 All right, I'm gonna go ahead and get us started today. So I'll stall for time since I know people are still eating food downstairs. But the first thing I wanted to say is just, to all of you with an earshot, I wanna thank you for getting through this tough year. I think all of us can look back at this year and go, this was not the year we expected it would be. It was still kind of mixed in person and not. There were a lot of stressors and students and staff on faculty, but I really appreciate the effort that every put in, and now that we're in, or getting into the finals period that you're now going to have to put in as we go through final exams. One of the exciting things always in a department is to welcome new young people into the field. They're gonna be the future of the field and we were very pleased to welcome three new postdoctoral researchers into various programs in the department this year. So we had Joe Ryan working with Joel Myers in the cosmology area. And then of course we have Xi Jinping and Yi Lin Yang working on Atlas. Yi Lin is now relocated to CERN, so that's exciting where people are starting to move back to the places that we had to, you know, free from during the pandemic. So that's something we expect to see more of during the summer. It's also been a year of some goodbyes as well, but for good reasons. So we saw our research faculty members in the Opti-Electronics Lab, Datal Gong, and now Andy Liu, both got really nice positions at national labs. And of course they were offered those positions and said, yes, this is what we want for them is to keep moving up. So Datal has started already at Fermi National Accelerator Lab and Andy will be joining Brookhaven National Laboratory. And we just got news from Joel that Wichigo Cynthia has been offered and accepted a Center for Astrophysical Surveys or CAPS postdoctoral fellowship at the National Center for Supercomputer Applications at University of Illinois-Urbana-Champaign. This is a very, this is excellent, right? I guess no other way I can say it. These are competitive positions. And I think it's important that we celebrate all of the successes of these people. So let's do a round of applause for all the new faces and all the faces moving up in the world. All right, now to the main event. All right, so I am very pleased today to introduce our distinguished speaker. Bonnie Fleming is a professor of physics at Yale University. She earned her PhD at Columbia University where she studied proton structure, something near and dear to the hearts of many people in this department by observing the interaction of neutrinos and nucleons using the new TEV experiment at Fermilab. She earned a Letterman Fellowship at Fermilab where she then worked on Miniboon, another experiment there, studying the changing of one neutrino species into another. This is known as flavor oscillation. And she earned a prestigious in competitive national science foundation early career award in 2006. Professor Fleming is the founding spokesperson of the Microboon Experimental Collaboration at Fermilab. And she served until recently as its scientific co-spokesperson. She's a recipient of the 2018 American Physical Society's Division of Particles and Fields Mentoring Award. And as an example of her outstanding mentoring, she initiated using the NSF Career Award, an organization called Girls Science Investigation through that award. And this organization provides a Saturday program for sixth to eighth grade girls in New Haven where they engage in hands-on science experiments at Yale, guided by a team of volunteer mentors. And one of the things she shared with me today is that she's currently working on a book about the curriculum for that program to be shared more widely about how to pair experiments and so forth to engage young people in these investigations. So we are very pleased to be able to welcome Professor Fleming to give the spring colloquium today, a real leader in the field of neutrino physics. And her lecture today will be on the subject of the so-called phantom particle, the neutrino, and the lessons about physics beyond what is known that it can present. Please welcome our distinguished speaker. Thank you for the lovely introduction. Still, can everyone hear me? Yes, and that's good. And I'm pleased to be able to be here today. This is the second time in the, I wanna say post COVID era when I've been fortunate to visit another institution and it's a joy to see people in person, to talk to young people who are studying undergraduate, had lovely lunch with undergraduates and graduate students today. And so it's really my pleasure to be able to visit and to talk about neutrinos, which I absolutely love to talk about. So that's what I'll be talking about today. Please stop me if I'm going too fast or too slow. And I'm happy to answer questions in the middle of the talk and at the end. So as Steve said, I study the phantom particle. Neutrinos have presented many puzzles to us right from their postulation by Wolfgang Pauly, which I'll mention in a little bit and even through to today or on the microboot experiment, we look at short baseline neutrino oscillations to try to understand neutrinos. And even beyond that, we're getting into an era, we're really in an era where precision neutrino physics can teach us about the rest of the universe. So using neutrinos to teach us about things other than neutrinos, which is I think a turning point for neutrino physics. And I'll talk about that in the context of the Dune experiment closer to the end of the talk. Okay, so let me get started. Nope. So I was looking so fancy, pressing the button. All right, now I'm not a Mac person. So you don't want me touching the second. I think if you just click on that slide, let's see what happens. Okay, and then let's try it again. All right, excellent. Good, got the snafu out of the way right in the beginning. Okay, so what do we know about neutrinos? We can start with what we know about the universe, which is really not that much. Most of the universe is made up of things that we call dark because we don't know them. 70% of that is dark energy, which we know is there because we can see the universe expanding and accelerating. And that's about 70%. 25% is made up of what we call dark matter. Again, dark means we don't know what it is, but we know it's there because we can watch the rotation of the galaxies and see that it must be there. And there are people like Professor Cooley in your department and one of the graduate students I spoke with at lunch actively looking to figure out what that dark matter is. What do we know? We know that 4% of the universe is made up of ordinary matter. And that is what particle physicists like me do try to understand that measly 4%. I focus on the neutrinos. Neutrinos are special in the ordinary matter in that they are second in abundance, second only to light particles. So they're really everywhere. I love doing this. Everybody hold out their hand. Count to three, one, two, three. Three trillion neutrinos just pass through your hands. So through each of your hands. So there's really a tremendous number of neutrinos in the universe. They don't interact much, which makes them challenging, but luckily at least there are lots of them, both so that we can study them. And as I said before, so we can use them to tell us about the rest of the universe. Okay. How do we actually figure out how neutrinos work and frankly, other particles? My job as an experimental physicist is to break the world down rather brute force using particle accelerators to the tiniest building blocks of matter. And when we come up with something that, whoops, that we can't, when we come up with something that we can't break apart anymore, we call that a fundamental or elementary particle. So what are the fundamental and elementary particles? We have 12 building block matter particles, six quarks and six leptons. And then we have the force carriers that help them talk to each other. So I'll focus for this talk on the neutrinos. They make up three of the 12 building blocks of matter and they're associated with their charge lepton partners. So for the electron, there's the electron neutrino, the muon, there's the muon neutrino in the tau, there's the tau neutrino. Let's try going over here. Neutrinos are tiny. When I was a graduate student, we weren't sure if they even had mass at all. We now know that they do have mass, but they're at least extremely tiny. They're neutral. That makes them challenging to see in a particle detector. We see particles when they deposit light and charge. Neutrinos do neither one of those things. And they're weakly interacting only and they don't call it the weak force for nothing. It's weak. It means that neutrinos don't interact that much. We can see a neutrino through a neutral current interaction like a billiard ball when a neutrino comes in and a neutrino comes out. And we see what happens when they've hit whatever target you've smashed them into. We also have a very forgiving flavor-changing interaction, a charge current interaction where a neutrino comes in and out comes the partner lepton particle. So if it was an electron neutrino in, it's an electron out. And this is very forgiving because it allows us to tag a neutrino's flavor. It's the only way we can tag a neutrino's flavor by looking for the charged lepton partner that we can see that comes out of the interaction. How did we even discover neutrinos to begin with or how were they even postulated to begin with? This is the first puzzle of neutrinos which have been puzzling and frankly surprising since their inception. In the early 1900s, scientists were measuring nuclear beta decay and they saw a proton and an electron come out of a neutron decay. They expected it to be a two-body interaction where they would see a mono energetic energy spectrum. And instead they saw less energy than they expected and it was a continuous distribution as if something was stealing away the energy. And Niels Bohr is famous in this case infamous for being ready to abandon the concept of conservation of energy which is definitely one of our main tenants in physics. So Wolfgang Pauley came along and proposed what he called a desperate remedy, a particle that we couldn't see and that we couldn't measure that was stealing away some of the energy. And he even called it the neutron which was then discovered and renamed shortly thereafter. And eventually Enrico Fermi came along about two years later and laid out the theory of the weak interaction which predicted the neutrino. And then it was dubbed the neutrino which stands for little neutral one which is exactly what it is. But he famously said, I have predicted something which shall never be detected experimentally. Fortunately, we now have many, many, many neutrino experiments across a broad range of energy scale and across a broad range of size and technology that measure neutrinos from the lowest energies to the highest energies. So here's cross-section for interaction as a function of energy. At the lowest energies, there's neutrinos which we have not yet detected but which we think are leftover from the Big Bang. We call them relic neutrinos. It's like the cosmic microwave background, but for neutrinos, very hard to detect them. At the highest energies, we have neutrinos that are produced by the highest energy cosmic accelerators in the world. And in between, we see neutrinos coming from the sun, from the center of the earth, from supernova. We produce neutrinos and reactors on earth in particle accelerators on earth and much in the way they're produced in particle accelerators, they rain down on us from the atmosphere. And each of these words is the name of an experiment that uses that variety of neutrinos source. So just to emphasize, thankfully, Pauly was not correct as an experimental neutrino physicist. There's many different ways to detect many different kinds of neutrinos that teach us many different things about neutrinos and about the universe. Okay, what is a fundamental property of a particle that a particle has mass? And whether or not the neutrino have mass was something that we didn't know when I was a graduate student. When I was a graduate student in 1998, the super-Caminconde experiment produced results that hit the front page of the New York Times. When does that happen in physics? Only now and again, that was convincing evidence that neutrinos had mass. However, it's a little bit more complicated than that. We couldn't measure neutrino mass in the usual way by looking by conservation of energy or conservation of momentum in an experiment. We had to look for a quantum mechanical effect or a behavior that was associated with mass. And that is neutrino oscillations. So if neutrinos oscillate from one flavor to another flavor, frankly, like the corks do in the weak interaction, then we can say that they have mass. And that's what experiments did. They saw convincing evidence for neutrino oscillations. And now it's something that you probably learn in your particle physics class, neutrinos have mass. So what does it mean that neutrinos oscillate? You can start with one flavor of neutrino, let it travel over some distance or evolve over time and there's a non-zero probability that it changes to another flavor of neutrino. And as an experimentalist, you put your detector somewhere along this distance, or we call it baseline, and measure what number of your original neutrinos have disappeared and what flavor of neutrinos have appeared. And it's that way at different energies and different baselines that we've, over the last couple of decades, mapped out the parameter space for neutrino oscillations. So neutrino oscillations depend upon the neutrino flavor, which you can see here. This is an example of a new mu oscillating to a new tau and the neutrino energy dependent upon the energy of the neutrino. And oscillations happen over different baselines or distances and many different neutrino experiments have a whole variety of different distances, short baseline, long baseline, et cetera. You have your own beautiful, beautiful picture. I had to stop Steve when he walked me across campus of neutrino oscillations. Here's that great wave sculpture in front of the museum that I just couldn't help but sit and stare at. Looks just like a neutrino oscillation. This is really new, I've said that already. When I was an undergraduate in my Griffiths great textbooks, Quantum Mechanics textbooks, there was the statement that present, this is a highly speculative, there's no experimental evidence for neutrino oscillations. And that's what my undergraduate advisor told me, don't go into neutrinos. There's a whole bunch of crackpots in that field who thinks that neutrinos oscillate. So anyway, so let me say a few more words about neutrino oscillations and how we derive the oscillation probability formula. It's very handy as a neutrino physicist. First, imagine the world where we have only two neutrinos, two-dimensional world, and you can describe neutrino space as two orthogonal vectors. You can equally well rotate that representation. So instead of new one and new two, you can just completely describe the space as new alpha and new beta. Then I can write the two different representations, alpha and beta and one and two in terms of a simple mixing matrix that depends upon the angle between those two representations. And I can time evolve an alpha state that's written in terms of the combination of new one and new two and see that at a later time, upon mechanical time evolution operator, there's a non-zero probability that the new alpha has become a new beta. That's the basic idea. And this is the relevant formula that you get the oscillation probability formula. I won't spend too much time on it, except to say that it depends on the mixing angle, so the amount that the neutrino mixes between these two different mass representations and flavor representations. And it depends upon the difference between the mass states, delta M squared. Those are two things that Mother Nature has decided. And then there's the baseline L over which you let the neutrino travel before you see if it's oscillated and the energy of the neutrino E. So there's two things that experimentalists can control, the baseline and the energy in order to probe delta M squared sine squared two theta space and see where there actually is a delta M squared and a sine squared two theta that corresponds to an oscillation. Okay, this is a Nobel Prize winning result. And in 2015, the Nobel Prize was awarded to Art McDonald and to Kai Kajita for the snow experiment and for the super chemistry and conda experiment for the convincing evidence that neutrinos mix and therefore have mass through this phenomenon known as neutrino oscillations. And we have now this well understood picture of three neutrino flavor oscillations, new mu, new E, new tau and how they oscillate and what delta M squared and sine squared two theta's. There's still lots of questions to understand. We don't know where those delta M squareds are compared to zero. So we don't actually know the mass of the neutrino, we just know their mass differences. And we don't know whether or not the mass differences, the smaller one is closer to zero or farther away from zero, that's mass hierarchy. We don't know whether or not neutrinos and anti-neutrinos oscillate at the same rate. And I'll talk about that at the end of the talk but it's enormously interesting because maybe it gives us a hint of the differences between matter and anti-matter. We don't know the nature of neutrinos, we don't know if they're Myrona particles or Dirac particles. And we don't really understand how neutrinos fit into the cosmos and what neutrinos can tell us about the cosmos. We know some but not everything. And as always, as an experimentalist, we look for beyond the standard model hints for what neutrinos can tell us about themselves or about other parts of the standard model exotic phenomena. And the first part of my talk where I'll talk about microboon is looking for beyond the standard model physics, specifically new particles. Okay, so that's where I start with the story of what we call the short baseline anomalies. I told you already we have this great picture of three neutrino oscillations from accelerator-based experiments, experiments that look at neutrinos from the atmosphere, experiments that look at neutrinos from the sun. We have this really beautiful picture. We also have these lingering anomalies from experiments that do measurements at relatively short baselines, somewhere from 10 meters to a kilometer. Those are the relatively short baselines. And they come both from accelerator-based experiments and experiments that use neutrinos that are produced at reactors. And in terms of accelerator-based experiments, there was this experiment that ran in the 1990s at Los Alamos called LS&D. And they observed an excess of electron-type neutrinos in a muon-type neutrino beam at 30 to 50 meters baseline. The mini-boon experiment followed on to try to address LS&D and also saw an excess of electron neutrino events, but in a little bit of a different energy range than was expected by LS&D. So a continuing mystery that I'll say a little bit more about a little bit later. In terms of the reactor neutrino experiments, these experiments looked fine for frankly decades and then refinements in predictions for how many neutrinos were coming from reactors. So the incoming flux moved those experiments away from the prediction. So these data points here are all data that was taken like when I was in elementary school and became an anomaly when we better understood some of the inputs to those experiments. Okay. So let me try to sum this up in terms of the fundamental parameters and the experimental parameters that I talked about before. When we talk about neutrino oscillations, there's that formula again. We talk about delta M squared versus sine squared two theta space. The experiments that look at neutrinos from the sun, like the Nobel Prize winning snow experiment are down here. And from long baseline experiments, like the Nobel Prize winning supercammy conduct experiment are here and the short baseline experiments are up here. So for a while, like when I was a postdoc, there were theorists trying to reconcile these with our three neutrinos. And then the phase space for these experiments got smaller and it didn't work anymore. And the main point is one of these delta M squared is 10 to the minus five. The other is 10 to the minus three. And those two are never gonna add up to one. So somehow we had too many delta M squareds. And the best resolution to this, and here you can see the different delta M squareds, one there, one there, then the big one here, is to suggest the most convenient way, best way, if there's the theorists in the room we'll argue if I say best, is to propose a new particle, which people call the sterile neutrino. So somehow there's a neutrino that doesn't interact weekly, but can oscillate between the standard model neutrinos. And so the suggestion for the reactor neutrino anomaly and for the accelerator-based neutrino anomalies at short baseline was that perhaps there's this new particle called the sterile neutrino. If you look at those experiments here, I have them in a table, LS&D I mentioned, MiniBoon I mentioned, the reactor experiments I mentioned, the significance of those experiments to suggest a new neutrino is all at the two to four sigma level, which is not very convincing, but they could all be explained in the same scenario of having this one extra, or even two or three extra sterile neutrinos. So the point of the microboot experiment and follow-on to the reactor experiment were to try to do a better job of looking to see whether or not those anomalies were real or not. In terms of the reactor experiments, there is a continuing worldwide effort to reproduce those reactor neutrino, typically disappearance measurements from a variety of different kinds of reactors, so different fluxes of neutrinos coming in and a variety of different kinds of neutrino detectors. And I would say many of these experiments have shown that there is not anything to write home about there, and some have shown hints that there could be a sterile neutrino. So the jury is a little still out a little bit on that. In terms of addressing the MiniBoon low energy excess, that was the purpose, is the purpose of the microboot experiment, which as Steve mentioned, I served as the founding spokesperson and the co-spokesperson up until February. So I've been a little freer as of late. Not that we've answered all the questions on microboot and we still don't know what the MiniBoon anomaly is, but we have ruled out some of the most common interpretations, but I'll get to that. So here's a plot of the money plot from the MiniBoon experiment, which shows number of events as a function of energy and the stacked different colors are the different backgrounds. And you can see there's an excess on the order of five sigma above this, rising as you go to lower energies. Here you can see that same excess when you look as a function of the direction of the outgoing particles in the neutrinos in the detector. The problem with MiniBoon is that the detector, a big vat of oil, Trenkov imaging detector, a lot like super K-May conda, where you see rings in the detector, you can't distinguish between electrons and photons. And these backgrounds in particular, that orangish one are all single photon backgrounds. So you have an excess of signal events that could be photons on top of a rising photon back and scientists were rightfully very skeptical about that. So the purpose of the MicroBoon experiment is to try to sit in the same beam as MiniBoon, but use the right detection technique to be able to do precision physics that can to first order differentiate between electrons and photons and say, is this excess still there? And is it electrons? Which would mean that it came from electron neutrinos and therefore an oscillation, or is it photons? And which could also be very interesting beyond the standard model physics or could be a background that's not understood. Okay, so I mentioned that you want the right detector technology to do that. So part of the story of MicroBoon and the story of Dune as it turned out is developing a new kind of technology in neutrino physics. Neutrino physics in the 1970s was dominated by bubble chambers. You've probably all seen the beautiful pictures of bubble chamber events where you can see particles in the detectors, the bubbles from the tracks that they leave, many of them with magnetic fields. So you can see them curl in the magnetic fields. I think this is the 15 foot bubble chamber that ran at Fermilab in the 1970s. The problem with these, albeit beautiful detectors, is that they're very hard to scale to larger sizes. Where's the mechanical engineer? I talked to at lunch about this. So very challenging to scale these detectors to larger sizes. So in building bigger and bigger neutrino experiments, people move to things like water-trenkov imaging. And here's a picture of the super-camry condi detector. If any of you have seen the high-rise at Fermilab, it can fit inside this detector. It's a huge detector, but not very precise. You can only measure the neutrino interaction point in the detector to within something like 20 centimeters. And you're looking at these fuzzy rings. Again, where it's difficult to differentiate electrons from photons. Tracking detectors came after this. This is an image of the NOVA detector, which is more precise because you can track in a centimeter scale volumes of liquid scintillator across the face of the detector. This is also absolutely huge. I think you can fit a couple of airplanes and like the Taj Mahal in there or something like that. So thank you. But the Taj Mahal on a couple of airplanes is a nice image also. Okay, 60 meters on. And what we move to with the microboon experiment, and here's a picture, bubble chamber-like picture of an neutrino interaction in microboon, is a liquid argon time projection chamber. So back to the precision of the era, the era of precision of the bubble chamber detectors, but scalable to very large sizes. And here's a schematic of the DUNE experiment, which is 15 meters by 15 meters by 60 meters long. And that's one of four modules of detector. And you can see for scale a blow up of the corner of this and there's the person down there. So we capitalize on the precision of bubble chamber detectors. We move to something that's easier to read out that gives us calorimetry, which is really important. And that can be very large. Okay. So how did a liquid argon TPC come about? Belka Radica and Bill Willis wrote a seminal paper in 1974 that suggested reading out charge in liquid argon without amplification. So really that's a challenge in low noise electronics to first order, where typically when you're reading out ionization charge, you need to amplify it, you can't do that in liquid. Dave Nigren proposed the concept for a time projection chamber several years later in 1976 and Carla Rubia wed the two ideas to suggest a liquid argon time projection chamber for neutrino detectors sometime after that, a couple of years after that. So how do these detectors work? You have an incoming neutrino into a big vat of liquid argon on the order of meters wide and many meters long. The neutrino interacts with the argon and produces charged particles. Those charged particles travel through the argon and knock off ionization electrons as they go. So these tracks are tracks of ionization electrons directly along the path of the charged particles. You set up a big electric field, big like 500 volts per centimeter, really big, and you drift those ionization electrons to the edge of the detector and through planes of wire, charges induced on the wire and you read out the charge and from that you reconstruct a 2D image of the event which you can see in these waveforms here and then you project back in the Z direction to figure out in 3D where it was in the detector. That's the projection part of it. You know, when the beam goes through, that's your T zero. You know how long it takes to drift to the wires because you set up the electric field and therefore you can project a 3D image of the event. Okay, nonetheless, seems like a simple concept. It took a long time to get all the ingredients together to make the detection technique work. In the 1980s and in the 1990s, the efforts were primarily in Europe through Karla Rubia and the Icarus collaboration to start building test stands to make it work and eventually put small devices, 50 liters in neutrino beams to try to see interactions. In the United States, there was some early work in the 1970s and then nothing until the 2000s when people became interested in building these detectors for neutrino physics. So some of the challenges are achieving that high voltage, getting ultra pure argon. You need ultra, ultra pure argon, tens of parts per trillion impurities and no larger so that when you drift the ionization electrons to the edge of the detector, they won't get gobbled up by oxygen or hydrogen impurities along the way. And you need to keep develop the cold, low noise electronics to be able to pick up the very small signals. So we did that with a test stand in 2007 when I first got to Yale to see first tracks, embarrassing tracks compared to what had already been seen, but it turned out to be the same month in which first results from Miniboon were presented. So April of 2007, we see this confusing excess of events at low energy. And we also saw first tracks and technology transfer in my lab at Yale. And that was the birth of the microboot experiment. Could we actually use this technology, which at the time had been developed and on test stands, but had never been used for physics experiment. And there were still these lingering questions about high voltage electronics, purity. And could we combine the two ideas into a physics and an R&D experiment? So physics to understand if the Miniboon low energy excess were something interesting like a sterile neutrino and R&D, could we develop this technology, not just for microboon, but potentially for beyond? For example, for the Dune experiment, which turned out to happen. So let me now get into the details of microboon. I can pause for a minute. Anybody have any questions, comments, thoughts before I start talking about microboon? I'll start with the physics of the experiment, pepper some technology along the way. Microboon is named as such because it runs on the booster neutrino beam line at Fermilab. Here's a picture of Fermilab and the booster neutrino beam line is here. The Miniboon experiment is here and microboon is just upstream of it. It's kind of a terrible name, I have to admit. Micro, the idea is that it could be an order of magnitude smaller than Miniboon, but still more sensitive than Miniboon. And that's sort of true, always confusing to say microboon and Miniboon, but so it goes. Here's a picture of the Miniboon detector, which was 800 tons of liquid simulator, turn off imaging detector, and the microboon liquid argon TPC as it was being carefully lowered into the building after it was constructed elsewhere on site. The key to microboon or one of the keys is to be able to use the same change as little as possible for Miniboon except the detection technology and see if we see the same thing or not. So that meant being on the same beam line, as I said, just upstream of Miniboon and seeing a muon neutrino beam, which is primarily muon neutrinos and very small component of electron neutrinos or half of a percent of electron neutrinos so that we can easily understand if we see electrons appearing in the detector, they're coming from neutrino oscillations above the intrinsic electron neutrinos produced in the beam. Here we are when we started microboon, here's our publication record, which I like to show, it's a nice curve, culminating in our signature results for electron neutrino appearance and photon, first photon neutrino appearance results last October. So I'm sure you may have heard about it already. We tried to make a splash. First, we had to, from the R&D side of the experiment to actually make it work. And that started in June of 2012 where we constructed the time projection chamber, you can see a picture of it here, installed the detector filled with liquid argon, turned on the detector, that's the day we turned the high voltage on, it was a bit of a nail biter and started taking neutrino beam data, which we took for five years until we, while analyzing, until we presented first results. Okay, as I said, lots of R&D along the way. Can electronics have low noise? And here's the noise level of the electronics as a function of temperature and they're particularly good at liquid argon temperatures, which is nice. Can we keep the liquid argon pure over time? We did great at this. This is at least six times better than how we thought we would do. That's great because we couldn't raise the high voltage to our nominal high voltage, which felt like a big setback in the beginning. We were supposed to run at 128 kilovolts, we never got above 70 before the detector tripped off. Thankfully, so we were drifting a lot slower than we wanted to. Thankfully, the purity was so much better that it counteracted the fact that the drift was so slow. After we finished data taking, we did crank up the high voltage. Finish the physics run, see if we can crank it up. Who cares if it's first order if the detector breaks? And we got to nominal voltage. So we learned something there too. Okay, and what we got are these beautiful event images. I have to say they speak for themselves. I love showing them, they're absolutely beautiful. They're much different than the kind of fuzzy rings to interpret the neutrino interaction from a chair and craft imaging detector. You can see not only the charge particles coming out in straight tracks like muons versus showers like electrons and photons, but going beyond the bubble chamber era events, you have calorimetry. So you can see the red parts are where you have, for example, highly ionizing protons that comes out of the vertex. In the case of a big cosmic ray shower, you have also enormous amount of energy deposited, even though it's a showering event. So you can see lots of great details. You can see a photon produced here and converting here into an E plus or minus pair. So the key on MicroBoon was to take advantage of this beautiful imaging and tell the difference between an electron neutrino candidate and a photon candidate. And you can take advantage of a couple of different things in trying to do that. The first is topology. In the case of an electron neutrino interaction coming from a neutrino, you'd see the outgoing electron attached to a proton. In the case of a single photon event, the photon travels for some distance away from the vertex tagged by the proton in some cases, and then pair produces. So this helps you differentiate between electrons and photons. In addition to that, you can use calorimetry. So, oops, I did it in the wrong order. You can look at just the electron versus just the converted photon. And before they evolve into a shower, you can look at the first two centimeters and see that the energy deposited in an electron is about one mip, and a photon, which is an E plus or minus pair, is about two mips. So even without the added benefit of the topology of the rest of the interaction, the idea with liquid argon is you can use calorimetry deposited energy to tell a single photon from a single electron, which works pretty well. Okay, so let me sum up what our goal was. Here's the mini-boon signal shown again. This is either electrons or photons, can't tell the difference. When we wrote the micro-boon proposal in 2009, our goal was to differentiate photons from electrons. What we actually presented a number of years later, last fall in first results, was differentiating many, many, many different channels. There's nine channels here. The first series of our results presented the channel shown in yellow, and we still have a lot to do with these other channels. So what we actually learned, thankfully, was that the technology gives you much more than just separating electrons from photons, but a really rich variety from the hydronic side of different channels that can help us tease out what the low energy excess is, we hope, and means that there's a wealth of interesting cross-section measurements to do as well. So I'll show you this at the end of the talk too, with a sort of theory rundown of what they are and might be telling us, but we in the end learned a lot more than we thought, which is a nice thing in science. Okay, so let me say a few words about the results. When did I start? At three? Okay, so I have another 20-ish minutes left. Okay. Okay, okay, okay. I just thought, I don't even know when I started. Okay, so our first results focused on one analysis that tried to interpret any excess we might see as single photons, and three different analyses that looked for any excess we might see as single electrons and therefore from an electron-neutrino interaction. So the single photon analysis is slightly model dependent. It assumes that any single photons we see are produced from an interaction where a proton is excited to a delta state and the delta state decays into a single photon. And we didn't really expect, or many would have been very wrong in their own calculations if that would have seen an excess, and we did not see an excess. At the moment, we're doing many more analyses to make the single photon channel more inclusive. How can we do analysis that looks for a single photon event produced from a neutrino interaction in any way at all? The electron, and here are the results of that, where you have two different channels, one where you see just a single photon in the detector and another where you see a single photon with one proton coming out. It's both nice and challenging that the hadronic side of these interactions are very rich in physics in terms of the physics of what you can learn. What do I mean by that? Protons can be absorbed in the nucleon before they can exit. So you might have an event that ought to be a single photon and a single proton, but you don't ever see the proton. You can also get other stuff coming out of the nucleus like pions and things like that. That allows us to separate into these different channels, but it makes it a little challenging to do the physics. In both of these cases, if you look at the fully constrained side, you can see that the data points agree with the prediction, which means we do not see an excess of single photon-like events in this scenario where they were coming from a delta production to a single photon and gamma, as we say. Going back to the electron appearance results, we had three different electron analyses, that focused, that had three completely different reconstruction techniques, which is nice for checking things in an independent way, and three different ways of looking at the data. The first was the most exclusive, a single proton and a single electron, and this analysis took advantage of a number of new deep learning techniques. The second analysis was semi-inclusive, which is really electron neutrino appearance without any other pions, which is the closest to what Miniboon was actually measuring as well. And the final is fully inclusive, so an electron and anything else in the event. So what's the answer? This is the answer. All three analyses, the most inclusive, the semi-inclusive, and the fully inclusive, saw the same thing, essentially, for the most part, agreement with the prediction, so no excess of electron neutrinos, and therefore no hint for a sterile neutrino. So that was heartwarming. I say that, well, it was, maybe it was heartbreaking. People over the years asked me, what's your gut feeling, what's your hunch? My answer was always, I don't think that's my job to have a hunch. I think my job is to take the data and look at the data and see what it says. They didn't like that answer very much, but I stuck to it. What's heartwarming is that all the analyses agree. We did all these analyses blind, so we only looked at about 8% of our data for the whole five years while we were developing the reconstruction techniques, publishing other papers on cross-sections, and making sure that we understood every aspect of the detector. And then over the course of about a month, we opened the box on all of them, and it really would have been messy if one had seen an excess and one hadn't. That's what was heartwarming. Heartbreaking, who knows? You get different people think different things about whether or not there should be a sterile neutrino. We don't see a hint of one. Okay, let me talk now a little bit more about the most inclusive channel selfishly because that's the one that my group was involved in. And what was the reason for being fully inclusive to try to be less cross-section model dependent, topology agnostic and high statistics, and as close as possible to what the DUNE experiment will do. So including the lower energy interactions that tend to be more inclusive up to the higher energy interactions that tend to be messier, frankly, electron plus anything. I won't spend too much time, but there were many different steps in the chain, starting with noise filtering and signal processing, imaging, clustering events, folding in trajectory and energy deposits, fitting, tagging cosmic ray muons. And finally, organizing all of the different tracks into events and classifying them as different kinds of events. And it was a mess. We started with a mess. So microboon is effectively on the surface. It's 25 people low grade. And that means that we had a whole mess of cosmic rays concurrent with the events. Normally, like with a Trenkov imaging experiment, the readout from an interaction is prompt because you're just measuring light. So a bath of cosmic rays doesn't matter. You can use timing to minimize concurrent cosmic rays. Look what Argon TPC has the feature of this slow drift, which happens on the order of milliseconds, three milliseconds to drift an event to the edge of the detector. That's like an eternity in which you can have many, many, many cosmic rays concurrent at the same time on top of the event. So our events started, we started with this, with many cosmic rays. And the goal was to be able to, before we do any analysis, get rid of this huge bath of cosmic rays. And we did that by taking slices in time of the detector, which you can see one slice in time, and taking all of these different slices in time and combining them together to make one image of this very messy event with lots of cosmic rays and one neutrino interaction. Most beams bills had no neutrino interactions. We knew if there was a neutrino interaction because we also had a light collection system in the detector, which could see a prompt light. But we ended up with this and the goal was to figure out which of these interactions were from cosmic rays and which were from neutrinos. And we did that by, as you can see here, colorizing the different events in terms of categories. Wasn't as easy as just colorizing them. We had to figure out what was what, but eventually would end up with one neutrino interaction which you can see here. So just to give you the numbers, that cosmic ray rejection level, we started with one to 20,000 neutrinos to Cosmics and we ended up with 5.2 to one neutrinos to Cosmics. So that's a was, I guess I belabor it too much, but it sure took a lot of time on the experiment to first get rid of all the cosmic rays. Overburden, we did not. We built, the question was, was there any overburden? We actually built the building to be able to accommodate three equivalent of three feet of overburden which would have gotten rid of soft stuff which is also a problem. Photons and things like that would not have gotten rid of the muons. So we still would have had to get rid of the muons. We never put it on because we figured out how to get rid of the Cosmics through data analysis. The future SBN experiments do have overburden on them to make it a little easier. So there you go. In the end, we have this generic neutrino selection to first pick out the neutrino event. And from that, we keep analyzing to separate tracks versus showers, subcluster, again, fold in a DEDX information and finally come up with classification for events. We have, I won't go into the details but you can ask me later about how we determine energy resolution for the different classes of events, how we use different particles in our detectors. Cosmic rays are a big pain but they're also very helpful for calibration and what standard candles we use in order to determine energy in the detector. But in the end, we end up with efficient and very pure sample of new muons and an efficient and very pure sample of new ease. And we use the new muons to help constrain the new prediction. That's a key part of reducing errors in the analysis. We had a seven channel fit to categorize every event we saw into seven different categories. Let's skip the systematic errors. And in the end, this is the distribution we saw before we opened the box. The red line is what we expected to see if there were mini-boon like newie appearance. And we developed a way to use our new mu data to constrain the newie data. So we would end up with a picture like this. Okay, we had many different data sets to try to make sure that we knew what we were doing. There's two neutrino beams on site at Fermilab. There's the booster neutrino beam which is what on axis, what microboon saw. And there's the new me neutrino beam which ran first for the MEDOS experiment and then for the NOVA experiment. And as luck would have it, the new me neutrino beam goes right under the microboon detector on its way to Minnesota. So we see a really sizable and enormous in fact flux of off-axis neutrinos from the new me experiment. That's almost the same energy distribution as our own. And we took advantage of it. We still are taking advantage of all of those neutrino interactions timed differently than our own beam to use as a calibration source and for other physics. And so some of these are, this is data from the new me experiment which we use to say, do we really understand? Newies from new me? Yes, they fall on top of our prediction. Do we understand if we produce fake data in the detector? Do we get back what we put in? Yes, we got back what we put in. Whole bunch of different checks. And we did all of this while only looking at data outside of the signal region and eventually opened up to be able to see that the data was in good agreement with the background predictions and not with the red mini-boon proposed curve as shown. So no excess of low energy new candidates observed. And here's the plot from the wire cell analysis that my group worked on, but as I showed you before the same is true for the other analyses. I showed you this plot before. Okay, we sum all that up into one big money plot. This is the most inclusive analysis. These are the two semi-inclusive analysis and this is the fully inclusive analysis. And you can see the data points all are in even with some deficit which is still not quite understood but in agreement with one. So events observed are like those predicted with no low excess of no low energy excess. Okay, we still have a ton of work to do. We've answered one question. There doesn't seem to be a hint for an excess of electron neutrinos and therefore sterile neutrinos. We've only looked at half of our data set. We still have another half that we're calibrating. And we have these other channels. Here's that same top row that I showed you before that are in white that we haven't looked at at all. Theorists are clever. They think of many, many, many different scenarios that could explain anomalous data. And down the left side here, you can see all of the different models that might explain in some cases, just Miniboon and in other cases, both Miniboon and the anomalous reactor neutrino data. And so our goal is to try to address as many of these as we possibly can over the course of the next couple of years. We've taken a first stab at it and we still have more to go. And if you're interested in what some of those check marks are, there's very small prints. So you'd have to go to the talk to see it, but you can see a whole variety of theory papers that suggest the low energy excess could be just electrons, just photons, or for example, E plus, E minus pairs. And in some cases, a combination of these different channels. Here are the different final state topologies we could now draw of the things that we've looked at with the first half of our data and the things that additional analyses for these different channels, both with our current data and then a repeat of everything once we have the full data set analyzed. So stay tuned. As I said at the beginning, we've answered some questions, but there's still a bunch remaining. We still haven't set out what we intended to do, which is to understand what the excess is. And that'll come with more of our own data and more analyses, but also with the rest of the short baseline program at Fermilab. We're the first in a series of, in the end, three detectors that will run on that beam line. Here we are, the microboon detector. Some distance beyond us is the SBN-FAR detector, which is much larger than ours, also a liquid argon TPC. It is the Icarus detector shipped from Italy refurbished at CERN and now here at Fermilab and a near detector, the SBND near detector, so that you can really do a good job measuring the flux before it has a chance to oscillate to get to one of these detectors and to have really high statistics interactions. If the low energy excess is due to photons and something that's hard to tease out from background and potentially a rare-ish process, you want lots of statistics to be able to understand it. And so the SBND detector can give you that just downstream of the neutrino source. So lots to add there. Okay, I have, I don't know how much time I have left, but I'm gonna talk about Dune for a little while because that's really exciting. Let me go back to that picture of the universe. And as I mentioned, neutrinos, I've just talked about what they're telling us about themselves, new kinds of neutrinos, neutrino interactions. What can they tell us about the rest of the universe? Which is wonderful when you get to the point in a field where you can start to look beyond a sort of what's in front of you. And in particular, what can they tell us about how the matter in the universe came to be? And here I love a quote from Ray Davis who won the Nobel Prize in 2002 along with Mesutoshi Koshiba, in that case for the detection of cosmic neutrinos. He ran the homestake experiment in South Dakota which looked for neutrinos coming from the sun and saw a deficit for decades upon decades. And people told him, he was a little crazy. We've never, those weren't neutrinos oscillating. We probably didn't understand the sun. And he and John Bacall persisted. And eventually, in fact, it was understood that his data was due to neutrino oscillations. And he won the Nobel Prize, did not share it with John Bacall who passed away before 2002. But he said, to my surprise, a whole new field of neutrino physics has developed in directions I never imagined in the homestake phase. And that is how science should work. Paraphrased, what Dune will be doing is our neutrino is the reason we exist. So what's the story there? Our universe is made of matter. But we know that for every matter particle, there is an antimatter particle shown here in this anti-standard model. In the laboratory, we're pretty good at creating antimatter, but we can only do so when we take energy and create equal amounts of matter and antimatter. And then that matter and antimatter comes together again and annihilates back into energy. It's a beautifully symmetric process. And thankfully, it is not what happened in the early universe. Otherwise, that 4% of ordinary matter would have annihilated away and we would just have a bath of energy in the universe. Something happened in the early universe to cause an asymmetry in the process which left us with a little bit of matter in the universe, which is the ordinary matter that we see. So scientists look for many decades at differences between quarks and anti-quarks to try to see if there were some hint to why we live in a matter-dominated universe and couldn't find enough to understand that. So in some ways, neutrinos are sort of the last hope. Are there differences between neutrinos and anti-neutrinos that somehow can explain to us what happened in the early universe to leave us with a matter-dominated universe? And so the point here is to look for differences between neutrino oscillations and anti-neutrino oscillations for these matter-antimatter differences. And that's the point of the Dune experiment that you may have heard about. It will be the biggest US-based particle physics project ever. Dune deep underground neutrino experiment starts with a neutrino beam at Fermilab directed down into the earth to come up in South Dakota at the Stanford Underground Research Facility at the Homestake Lab in South Dakota. This is the place where Ray Davis, when Homestake was a running gold mine, put his experiment to study solar neutrino oscillations. It's no longer a running gold mine. It closed in 2002 and it's been fully transformed to an underground research facility and it will house Dune. It'll house Dune a mile underground and it's still this ginormous detector that I showed you before and it's filled with an enormous amount of liquid argon. It never fails to shock me when I show a slide like this. We're actually gonna do this. We're gonna build these 15 by 15 by 60 meter long detectors filled with liquid argon with high electric fields, ultra pure liquid argon and precision cold electronics and we're gonna put it a mile underground so that we can take advantage of a mile of overburden to get rid of the cosmic rays. So that's really exciting. I'm gonna end there. That was so exciting to end with. I hope I've given you a picture of the first results from Micro Boone what we've answered, what we haven't what we still have to do and a lot about the power of the technology which we're now using for Dune and Dune is on the horizon. Very exciting future experiment. You know, I started giving talks on Dune when I was pregnant with my first child who's now 15 and I used to joke. I really thought it was a joke like somebody's gotta be around to run this experiment. Well, things take time. Dune, the current plan is to have Dune running in 2030, which isn't that far away. Eight years, but it's still as a place where I hope many of you will do good science and myself too, and maybe my son. Thank you. We've got plenty of time for Q and A. So I think we'll start with the folks in the room. You're gonna help the crowd here. Any questions? Yeah, Tom. I'd ask you a question at the comment based on the advice or undergraduate advice you gave me. Yeah. Paulie's speculation, David's comment, is clearly young to consider the advice from their elders very skeptical. I couldn't agree more, 100%. So just to repeat for the benefit of the folks online, basically if you're young, ignore old people. That's what we heard from a more senior member of our audience. There's a stage. They're not young person. They're not young person. There's a stage when graduate students, you know, hopefully young, but maybe third, fourth year, they start arguing back at you. That's my favorite stage, because I'm like, okay, bring it on. So skepticism, healthy skepticism is really important because otherwise we wouldn't learn new things. Thank you, great comment. Oh yeah. Well, it doesn't cosmology tell us that the number of active flavors of neutrinos is only three and the picture works well like that. Is that what you mean? Is that what you mean? Or could the sterile neutrino help us explain? No, I don't think so. In fact, it only mucks things up to be frank. If in fact we'd found a sterile neutrino, it would make the DUNE experiment, for example, much more complicated because you have just that many more oscillations going on. I think the biggest challenge in general for neutrinos and connecting it with the matter, anti-matter asymmetry is doing just that. In fact, theorists will often, you know, heckle me. I'm married to a theorist, so I understand that. That's a positive thing about wealth. You know, we can't really connect it, true, not yet. If we do see a difference between neutrino oscillations and anti-neutrino oscillations, it has to be a clue for the differences between matter and anti-matter that could explain whatever happened in the early universe to explain why we live in a matter dominated universe. Sterile neutrinos, I don't think make much difference unless there's something that we don't understand yet. Yeah. All right, any questions from the folks online? I know we've got a few people online. You can just unmute your mic and ask, I think. It is true, yes. Okay, any other questions from the room? Oh yeah, the cosmologist has a question. Microboone, you mean? It's on the order of one point something, Sigma. I can go back to it if you want. It looks alarming, doesn't it? But it's, and I have to tell you, I'm happy to tell you that I will, I don't ever want to open near side bands again because where's that great picture? Here we sat for a long time with this event diving off and we thought we were so clever for a while all this was blinded and then we opened one box down to here and then we opened another box down to there and then we sat for a long time. Although we had planned to not change anything in any of that process, we still sat there deciding that reminding ourselves not to change anything, but better to have just ripped the bandaid off. That's what we should have done. In the end, that deficit corrected when you get to the lowest energies, but it's on the order of a one point something I can't remember exactly, deficit. And we see it in all three analyses and we'll see what happens when we have more data. It's not significant enough to worry about, but it does give you pause. I have no doubt theorists are already writing papers. I have. I want to have sigma. Yes, they have. Disappearance. Disappearance. Other comments or questions? Yeah, well. Hands are hard. Protons from pions we do pretty well on. We just don't get a lot of cans to be honest, we're too low in energy, but there are ways to use DEDX to identify cans. But to first order, we haven't worried about it much because they're on the higher energy of our interactions. No, we don't think so. You mean by mis-ideating them? There's not enough of them to worry about that. So my answer would be no. Yeah. In the other round? Yes. Yeah. So micro moon is about the size of a school bus. Two and a half meters by two and a half meters by 10.5 meters long. Icarus is bigger than that. How big is Icarus? 600 tons. Microboon is about 100 tons. So that's the scale of Icarus. The prototype detector for dune, which is in the CERN test beam, is on the order of a kiloton. And so one can argue that it's easily, that it's a big jump when you get to 40 kilotons for dune. There's a lot of thought that goes into how you scale, meaning how, in some sense, the dune detector is modularized into drift regions. Since we're confident we can drift five meters, it's two and a half meters of drift and microboon, it's five meters in proto dune. So in module one of the dune detector, there's these sort of dry cleaning style hanging curtains of cathode wire chamber plane readout, can you get a detector that has that much liquid argon pure and keep it pure? So that takes cryogenic engineers to convince us that the number of exchanges of liquid argon through constantly running pumps, you turn the pumps off, you lose purity, so you have to keep the pumps on, is adequate to keep the purity. Will the cold electronics work in the detector? Yes. Will they work in a conductor? Will they work in a configuration where you have them near the bottom of the detector? How do you get all the signal cables out, many more in a detector than, for example, in microboon? The electronics, thankfully, have evolved. There's more in-detector electronics. In microboon, the motherboards are outside of the detector. There's a continuing effort to bring more and more inside the detector, which is good and bad, you can't fix it. But as Velco Radica always said, the physicists can't get their hands on it if it's inside the detector. So a bunch of different things. I think it's absolutely, you can work through the process that it's scalable, that doesn't mean we won't see challenges. Yeah, please. Absolutely. I wasn't sure what the time was. Maybe I ended very early. I thought I was very late. But anyway, let me- We're all counting down the seconds to the reception in 20 minutes. It's fine. Okay, good. It's a two-minute walk. Okay, great. Because I rushed through this. Okay, so let me go through this because it's all quite interesting. We do neutrino energy reconstruction based on calorimetry in the detector. And we end up with a 15 to 20% resolution for new mu events and 10 to 15% resolution for fully contained new events. Things get more complicated when they're not fully contained. In terms of calibrating energy scales in the detector, muons are great because they deposit one mip. We see them easily. And we have a ton of them because of cosmic rays. In terms of other standard candles, higher medium energies, we can look at a sample of pine knots produced from neutral current pine knot interactions, measure the invariant mass and see that our invariant mass is what we expect. That's a number we know. And at the lowest energies, we can look at electrons, Michelle electrons from decaying muons and use them as a tag. Those are pretty low energy. So they're not as useful. And only 25% of our muons stop and decay. The others are captured, which is what happens in liquid argon. But really I would tell you that the pine knots are one of our most important standard candles. Any other questions that can't be asked over food? All right, I see no other questions. So why don't we thank the speaker one more time? And if you're within your shot right now and you don't know that there's a reception in 20 minutes and you would like to go, you're welcome to go. It's a reception for Bonnie and a celebration of this being the end of the academic year, which is always, yes, I see some junior faculty that are very happy about this. So say we all, all right? All right, so thanks everybody. And hopefully we'll see you with the reception in 20 minutes. If you don't know where it is, come talk to me, okay? All right.