different ways of detecting it. We are investing in the R&D to expand the sidewalk, which is illuminated, but we started out where it was natural to do it and where you could do it relatively expeditiously and economically.
Well, Kevin, now that we’ve worked right up to the present, for the last part of our talk I’d like to ask some broadly retrospective questions, and then we’ll go back to speculation, looking to the future for the end. So, my first question is a theme of your career, geographically at least, has been LBL has been home base, but most of your major research has been done elsewhere. What have been some of the inherent advantages and disadvantages of always having that geographic duality in all of your research?
Yes, it’s curious because as an undergraduate and graduate student, I always thought I’d be working close to home. So did my wife. [Laughter] But I went where the most interesting physics would take me, where I could do it, and made use of facilities around the globe to do it. That’s where I had to go to make it work. It took a lot of understanding from my family about me being on travel with a packed suitcase at the foot of the bed.
I mean, even scientifically, the fact that you have an intellectual home base at Berkeley…right? So you always have that to come home to, literally, but then you have all these interactions elsewhere on the planet. This is not just a geographic challenge that it presents you; there are also certain advantages to having this both home base and these partnerships far away.
Yes. I have benefitted from the exceptional support from my home institution for the actual extraction of physics from the data. Data analysis doesn’t take place at the 4850 Level at SURF. For me, it always takes place at Berkeley. My group has always been focused on building the detectors, making use of the strong engineering resources at LBL and computing resources, and then doing the analysis with the group there. Over the years I’ve had tremendous support from the institution to make sure that not only could I establish an experimental program, but I would also have the students and postdocs to extract the physics and have the infrastructure from the lab to build it. The home base there to do the physics is natural, and a great advantage for my group.
Berkeley is a great location. It’s a crossroads of physics intellectual discussion, and there are more meetings and seminars and discussion than I can fit in a normal day. People, before the pandemic, always visited Berkeley, and we would have an opportunity to exchange ideas and interact with people from around the world very easily. There are a lot of strengths with LBL’s long history of pursuing the highest quality science. Their support of their scientists I have witnessed is remarkable, and really has enabled me to have a career of very successful physics—much different than I ever thought I would be doing in 1978. [Chuckles]
What have been some of the major technological advances, either in instrumentation, in materials, or in computation that stick out in your memory as the kinds of advances that really move the needle forward for the science in short bursts?
Many of today’s most interesting experiments, the ones I’ve been involved with over the past 20 years, are rare-search experiments. You’re looking for a couple of events a year. When you think about SNO, when it got started, the total number of neutrinos that had been observed was a dozen, maybe 20. There were a handful of events, and then building a detector where we were detecting 20 a day was a huge advance. A critical advancement that enabled us to build detectors was our ability to control the backgrounds, so we weren’t swamped with cosmic rays or trace quantities of radioactivity. This required us go deep underground, and required very careful selection of the materials, which in turn required low background assay.
Our ability to assay to increasingly lower levels of radioactivity, developing deeper and better shielded laboratories are what allowed us to so expeditiously understand the neutrino and pursue dark matter.
Given that so many of your experiments are rare, only a few times a year, I wonder if you’ve ever reflected on the virtue of patience as a scientific…skill.
I think patience in analysis is great. We want to make sure that what you’re seeing is correct. We want to test it many, many different ways. Patience in designing and building experiments is not as desirable. By this, I mean we must learn when something is good enough and stop tweaking and perfecting aspects of the detector. We want to get your instrument up and running as soon as we can. Certainly, you want to make the right choices, but I’ve seen several experiments spend too much time perfecting their instruments rather than begin looking for physics.
Kevin, last question looking to the future. I can’t yet ask you what we know about the universe now that we’ve detected dark matter; we can’t ask that question yet. But what do we know about the universe so far as a result of not yet understanding dark matter in terms of ruling it out, in terms of recognizing its few characteristics that we do understand?
I think the most valuable aspect of this is sparking our curiosity. It’s actually having people ask the questions, pursuing different experiments. Having something which is a mystery and is a problem for the theorists, is great for experimentalists. It sparks our curiosity. It sparks investigations, technology advances. Having a mystery is exciting. Filling in the details and understanding the 17th decimal point of some number—that’s, to me, not as exciting, but having a big mystery where what makes up 95% of the universe is a big one. And very exciting.
It’s even the biggest one, perhaps!
[Chuckles] And it keeps the next generation of scientists interested. It’s easy to talk to kids about underground physics, and they get excited about it. So, I find the biggest value of talking to the younger students is to spark their curiosity, build their excitement about science, excitement about understanding the universe. This is easier if there are big things you don’t understand.
Kevin, this has been so fun talking to you. I’m so glad we were able to do this. Thank you so much.
Well, thank you.