Deborah Harris

Zierler:

Okay, this is David Zierler, oral historian for the American Institute of Physics. It is October 12, 2020. I'm so happy to be here with Professor Deborah Harris. Deborah, thank you so much for joining me today.

Harris:

Thanks for inviting me.

Zierler:

Okay, so to start, would you please tell me your titles and institutional affiliations? And you'll notice that I put an S on both of them, because I know you have more than one.

Harris:

Right now I am a professor at York University, and I'm a senior scientist at Fermi National Accelerator Laboratory.

Zierler:

And how long have you had this arrangement? And what would you consider to be more your home institution?

Harris:

I've had this joint appointment since July 2019. And my home institution is technically York University. But I am supported by both institutions, and right now I am based in Canada. Because of the pandemic there's more of a border there now than there used to be.

Zierler:

Right.

Harris:

Actually, since the pandemic started, I've been to Fermilab twice, but I've only been to York once. I've only been to campus once. I was working at Fermilab for almost 20 years before starting this joint position, so it is strange to be away from Fermilab for so long.

Zierler:

And is it essentially a 50/50 split both in terms of salary and expectations and how you spend your time?

Harris:

For the five years of this position, I have half as much teaching as a normal professor at York University would have, but both York and Fermilab expect me to do research so thankfully there are overlapping expectations. It is hard to divide up and quantify how much time you spend on one thing versus another. Because your brain is often trying to work out problems even if you’re doing something else like cooking dinner.

Zierler:

But maybe, Deborah, that maybe that's an important point that your work sort of does not neatly fit into one institution or the other, you have an overall research agenda. And perhaps some of it you do wearing your York hat and some of it you do wearing your Fermilab hat.

Harris:

Right. There's research that I do that is a benefit to both Fermilab experiments and experiments not at Fermilab. So how does that count?

Zierler:

So nowadays, in the pandemic, of course, the theorists are pretty thrilled they're being more productive than ever, right? They're just sort of home and plugging away on their calculations. But in the world of experimentation, how, how are you keeping productivity up? How are you continuing to make sure that everything is operating the way you want it to?

Harris:

Well, for me, personally, it's strange, because my efforts right now are split among experiments which are not operating right now. So for example, the MINERvA experiment, an experiment for which I'm the co-spokesperson, stopped taking new data in February 2019. We are mining that data as best we can to make more measurements. Everything we do is online although it's much nicer to be able to meet with people in person and discuss things face to face. I find that in person you can bounce ideas off each other in a more informal way. But we don't actually need to be physically anywhere to do this work. On the other hand, to make progress on an experiment which is in the design phase, you also can do a lot “online.” But at some point, you need to test those designs. The challenge in doing research on neutrinos though is that the bigger your detector is, the better statistics you get, and the better measurement you can make. But to test a design you would rather not have to make a full-sized detector. The DUNE experiment is planning to build four warehouse-sized detectors to run far from the neutrino source, but it is also making a four-closet-sized detector that is running near the neutrino source, and both are with a brand-new technology. For the near detector, the original plan was to build a prototype that is half as big as the full thing and test it on cosmic rays in Switzerland and then in an existing neutrino beam at Fermilab. And we would go to Switzerland and or Fermilab to learn what we can by operating that particular prototype. But that doesn't really work so easily right now. So that part of the project has been delayed, although they are managing to do tests in Switzerland. But another idea is to make cubic foot-sized prototypes and send them around to different institutions so people can do tests at their home institutions.

Zierler:

Specifically, as a result of the pandemic?

Harris:

Yes. So we are investigating what aspects can you test with a smaller version? And planning for those tests at York University.

Zierler:

Deborah, on the on the mentoring side of things, where would you be more likely to have graduate students, at Fermilab or at York?

Harris:

Well, that's a funny question. The first person I brought into my research group at York is a PhD student, and I'm hoping to bring in one or at most two more students to the group, where I am their direct supervisor. But when I was at Fermilab, I was an informal or even sometimes formal mentor to up to a dozen students who were enrolled in universities around the world. So I'm actually still the co-supervisor for two students at Aligarh Muslim University. So at least before the pandemic, I was mentoring students informally at Fermilab and formally at York.

Zierler:

Now, that's probably a great way to get back to that question about how integrated your research agenda is, which is, would graduate students that you take on at York, would they be pursuing research that was strictly relevant to your projects at Fermilab, or not necessarily?

Harris:

The student in my group has a few different projects in his portfolio, and most of them are strictly relevant to the DUNE experiment at Fermilab. For example, he is developing an analysis to be able to identify tracks in a simple way so that when you first turn the detector on, you can really make sure it's working, diagnose the things you find wrong when you first plug in a device.

Zierler:

Well, Deborah, at this point, let's – take it back to the beginning. I want to hear first about your parents tell me a little bit about them and where they're from?

Harris:

My parents?

Zierler:

Your parents.

Harris:

Okay, well, so my dad is from Cleveland, Ohio, and my mother's from New York City. And they, as far as Fermilab is concerned, my parents moved to Illinois in 1972.

Zierler:

Where did they meet?

Harris:

So where did they meet? They met while they were in grad school at Harvard. In fact I was thinking you should really be interviewing my dad for an oral history of Fermilab. My dad was working on experiments at Nevis in New York, and Leon Lederman recruited him to come to Fermilab. So he came to Fermilab in 1972 and I grew up as a kid going to Fermilab family events like potluck dinners and holiday parties.

Zierler:

Is your mom in physics as well?

Harris:

No, she's a French teacher.

Zierler:

So what graduate degree was she pursuing?

Harris:

Sorry?

Zierler:

What graduate degree was she pursuing at Harvard?

Harris:

She was pursuing a master's in French.

Zierler:

So you grew up in a physics household from your dad essentially?

Harris:

Well, yes and no: we did not talk about physics around the dinner table. But I did grow up thinking that being a physicist was not some unusual thing. I definitely grew up thinking I knew a lot of physicists. It just seemed like the thing a lot of parents did. And my kids had the same experience.

Zierler:

Besides the dinner table, was your dad's style as a dad to involve you in his career? In other words, did you know, have a good idea of what he did? Even when you were a child?

Harris:

I don't think I had a good idea of what he did when I was a kid. What I remember is that he would bring home computer paper with the holes on the sides for us to color on and computer punch cards: we used to color on those too. But I don’t remember talking about what the plots on the other side of the paper meant.

Zierler:

What is your father's name?

Harris:

Jeff Appel.

Zierler:

I'm gonna have to check to see if we have an oral history with him because one of the great pleasures of our oral history collection is we have several child parent oral histories, like the Kivelsons and the Drells, for example. Maybe if your dad is still around?

Harris:

Yeah.

Zierler:

Well, then, I mean, hey, we're gonna have to talk to him, too, if we don't already have an interview with him. That's very exciting.

Harris:

Yeah, yeah, I'd love to hear how he answered some of these questions.

Zierler:

And what, what was his field? What did he work on?

Harris:

He worked on charm physics. And he worked on fixed target experiments. And then towards the end of his official time at Fermilab he was in the directorate. He was probably at Fermilab from 1972 until 2012. So he was there a good long time.

Zierler:

Yeah.

Harris:

And yeah, that's 40 years.

Zierler:

And he did his doctoral work at Harvard?

Harris:

Yes.

Zierler:

Who was his advisor?

Harris:

Actually, that's a funny story, too. My dad’s advisor was Dick Wilson, who is the father of another physicist who's currently at Fermilab. Peter Wilson, so...

Zierler:

It's a real family affair.

Harris:

Right. And Peter Wilson is a friend of mine, actually, and we met because we have kids who are the same age, and they went to the Fermilab daycare together.

Zierler:

I wonder if your dad made physics seem accessible to you? Like it was something that you could do yourself?

Harris:

Yeah, as a kid you always put your parents on a pedestal, but it's not like some unknown pedestal, you know what I mean?

Zierler:

And did you go to public schools throughout in Illinois?

Harris:

Yes.

Zierler:

And at what point did you realize that you had your own talents in in math and science? And you weren't just the daughter of this physicist?

Harris:

I didn't think of myself as the daughter of a physicist until after I went into physics. I knew by high school that I had an easier time in math and science classes than in my English classes. And I really liked studying languages, for example, and I also knew that solving math and science problems was easier for me than writing essays.

Zierler:

Now when you were thinking about college were you thinking about physics programs, specifically?

Harris:

I actually started as a chemistry major.

Zierler:

Oh.

Harris:

I knew that I liked chemistry and physics right when I started college, and I started taking chemistry classes, because the chemistry classes at Cornell are really popular. Then, after my freshman year of college I worked at Fermilab over the summer. That's when I realized that I enjoyed making these detectors that could see particles that you can't see with your eyes. And that's what got me into physics. The funny thing is that only by working at Fermilab that first summer did I realize that physicists really love their jobs and are driven by wanting to solve these problems. It's not like I'm doing this so I get paid so that I can do something fun over the weekend. It's that you really are trying to solve these problems and learn more about the universe. You could ask why didn't I get that sense from living with my dad for 20 years. But maybe that’s just a measure of how little we talked about the technical parts of his job at home.

Zierler:

Now, did your father encourage you for that formative summer at Fermilab?

Harris:

Well he drove me there each morning. It's funny--if you ask him, that he'll tell you “Well, I encouraged Deborah to go into medical physics because I thought that was really the thing of the future, but she didn't listen!”.

Zierler:

What was your project that summer? What were you working on at Fermilab?

Harris:

I was soldering the same thing over and over and over again. I was just following instructions, but I was building a detector, it was called the vertex time proportional chamber. And so it was a device that was going pretty close to the middle of CDF, an experiment to measure proton antiproton collisions. That device was later replaced by fancier detectors that were more resistant to radiation damage, but the point is that the other student I worked with, we joked that we were like spiders weaving these webs because it looked like a wheel with like spokes coming out. And we had to solder these very, very thin wires, you know, across each of the spokes over and over and over again. And in fact, I remember I couldn't even drink coffee during lunchtime, because my hand would shake too much to thread the wire if I had coffee after lunch.

Zierler:

Did you know even at that early age, what some of the bigger research questions behind CDF were or were you sort of more? I'm soldering, I'm in my zone, I'm focusing on this?

Harris:

They did a good job at Fermilab trying to get the summer undergrad students to appreciate what was happening at the lab. And I think they still do a good job trying to give students a chance to go to lectures that are aimed at their level. During the summer, there's a big summer Student Program at Fermilab, (except this summer, of course). They have all these students doing very simple technician work. And then they introduce the students to particle physics and detectors and accelerators, you know, so they can get a sense of the big questions.

Zierler:

When you went back to Ithaca, did you switch majors right away?

Harris:

Yes. Well, the other thing was that I took organic chemistry, which was really hard.

Zierler:

You're not the first physicist who said that they switched over because of organic chemistry.

Harris:

Really?

Zierler:

Yeah. I've heard that. I've heard that a few times before.

Harris:

That's really funny. Yeah, physics just comes easier to me than chemistry.

Zierler:

When did you transfer [to Berkeley]?

Harris:

I transferred to Berkeley in the middle of my junior year.

Zierler:

Did you feel like you could get a better degree in physics at Berkeley?

Harris:

No, actually I married a graduate student at Berkeley. But Berkeley had a lot of great physics classes, as did Cornell. So I didn’t feel like my physics degree was any worse or better at Berkeley than at Cornell.

Zierler:

Did you finish out with a senior thesis at Berkeley?

Harris:

No because I had to catch up—since I started as a Chemistry major at Cornell, I was behind in required physics classes. I took only physics classes in my three semesters at Berkeley, with the exception of one women's studies class.

Zierler:

You were, you know, your framework is rather narrow as an undergraduate and as a transfer student, but I wonder if you saw if physics was done in any particularly different ways, at Cornell versus Berkeley?

Harris:

No, you know, I took very few physics classes at Cornell. I took a physics class for pre-meds at Cornell. And then I jumped over a track and took the physics class for physics majors at Cornell. So in fact I only took two physics classes at Cornell.

Zierler:

And Deborah by the time you graduated, were you 100%. on the experimental track? Did you give a consideration, no, you never really thought much about theory? about pursuing a degree in theory?

Harris:

No, I didn't. I really liked doing experiments. I like building detectors. And so that was definitely the thing I wanted to do.

Zierler:

Did you have any good lab experiences at Berkeley?

Harris:

Yeah, there was a great undergraduate lab. We got to make a hologram--that's the one I remember the most, but that wasn't the thing that made me want to go into experimental particle physics. The most fun part for me was building new detectors. That's the thing that got me started.

Zierler:

Where did you apply to graduate school?

Harris:

Wow, a lot of places. I don’t even remember them all. I do remember applying to Berkeley, Harvard and Chicago.

Zierler:

And were you motivated by particular professors you wanted to work with or experiments that you wanted to join? Or just by reputation? How did you make those choices?

Harris:

No, just reputation. I probably asked my dad. I think by then I knew I wanted to do particle physics. And maybe that's the thing is because I started at Fermilab that got me to make the jump from chemistry to physics, but I never looked around at all these different subfields of physics before jumping into particle physics.

Zierler:

Now was part of the attraction with Chicago that you would have closer access to Fermilab?

Harris:

That certainly helped, but the truth is that I was married, and my husband was applying for a postdoc job to work on an experiment at Fermilab. It's much easier if you live near where your experiment is, especially if you don't like traveling. And so he got a postdoc job at Fermilab and I wanted to be near him. I have to say though that Chicago was great—I really liked the school.

Zierler:

What is the curriculum like for graduate students on the experimentation side of things, how much coursework did you need to take?

Harris:

Well, it's different for different institutions, but at Chicago I took two years of classes, probably three classes a quarter. It wasn't like being undergraduate where you take five classes a semester. So you take two years of classes, and I was a Teaching Assistant, for a year and a half. And then after that I was doing research full time.

Zierler:

And how did you develop your relationship with your graduate advisor?

Harris:

Well, it’s kind of a funny story.

Zierler:

There you go, good example.

Harris:

When you are in experimental particle physics, when you get your data depends a lot on when the experiment on which you're going to get your data, actually takes the data, right? And it really depends on events that are well out of your control: even people who are working on the accelerator, it's out of their control. So when I started graduate school, the experiment that I wanted to do my thesis work on, KTeV, was in the planning stage, but people were starting to operate a different experiment with a related physics goal. The new experiment was supposed to start taking data a few years after I started graduate school. During my first year at Chicago I had a conversation with Bruce Winstein, one of my mentors at Chicago who I hoped would be my advisor. And he said, well, your thesis should be on KTeV, unless the schedule is delayed much longer than anybody expects it to be delayed. And then I thought, aha, that means I'm going to get out on the earlier experiment’s data. So, I ended up getting my thesis on the earlier experiment, which was the last in a series of experiments that were done with the same detectors. Those detectors were getting older and older, and some were suffering from radiation damage. But we still could get physics out of that setup, so we did. When I switched from the fancy new experiment to the older experiment, I also switched advisors to Yau Wah. Although the two mentors were very different in personality, they both were fantastic mentors.

Zierler:

And what was that experiment that ended up being yours that you worked on?

Harris:

The name of the experiment was, was E799.

Zierler:

Sounds like a tax form!

Harris:

Once upon a time at Fermilab, experiments only had numbers, they didn't have names that went along with those numbers. And so there was “E799”. But between that one and the next one the rule was introduced that you needed a name not just a number. So KTeV stood for “Kaons at the TeVatron.”

Zierler:

And just to zoom out for a second, Deborah, I mean, as a graduate student, of course, you're focused on your own research and not the bigger questions. But maybe at the time, or looking back, you might think about what some of the bigger questions surrounding this experiment were, and how they were responsive to, you know, the larger questions in experimental particle physics at the time.

Harris:

Yeah, well, it's funny because my thesis was to study a certain particle that is very unusual in that it is (almost) its own antiparticle. And so people were studying the decays of this particle as a way to probe whether or not there's some asymmetry between matter and antimatter—that was a big question then, and it's still a big question now. And now we are looking for differences between matter and antimatter in the way neutrinos change flavor. So even though it's a totally different particle from a Kaon, we are still trying to figure out why there is so much more matter in the universe than antimatter, when in particle physics they seem to be produced in equal amounts and they behave exactly the same way.

Zierler:

And what specifically in terms of your dissertation, in what ways did your work respond to these larger questions?

Harris:

My dissertation was a search for two different possible decay modes: I am looking for that particle to decay into a very specific set of other particles. And basically, if there is a prediction for how much that would happen, if there is, if particles and antiparticles have exactly the same decay rates. If I had seen either of those decays, then it would have actually been at a much higher rate than the standard model would have predicted. And that would have pointed to the fact that there must be a mechanism to change the balance between matter and antimatter. But I didn't see either mode. So my thesis result consisted of two different limits.

Zierler:

Who was on your committee?

Harris:

Who was on my committee? Yau Wah and Bruce Winstein, who I mentioned earlier, were on my committee, as well as Tom Witten, who is not in particle physics. Tom asked me so many questions at my thesis defense that at one point Wah said to him "you know, like, can you give her a break already." Tom was just really interested, so he kept asking.

Zierler:

Soft matter physicists really like particle physics, sometimes, I guess. Of course, you're continuing to contend with the two-body problem after you defend. So what are your opportunities and availabilities after you defend your dissertation?

Harris:

I started looking for postdoc jobs, but by then my husband was already a staff scientist at Fermilab. So I applied to lots of different places, but the idea was that my postdoc job would allow me to live near Fermilab. It is not unusual in particle physics for an institution to hire a postdoc, and then have that postdoc live near the laboratory. So I got a postdoc job from Arie Bodek at the University of Rochester. Actually there's a funny story about that too: it was the last postdoc interview I had out of at least 5 or 6. The way these postdoc interviews go, is that you talk to a bunch of people, you give a talk, you go out for dinner, and then they say, “thank you for visiting we’ll let you know, don't call us we'll call you.” After I finished giving my talk, I walked back with Arie into his office, and he hands me a job offer letter: it was the first job offer I got out of several, but the last job I interviewed for.

Zierler:

Obviously, they brought you out there knowing full well, they already made up their mind,

Harris:

It sure seems that way.

Zierler:

What was his research? What was he working on at the time?

Harris:

Arie was working on an experiment that used very high energy neutrinos to try and measure one of the fundamental parameters that governs the weak interaction. So there are several different forces that we know about in the universe. One of them we call the weak force, because we're not very creative with names, but it is much, much weaker than the strong force and it's the force that is involved when nuclei fuse to make, you know, higher atomic number of nuclei but with different protons and neutrons than what you started with, or when unstable nuclei decay and produce electrons or positrons, that's the weak force. There's a parameter of that weak force that you can measure in lots of different ways. And so one of the ways to measure it is by studying neutrinos since they only feel the weak force. The weak force can still manifest itself in two different ways: when a neutrino interacts, it can just bump into other particles, transfer some energy but leave as a neutrino, or it can change from a neutral particle to a charged particle. When that happens, the weak interaction also changes a proton into a neutron, or a neutron into a proton. Remember that’s how radioactive decay happens. If the neutrino transfers energy but leaves as a neutrino it’s called a neutral current, but if it changes to a charged particle then it’s called a charged current. The NuTeV experiment was designed to measure the ratio of neutral to charged current interactions, not only for neutrinos but also for antineutrinos. Both ratios, and also the difference between them are functions of the strength of an important piece of the weak interaction, it's called the weak mixing angle. The funny thing though is that this weak mixing angle can be measured in many different ways: it’s also related to the ratio of the masses of the charged to neutral force carriers in the weak interaction. And so the charged carrier (called the W) mass was being measured by CDF and D0 actually at that time and the neutral carrier--called the Z or the Zed in Canada, its mass had been measured really well at CERN. And so the question is, if you measure those two particle masses, then take the ratio, and how is that related to the weak mixing angle measured with neutrinos? If you measure the weak mixing angle in a totally different way, at a completely different energy scale with all different particles? Do you get the same number? And that would tell you first of all if the model was right, and second of all, is there some other thing going on? That's sort of a theme in physics, that's one way of trying to find new physics is you make very precise measurements, given theoretical predictions. And if you see very different numbers than the theory, then that means you've uncovered some new physics outside of the theory. When I was looking for these rare decays in my thesis, I was looking for some way that the theory was broken. When I was measuring this weak mixing angle with neutrinos and antineutrinos and comparing it to a different measurement of the same quantity, that was also a way to understand if there's something else going on besides this standard model of weak interactions. So on NuTeV, we used an old, refurbished detector and at the time, we had the single most precise measurement of this weak mixing angle. Since then, other experiments have measured the W mass much more accurately, but we actually did see something that didn't fit in with the standard model at several standard deviations. And, you know, the jury's still out on trying to understand what that could be from...

Zierler:

Right to this day, the jury's still out.

Harris:

Yes.

Zierler:

That must be automatically exciting when something doesn't neatly fit into the Standard Model.

Harris:

It was very exciting!

Zierler:

Deborah, did you see intellectually was this a very reasonable or seamless transition from your dissertation research the kind of work that you were doing at Rochester?

Harris:

The work was very different: it was a different experiment, it was a whole new area of physics to learn about, a different detector technology too. So there was definitely a seam there. But it was good, I think I think it's healthy to move from one experiment to another. And as painful as it is for me to do, I always encourage students I work with to do something very different in their postdoc positions. I was happy about the new job and new situation though: the new experiment, and the new collaborators.

Zierler:

And to go back to earlier in our conversation where it was obvious, you know, talking about doing experimentation during the pandemic, obviously, computers are central to this. And so I'm curious, you know, perhaps during graduate school, or as a postdoc, at what point for you, did computers really become front and center in the way that experiments were run, the way to understand data? Where, where do you see your early career in terms of those larger questions about the increasing role of computers in experimental physics?

Harris:

Well computers were already very important when I was a graduate student because we would use computers to analyze the data we took with our detectors. But the way we checked that the experiment was operating was quite different. When we were recording the data, we had to take shifts where we depended on a large array of devices that you had to actually look at to see that things were working, and then we wrote things down with a pen in a logbook. That was still the case when I was a postdoc. But then when I started working at Fermilab as a staff scientist on the MINOS experiment, in that case one of the detectors was all the way in northern Minnesota, and the other one was 100 meters underground. So you would take shift by sitting in front of a computer terminal in a control room on the 12th floor of the high rise, at Fermilab. The thing that's changed is that by the end of the MINERvA experiment, we were taking shifts by looking at a computer terminal that could be anywhere in the world, as long as you have the bandwidth and right computer security clearance to access the computers recording the data at Fermilab. So, you know, people could take shifts from Brazil, Mexico, Peru, Chile, or from the UK, we had people all over taking shifts.

Zierler:

How big was the group at Rochester that you were a part of?

Harris:

There was a professor, a senior scientist, myself as postdoc, and one or two graduate students whose terms were staggered. But other than the professor, the rest of us lived near Fermilab. I never lived in Rochester, but I would go there once a year for the Department of Energy site visits. When I think of “the group I worked with as a postdoc,” it was actually a group of 30 people, most of whom were based at Fermilab. I would only meet Arie when he was in town which was roughly once or twice a month.

Zierler:

Were you looking, you know, as you are transitioning out of postdoc, specifically to pursue a long-term career at Fermilab?

Harris:

When I was transitioning out of my postdoc job, I certainly wanted to continue doing research and teach if possible: that meant applying for jobs at Fermilab and at universities.

Zierler:

Did you ever look at any other national labs?

Harris:

I didn’t look at other national labs because there was plenty of interesting physics that I wanted to do at Fermilab, and again because I had a “two-body problem” as they call it: my husband was a staff scientist at the lab by that point.

Zierler:

But what would he have done if you got a university position?

Harris:

I only applied for university positions at places with large population centers where there were lots of other potential jobs, lots of other institutions. I think that today universities are more willing to think about dual hires and how to solve two-body problems than they were 25 years ago.

Zierler:

I asked about that, because, I mean, you could have applied to a Brookhaven, and there would have been a Stony Brook option or a SLAC to a Stanford option or something like that. So I was reading the broader question there as, given how much experience even at this point you already had at Fermilab if you had a sense of what was going on at some of the other similar types of national labs across the country?

Harris:

The science that was going on at Fermilab was exciting to me. This was 1999: Neutrino oscillation physics was starting to be a bigger focus at the lab that point. After the announcement in 1998 that the SuperKamiokande experiment was seeing that neutrinos really do seem to be changing flavors, we were trying to figure out how to test this in an accelerator beam. Fermilab was a very exciting place to be if you wanted to work on neutrino oscillations, and I just spent four or five years working on different measurements with neutrinos. So even though they're different measurements, there were a lot of lessons learned from the NuTeV experiment that I could bring into this neutrino oscillation experiment that was based at Fermilab.

Zierler:

This is kind of an aside kind of question. But you are at Fermilab, sort of right at the end of the death of the SSC. And I'm curious if you felt how that might have impacted, you know, for better or worse, what was going on at Fermilab or the culture at Fermilab, or the general feeling at Fermilab?

Harris:

Wow, that's a hard question. Um, I think you would have to ask my dad about that, he would have a much better sense of that than I have.

Zierler:

Because there was a feeling of dread when the SSC was getting going, that it might, it might really harm the long term, you know, prospects for Fermilab. So on this, I'm asking you on the flip side of that, when the SSC was dead and gone, how that might have affected things at Fermilab.

Harris:

Ah, so the reason I don't know the answer to this question is because the SSC was designed to do experiments of the type that I've not worked on since I was an undergrad. The SSC was not trying to do neutrino physics, it was planning to study high energy proton-proton collisions. But the point is that the kind of physics I've been interested in, for whatever reason, has been done by these smaller experiments and, you know, trying to get at the standard model by comparing different precision measurements in totally different arenas, as opposed to trying to look at higher and higher energy collisions.

Zierler:

How much did your day-to-day change? When you switched over from postdoc to being a staff member and associate scientist at Fermilab? Was it a big change? Was it a clear transition?

Harris:

Well, it was a clear transition because it was a completely new project.

Zierler:

Was that the timing of the hire? Was the timing of your hire relevant to the new project or had the project already been ongoing by the time you joined?

Harris:

The neutrino beamline was being designed in 1999 when I joined Fermilab. This brand-new neutrino beam line was completed and started sending neutrinos to the MINOS detectors in 2005. I was hired to work on understanding what are the requirements on the different components that create the neutrino beam so that you can make an accurate prediction for how many neutrinos as a function of energy you're actually sending to this detector located 450 miles away? There was a lot of simulation work at the beginning. At the time, I wasn't working on building detectors, I was working on simulating beam lines to figure out how are we going to test that everything in the beam line was working. When I did work on detectors, they were used to measure the protons just before they hit the target to produce the charged particles that decay into neutrinos. Then, when the charged particles decay into neutrinos, they also decay into muons. I also worked on detectors that measured the muons that were produced at the same time as the neutrino as another way of making sure the beam is making neutrinos the way you expect. These were very different detector technologies from what I was working on as a postdoc. I also had to worry about the cost and schedule for building the detectors, which was new for me. We did get to do a beam test in Brookhaven for some of these detectors.

Zierler:

There's a lot of exciting neutrino physics that's going on all over the world at this point. And so I'm interested in what ways your group or Fermilab in general was involved in collaborating and aware of what was going on, or was it more this was a very Fermilab specific kind of endeavor.

Harris:

I feel like we were all very aware of what's going on actually because we were planning such a huge change in the intensity of these neutrino beams. It’s funny: there is a conference series that started around that time called “Neutrino Beam Instrumentation”, and the funny thing about it was that it was very different from a normal conference. In a normal conference, you get up and say, “here's this analysis, and we finished it, and here's our fantastic measurement. And here are the uncertainties on the measurement. And here's how we did it, and how wonderful everything is.” But at this new workshop, people would get up and say, “here's what failed, here's what didn't work.” We had to build these devices that have to withstand, huge numbers of protons hitting targets and huge currents going through these magnets. So, things break and you want to learn from what everyone else got wrong: you don't want to break something that somebody else has already broken, right? You can learn from the dirty laundry that gets aired, and this was the place where that happened. There was a brand-new laboratory at the time in Japan, called J-PARC, and one of the main focuses of that laboratory was to provide the neutrino beam to the SuperKamiokande detector 295km away, all the way on the other side of Japan. Plus there was a neutrino beam made at CERN that was sent to detectors in Italy, another 730km away. So we were all having these conversations about what worked, what didn't work. So at least on the beam line side we were all communicating.

Zierler:

And as part of that larger worldwide neutrino community, where were elements of sort of competition or collaboration? In other words, was your group doing things that were very similar to what was going on elsewhere? And there was, you know, healthy competition in some regards to, to find new physics first, or was what you were doing at Fermilab, you know, totally unique in that regard?

Harris:

So healthy competition between neutrino experiments is something that is very much in the forefront of my mind right now.

Zierler:

Yes, yes.

Harris:

I would say that both then and now there's healthy competition. The technique might sound the same: you make a beam of neutrinos in one place, you send it hundreds of kilometers away, you detect it both right after you produced the beam, and also somewhere far away and see what happened to the neutrinos on their journey. There are several different choices you can make: you could try and make a neutrino beam that is very, very narrow in energy. So you put all your eggs in one basket, if you like, and measure neutrinos at this one energy extremely well. Or you could choose to measure what happens to neutrinos over a broad range of energies. Another choice is detectors: you can use very different detector technologies to try and measure the neutrinos on the other side of their journey. So I think there's healthy competition but at the same time I think the future of this field is in experiments that are so complementary that you shouldn’t only focus on them as competition.

Zierler:

You said earlier that, you know, the attraction to teaching was part of your motivation to apply to universities after your postdoc? In what ways if at all, did you have opportunities to teach within the Fermilab environment?

Harris:

I worked a lot on a neutrino summer school series. This started for me in 2002 when I was asked to teach at a neutrino summer school. I really enjoyed putting together these lectures and working with students. I realized then that I could get involved with that at a deeper level. I taught at many years at different schools after that, but I also organized them and worked out other kinds of student learning sessions during those schools.

Zierler:

Now, the initial neutrino project, you said that it started taking data in 2005. Yes. And that's exactly when you switched over to MINERvA.

Harris:

Ironically, yes, that’s at least when I started putting efforts into making the MINERvA experiment a reality.

Zierler:

Was that baked into the plan? in other words, you had done what you set out to do, and you would not be part of the data collection?

Harris:

Not quite: I took plenty of shifts to collect data on the MINOS experiment. The switch from MINOS to MINERvA was much more gradual than my switch from NuTeV to MINOS. With the second switch I changed to a whole new detector and a new beam line, and a new crowd of people to interact with. On the other hand, when I moved from MINOS to MINERvA I didn’t change beamlines and had several of the same collaborators. Also, one of the things I started working on when I started working on MINOS was how do we have to align the different pieces of the beamline to make a precise measurement? how well do we have to know the current that we're putting in the focusing magnet to make an accurate prediction of the neutrinos that are getting to the detector? But then I started worrying about the fact that there's a lot that we don't know about the way neutrinos interact in detectors. So then I started doing these sort of toy studies, you know, the computer equivalent of a “back of the envelope” calculations: given what we don't know about the way neutrinos interact, how is that going to affect the precision of the neutrino oscillation measurement that we make? That got me interested in understanding the way neutrinos interact in a nucleus better, which then got me interested in MINERvA. I think of it as a progression of what to worry about, you know what I mean? To make the best measurement of neutrinos changing flavors you can, there's lots of things you need to understand. And after worrying about the beamline, I started worrying about neutrino interactions.

Zierler:

I assume you followed, at least from afar, MINOS. What have been some of the long-term things that have been learned from the MINOS projects?

Harris:

MINOS made very precise measurements of the difference between two of the three neutrino masses, and it did this to a new level of precision with a near detector that was as they called it “functionally equivalent” to its far detector. I mentioned earlier that you can make a lot of choices in how you make these measurements, right? In Japan there is a long baseline experiment where the choice they made for their near detector was actually a series of several different detectors, none of which look like their far detector. So anyway MINOS and then NOvA after it did a lot to develop ideas about how to use detectors both near and far that function very similarly. So MINOS started and showed how far you can get with that strategy. I would say that MINOS also was the one experiment which could make a high statistics measurement of what their neutrino energy spectrum looked like week by week to see changes as they developed in a neutrino beamline. You don’t always know till later what caused the change, but when you start to see a change, you watch it like a hawk.

Zierler:

You gave a great explanation of your sort of gradual transition to MINERvA, at what point was that transition complete? And how well developed was MINERvA at that point?

Harris:

When I started on the project, MINERvA was already a pretty well-developed gleam in a couple of people's eyes.

Zierler:

But not you, you were not part of that mission?

Harris:

No, no I wasn't one of the two main proponents of the experiment. But I signed on very early and I became the project manager in 2005. That was when we were doing serious preparations for getting approval from the Department of Energy, but I was still taking shifts on MINOS and involved with the operation side of things in MINOS for a few more years.

Zierler:

And so this idea, you know, 2002, 2003, when it's just an idea, it's a glimmer in someone's eyes. What was that initial idea? That sort of gave rise to MINERvA?

Harris:

The idea was this: Fermilab has just spent all of these resources making this incredibly intense neutrino beam. And you need that really intense neutrino beam because you have a detector that's 450 miles away. There's just no limit, you want as many protons as possible to be sent down the beamline to make the neutrinos. And the thing is that neutrinos interact so rarely that if you put a bunch of other detectors in the same beam line, you weren't going to compromise the physics of the main program. So I think people realized a couple things all at once. The first thing they realized is, wow, we really don't understand neutrino interactions well enough to take advantage of all the statistics that these long baseline experiments are going to be collecting. So we better understand neutrino interactions better. And at the same time, it's like wow, there's this incredibly intense neutrino beam passing through this hallway, going into the MINOS near detector, but there's all this space in front of the MINOS near detector that doesn't have anything in it. So why don't we put a detector there, we wouldn't have to build another beam line, but we get a whole new neutrino experiment program out of it that will help make better neutrino oscillation measurements. So these two different people, Jorge Morfin and Kevin McFarland, both wanted to take advantage of this existing neutrino beam to measure neutrino interactions but in different ways. It turns out there is a hallway off to the side that leads into the main cavern where the MINOS near detector was. So in principle, you could put a pretty good-sized detector in that hallway, or you could put one in the middle right in front of the MINOS detector. And so there were two different ideas. And again, it has to do with this strategy of do you try and look at a very narrow band of energy of neutrino energies and make great measurements, but just at this one energy? or do you measure across a broad range of neutrino energies? And so those were two different ideas that were presented to the Fermilab Physics Advisory Committee. And they said to Kevin and Jorge: the physics seems really important, and those are both good ideas. But can you guys just join together and just propose one experiment instead? So that's what they did. That’s what became MINERvA.

Zierler:

Why join it together? Why was that a more fruitful way to go about this than keeping them separate?

Harris:

I think the reality is that at the time the community was not quite large enough to support two separate experiments to do very similar physics. One idea was: why don't we build a detector that's small enough that it can fit in both places?

Zierler:

Yeah.

Harris:

And we'll run in one place, and then we'll move the detector over to another place. Totally unrealistic. But that's somehow how the two groups of people got together. I don't know what the overlap was on the two different proposals, for example. But I think that the detector technology being proposed, may have been similar for the two experiments and the physics goals were also similar.

Zierler:

But it doesn't, Deborah, it doesn't sound like the issue was one of budgetary support, where people were looking to streamline and cut costs by combining it. It sounds like it was more substantive than that. Deborah Harris Well, that was certainly part of it. Proposing a detector, getting it approved, getting the funding, is really hard. And I just don't know that the community at the time was large enough to go through all that effort for two detectors. As it turns out we built the detector incredibly quickly. We wanted to get data as quickly as we could, because we were really the little fish in the pond. MINOS was this big fish that cost over a hundred million dollars that was running, and we were trying to get our detector in fast and take data before MINOS turned off. So we were under this big-time pressure, and the more expensive you are, the longer it takes to get approved. So there was just there's a lot of politics and, you know, pragmatic decisions. Even now when the community is larger, there are only small efforts in Japan, where people will say, oh, here's a really cool little detector, let's put that into this already existing beam line and see what we can learn. But those detectors are much, much smaller than MINERvA. They take much a lot fewer people to operate, and their physics program is much, much narrower.

Zierler:

Related question: How, how much bigger is the community now? In other words, all else being equal, would the projects have remained, do you think that they would have still been fused together based on the size of the community or have things really grown in the past 10, 15 years?

Harris:

I think that in the past 10, 15 years, certainly in the past 10 years, the neutrino community has become more and more aware of how little we understand neutrino interactions in nuclei because of all the surprises both MINERvA and the oscillation experiments have seen. But at the same time, the community really wants to understand neutrino interactions for the sake of oscillation experiments. So the community feels bigger now, but I think it's because but it’s not because they're proposing their own stand-alone neutrino interaction experiments so much as they are proposing near detector suites that look that much more sophisticated than what would have been proposed if we weren't worried about neutrino interactions, if that makes sense. So the near detector, for the DUNE experiment, for example, has several different components. And they all address different aspects of being able to make the precise prediction for the far detector that you need, given the statistics that the far detector has to collect to reach its physics goal. The thing about these neutrino oscillation experiments now is that we have to compare two small oscillation probabilities for neutrinos and antineutrinos, and the two probabilities are only like a few percent at most. So you're going to have to measure things to one in 1000, or better. So you better have a good handle on what your uncertainties are and how you’ll get rid of them with the right collection of measurements at a near detector.

Zierler:

[Laughs]

Harris:

And that means, you know, making measurements in as many ways as you can to constrain these models of having neutrinos interact. And you want to do that in several different ways. So you, different technologies can give you different pieces of that puzzle.

Zierler:

You were named spokesperson in 2010. So I'm curious, both to broader audiences and to technical audiences. What were the main things that you wanted to communicate about MINERvA?

Harris:

As spokesperson? A spokesperson has a different meaning inside the particle physics community than maybe what you're thinking. One detail is that I was elected, it’s not just an appointed position. So to me, spokesperson means somebody who worries about everything, who figures out the physics priorities, the operations priorities of the experiment, and then communicates in two directions. First, a spokesperson has to communicate within the experiment to make sure everything that has to get done has enough people working on it, and also has to communicate to the outside world: either the funding agencies or the lab management, to explain this is how well the experiment is doing, and here's where we need more help to achieve our mission.

Zierler:

I see

Harris:

You know what I mean?

Zierler:

Yes

Harris:

So the message that I wanted to communicate to the outside world is that we are small but mighty, we're the only ones doing this neutrino interaction physics, this physics is really important for these oscillation experiments. And at the time, when we were trying to get approved and get funded, I was saying things like, “Look--we are going to make measurements that will enable you to run your oscillation experiment for a lot shorter time and get to the same precision for your oscillation measurement.” Because if you, you know, if you have a certain statistical uncertainty and you have a certain systematic uncertainty, and then the total uncertainty on your measurement depends on both of those. And if you have big, systematic uncertainties, then you have to run a long time, so that your statistical uncertainty is small compared to the total systematic uncertainty. But if it's the other way around, like if you have really small, systematic uncertainties, then the longer you run more your measurement improves. So, anyway, we're making these kinds of technical arguments, like, “for the low, low price of, you know, $15 million dollars, we can cause this billion-dollar experiment with a significant yearly operating budget, to run two years less and get the same physics out, get the same precision on their measurement.

Zierler:

So, given that you're making this case more on an intramural basis, does this involve DOE? Are you working directly with DOE people?

Harris:

We had to get approval directly from the DOE people. Most of the MINERvA project was funded by the Department of Energy, although some was certainly funded by the National Science Foundation. We went through several steps, critical decision steps, where you go, and you first have to show that there's a mission. And then you have to show that you have an idea of the range of how long it's going to take and how much it's going to cost. And then you go through another review that says, okay, this is exactly how much we think it's going to cost. And here's really how much time we think it's going to take. And then at the end, you have another review, to say, “okay, we did it. And here's how much it costs. And here's how long it took. And here's the proof that we built the detector we said we would build.”

Zierler:

What was the tenor of the conversation, like, with DOE, with your counterparts there? Would they be physicists? Would they understand where you were coming from? Would they be more? They would be?

Harris:

Yes, they are physicists who do understand where we’re coming from. But I think there's a tendency in our field to be worried. I remember, at one point, during the review process, we were trying to prototype the detector, build a small version of the detector. We wanted to install the prototype even before we finished building the last part, right? If you build the modular detector, and the neutrino beam is already running, in principle you can install pieces of the detector and start taking data before you're done building the rest of it. And I remember, we were worried: what do we tell the DOE? Do we tell them we're gonna start doing physics before we get our final approval that the construction is complete? So we talked about this internally, and we thought, well, we're just going to be honest, right? We're gonna tell them, this is what we're going to do, it makes more sense from the flow of construction, and we'll get everything done just as quickly if we install some of it and start taking data. And so we presented this plan to the DOE, and I forget his name, but he was the guy who was in charge of the people in charge of the people in charge of reviewing us, and I’ll never forget, at the end of the review he said, “Oh, yeah, that's a great idea. Of course, you want to get to the physics.” I realized then that I shouldn’t have worried, their goal is also to do the physics and not to put that off because you have yet to finish this last review. You know what I mean? So, anyway, that’s a long answer to explain that yet, they are physicists.

Zierler:

When did you get involved with DUNE? When did that start?

Harris:

I certainly started thinking about what MINERvA measurements will do for DUNE a long time ago. I can't even remember when that started, certainly it was well before the experiment was called DUNE.

Zierler:

Yeah, I guess I mean, really? That's my question. How did DUNE originate out of MINERvA?

Harris:

DUNE originated out of the need for an experiment to measure whether or not neutrinos and antineutrinos oscillate the same way, and to see whether or not the masses of neutrinos are organized the same way as the other charged fundamental particles. The only way to do the first of these two measurements, given all the neutrino sources we know about, is to compare the difference between muon neutrinos changing to electron neutrinos, and muon antineutrinos changing to electron antineutrinos. For a long time the field looked into trying to do this measurement in the “other direction”, for example, starting with a high energy beam of electron neutrinos and measuring their oscillations to muon neutrinos, but it turns out that is even more challenging than the DUNE measurement. But this has been a progression of attempts to study these transitions. MINOS was proposed to measure precisely how much muon neutrinos are “disappearing”: that was really what motivated the detector design. For the NOvA experiment, the detector design was motivated by wanting to whether or not muon neutrinos even change at all into electron neutrinos. So you want to build a detector that sees electrons very well, which is a very different detector from one that sees muons very well. NOvA has now seen that muon neutrinos are changing to electron neutrinos in a narrow energy range. And muon antineutrinos are changing to electron antineutrinos in that same narrow energy range. Those measurements are both very exciting and they show that DUNE should have the precision to actually measure if those two probabilities are different or not, and if so, what are the conditions that make them different? DUNE will measure the difference between those two probabilities as accurately as possible and over a much broader neutrino energy range. And so I would say that DUNE came out of wanting to test this whole framework, from wanting to see this pattern of oscillations over a broad energy range and see if the neutrino oscillation probability is different from the antineutrino probability.

Zierler:

And what were the limitations there on not knowing before NOvA started how much muon neutrinos would oscillate to electron neutrinos?

Harris:

I can think of two questions you might be asking: if you’re asking how do you design an experiment to look for muon to electron neutrino oscillations without knowing what to shoot for, you have to be ready for anything. If that oscillation probability is very small like one in a thousand or smaller, you have to worry about all the backgrounds to the measurement, and how precisely you can predict those backgrounds. If the oscillation probability is a few per cent which believe it or not is large, then you have to worry about how well you have modeled the interactions that make your signal, and how well you can measure the incoming number of neutrinos as a function of energy. Either way you need to understand neutrino interactions. On the other hand, if you’re asking what the limit was on that oscillation probability when NOvA started, that’s a more complicated story. It turns out that if muon neutrinos change into electron neutrinos, that means that if you had started with an electron neutrino beam, then some of those electron neutrinos should have become muon neutrinos. So, there were experiments at reactors, which are copious sources of electron antineutrinos. And so reactor experiments were done looking for electron antineutrinos to “disappear.” Reactors make very low energy electron neutrinos, and if those become muon neutrinos, they won’t interact in your detector because they don’t have enough energy to produce a muon. The only thing you can do is run an experiment and see if you have fewer electron neutrinos at some distance than you expected. So there was a series of reactor neutrino experiments that didn't see any disappearance. And so they set a limit on how small the probability for electron to muon neutrinos is which is also a limit on how small the probability is for muon to electron neutrino oscillation. So when NOvA was being designed, we only knew the oscillation probability was smaller than ten per cent or so. Now we know that that angle is not only big, but it was like right around the corner from the limit that there was right when NOvA started. So given how big that angle is, that means that we have a chance of measuring this difference between neutrinos and antineutrinos. The fact that this oscillation probability is big like a few per cent is what inspired the DUNE program, because we knew, wow, we actually have a way to get to this difference. Like if you had to measure if the probability for muon neutrinos to change to electron neutrinos was like, one in a thousand instead of one in a hundred? An experiment like DUNE could not look for this difference between matter and antimatter. Once you see that the muon to electron neutrino oscillation probability was a few per cent, it's like wow, we can actually get to this understanding whether we actually could measure the difference and get to an understanding why there is more matter than antimatter in the universe.

Zierler:

And just to bring this story of discovery, right up to the present, how close are we to achieving this?

Harris:

Well, the beam is being designed, the detectors are being designed, and people are talking about starting to take data towards the end of this decade, rough starting in 2028. DUNE is also sensitive to seeing whether or not neutrino masses are organized the same as the charged fundamental particles: for charged fundamental particles, the heaviest one is much, much heavier than all the other ones. And so, for example, the top quark is extremely heavy compared to the bottom, which is extremely heavy compared to, you know, the charm quark. So for neutrinos, we know about three different mass eigenstates. And we know the two of them are pretty close in mass, and then one of them is far away in mass. But we don't know if the two that are close in mass are the heavy ones or the light ones, compared to the third one. One of these possibilities is called the normal mass ordering, because it's like all the other charged fundamental particles, and the other one's called the inverted mass ordering. Knowing that will help us understand why particles have mass in the first place. Because if neutrinos are really different from the rest of the fundamental particles, then that will tell us maybe that there is more than one way to generate a mass. Maybe there is more than one process that's responsible for giving particles mass. That's something that DUNE should be able to see that within two years of taking beam data, and, but seeing whether the underlying theory has neutrinos and antineutrinos oscillating differently will take much longer. You started this question with asking me how long it's going to take? We should understand this mass ordering question around two years after we start taking data. But understanding if neutrinos and antineutrinos change flavor differently because of the underlying mixing matrix that translates between mass states and flavor states, is something that we will not be able to measure precisely until the middle of, you know, the next decade.

Zierler:

I wonder if history repeats itself, Deborah, when, when the project starts taking data, you'll have moved on to a new project by then.

Harris:

It's an interesting question.

Zierler:

When did the opportunity in York, how did that come about for you?

Harris:

I saw this job opening in the fall of 2017. And the job posting was a joint position between Fermilab and York, and the position was 50%, to work on DUNE. So the research part of the position was something I was already gravitating towards, and I thought it would be great to have an opportunity to teach.

Zierler:

Right? Of course.

Harris:

They offered me the job in the spring of 2018. I accepted obviously, but I told York I wanted to start a year later, because I wanted my daughter to finish high school in the place that she started. And they said, “Okay,” so that's why I started in 2019.

Zierler:

And to bring it right up to the present, what, what do you want to do in terms of the near term, thinking beyond the pandemic? How much given that you're pretty well able to continue doing this work remotely? What are the things look like, you know, when and if the pandemic ends, you know, next year at some point?

Harris:

I want to go back to going to Fermilab and meeting with people there. Experimental particle physics is a very group-oriented activity. There’s this collective energy that people get from working together and meeting in person. I hope that it's not just a luxury of the past, you know, I really hope that we can continue to meet in person. I really also want to be able to test these new prototypes in an existing neutrino beam, and there's just not very many of those around. This chance to test the new prototype for the near detector for DUNE in the NUMI beam line will be fantastic. So that's what I plan to do once the pandemic is over, and we can cross borders easily again.

Zierler:

Well, Deborah, I want to ask for my last question, you know, we were talking before about, you know, the audiences for these for these interviews, and who accesses them in the Niels Bohr Library. And you did such a phenomenal job explaining, you know, the technical details of how these experiments really work. But I wonder for that broader audience, especially for high school students, or people just getting, are interested in a broader sense of the kind of work that you do. Can you explain what, what do we understand better about how the universe works by understanding neutrinos through these experiments that you've been involved with for so many years?

Harris:

There are a few different things we are learning to understand better. I guess I'm always more focused on what questions am I trying to answer now as opposed to which ones have we already answered. Can I respond to your question that way?

Zierler:

Absolutely.

Harris:

One thing we are starting to understand better now because of MINERvA is what is going on inside a nucleus. You learn in high school that there's this periodic table of the elements. And you learn that these elements have different properties, depending on how many protons and how many neutrons they have. Right? For example you learn that there are these noble elements, they don't interact, they're very stable. The thing you never really learn about is this: what are those protons and neutrons actually doing when they are inside a nucleus? Do they behave differently if they're in a carbon nucleus compared to a lead nucleus? People have been trying to study this using beams of electrons actually. But by using neutrinos, you are actually looking at those particles in a different way. And so it's like having a new lamppost to look under, and here and even though we think we understand carbon, this is an element that is around us all the time. But we don't know what's going on inside the carbon nucleus very well. How is it that those protons, that are all positively charged, they should be repelling, what are they doing while they are inside this nucleus? So that's the kind of thing that we're studying, and we can do this better now because we have this incredibly intense neutrino beam. Because the beam is so intense can actually make a modest-sized detector that is affordable, and we can get enough statistics to actually make some completely new measurements. So that's one thing we are starting to understand better. The other area that we are starting to understand better now are neutrinos and the role they play in the universe. We know that neutrinos are all around us because we know there's so many examples of particles that decay into neutrinos. The current best model of particle physics, which is so accepted it’s called the Standard Model, that model says that neutrinos don't and even can’t weigh anything. But in fact, neutrinos do weigh something and because there are so many neutrinos in the whole universe, they actually will have a gravitational effect, right, because they weigh something. And so in fact, one of the things that we know now about the way the universe works is that, like, even neutrinos even have something to do with galaxies--we wouldn't be seeing the pattern of galaxies that we see in the universe now, if neutrinos didn't actually weigh something. So we certainly understand better now than when I started graduate school that neutrinos are not what we thought they were. All my textbooks said that neutrinos did not weigh anything but could be used to test predictions of the weak force and the Standard Model. So because neutrinos weigh something they can change flavors, and those changes are like a new lamp post we can look under, we're trying to see, you know, as much as we can about the universe by studying precisely how much neutrinos weigh and how they can change their identities.

Zierler:

And that's always an exciting thing, when in the world of experimentation, you come up upon something for which the theory really has nothing to tell you.

Harris:

Right? I mean, right. The fact that the experiments are leading the theories is what's been going on in neutrino physics for quite a while.

Zierler:

Why do you think that is? Why do you think that in neutrino physics, the experimentation has been running ahead of the theory?

Harris:

One possibility is just that the theorists are trying to make patterns of what they already know. So the theory was predicting, well, if the neutrinos did change, they would change kind of like the quarks do, which means that they don't change very much. The probabilities of changing from one kind to another would be very small, and you'd have to be able to measure probabilities like, you know, one in 1000, or 1 in 10,000, 1 in a 100,000. Because those are the probabilities of how much quarks change in the weak interaction. But in the neutrino world, some of these probabilities are huge. It turns out that you can put a detector up where 95% of your muon neutrinos have changed into tau neutrinos. But the short answer is that theorists are trying to find patterns and apply them to neutrinos, and neutrinos write their own rules.

Zierler:

Well Deborah, it's been so fun speaking with you today, I'm so happy that we were able to connect and that you were able to share your perspective and insights over the course of your career and, you know, most importantly, is to convey this ongoing sense of excitement about all that remains to be discovered, you know, for a long time to come and how, how real it feels that you and your group are absolutely on that path. So it's wonderful that we were able to do this. I want to thank you so much.

Harris:

Thank you very much for your interest.