Fermilab's Quest to Find the Sterile Neutrino

Fermilab has long been known as one of the world's premier accelerator facilities studying neutrinos, with the first beams being built way back in the 1970s. Studies of neutrino oscillations began at Fermilab about two decades ago and it is anticipated that they will continue for decades in the future. But the past is the past, and the future is the future. What neutrino mysteries are Fermilab scientists studying now? That sounds like an excellent topic for today's video. (intro music) If you were to ask where is the center of the universe, I would have to tell you that this

is a silly question. In a mind-blowing moment of physics Zen, the answer is both that there is no center of the universe and every place is the center. I made a video about that. However, if you were to ask where is the center of the universe for the world's neutrino accelerator research program, a strong case could be made for claiming that it can be found at Fermilab. The laboratory already has the highest energy and highest intensity neutrino beam facilities, delivering nearly a megawatt proton beam to make those neutrinos. In the future, upgrades to the accelerator complex will cross the megawatt threshold. The upgrade

is called the PIP-II program and all of this is in preparation for the Deep Underground Neutrino Experiment or DUNE, which will lead to crucial comparative studies of the behaviors of matter and antimatter neutrinos. I've made several videos about all of this- really- a whole bunch- and the links are in the description. But those facilities won't begin operation and those questions won't be answered for years. What about today's neutrino questions and facilities? Let's talk about both of these in turn. There's a lot of history here, most of which

I'm going to skip over, but we know of three different forms of neutrinos that interact via the weak nuclear force. Those neutrinos are called the electron neutrino, the muon neutrino, and the tau neutrino. Each one is called that because they are usually produced with their cousin particle. Since the late 1950s, scientists have speculated that neutrinos might not be immutable and that they might be able to change their identity in a process of subatomic switcheroo called neutrino oscillation. Between 1998 and 2001, a couple of measurements proved that this idea was true.

Basically, neutrino oscillation means that what started out as an electron neutrino could turn into one of the other kinds, much as if a cat could change into a jaguar, then into a tiger, and then back again. I've made videos on the topic and, as usual, the links are in the description. Researchers have worked out the probability that each type of neutrino can convert into the others. Most of the experiments told a more or less coherent story, but not all of them. There have been several experiments that measured more neutrino transformation than expected. This has led some to speculate that there exists

at least one undiscovered type of neutrino that is involved in neutrino oscillation. However, this fourth neutrino doesn't interact via the weak nuclear force. Because of this, we have a special name for this proposed fourth neutrino and it's called a sterile neutrino. By the way, I said that there could be one type of sterile neutrino, but there actually could be more than one. It's just simpler for our purposes to talk about the situation as if there were three ordinary neutrinos and one sterile neutrino. If there are more sterile neutrinos, this

doesn't change what I'll say in this video. One experiment that reported the possibility of sterile neutrinos was called the LSND experiment, performed at the Los Alamos laboratory in New Mexico. It collected data in the mid- to late-1990s. The experiment created muon neutrinos and looked for them to oscillate into electron neutrinos. The upshot is that they saw more electron neutrinos than expected. Other experiments have tried to replicate their observation, but without success. However, the follow-on experiments used different techniques,

so the fact that they didn't see the same electron neutrino appearance rate as LSND isn't definitive. Perhaps these later experiments employed methodologies that were too different. An experiment called MiniBoone was performed here at Fermilab to either validate or falsify LSND. It began operations in 2002. A first result published in 2007 seemed to rule out LSND, although a later paper in 2018 seemed to support it. The situation is a little murky because the experimental techniques back then weren't as good as we have today. Indeed, many of the MiniBoone scientists participated in a follow-on experiment called MicroBoone,

which employed more sophisticated detector technologies so that they could get to the bottom of this sterile neutrino question. MicroBoone used liquid argon to detect and characterize neutrino interactions. This is the same technology as will be used in the DUNE experiment. Liquid argon gives us a much more precise picture as to what's going on when neutrinos interact as compared to the older methodology. MicroBoone collected data from 2015 to 2021. It didn't see the same thing that either LSND or MiniBoone did, which suggests that maybe sterile neutrinos aren't real and definitely makes the

situation even murkier. We need to take the bull by the horns and get a definitive answer. So that brings us to the present. Fermilab has undertaken what is called the short baseline neutrino program. It's called short baseline because, unlike most neutrino oscillation experiments, the detectors are close to one another. Indeed, everything is located on the Fermilab site. So how does the SBN program work? Basically, it consists of two detectors, both using liquid argon, to look at neutrino interactions. One detector is called SBND, for short baseline near detector, and it's located about 110

meters from the place where the Fermilab beam hits a target to make neutrinos. The other detector is called ICARUS, and it's located about 600 meters from the target. So, the basic idea is that Fermilab scientists will extract protons from one of our accelerators called the booster. The proton will hit a target and begin a process that will result in a beam composed predominantly of muon neutrinos. The neutrinos will first pass through the SBND detector and then hit the ICARUS detector. This concept is really quite beautiful. The first

detector will measure the exact composition of the neutrino beam, precisely nailing down the fractions of electron and muon neutrinos in the beam. The beam energy is low enough that tau neutrinos aren't a consideration. The beam will travel to the second detector, experiencing neutrino oscillation as it goes. The ICARUS detector will then make a similarly precise measurement of the composition of the neutrino beam after the beam travelled from one detector to the other. The scientists will then have a solid measurement of the amount of the neutrino

oscillation that occurred in transit. This two-detector situation is optimum. Because the two detectors are located near one another and utilize the same sophisticated detector technology, it will allow for a very precise measurement. This is because whatever instrumental effects occur in one detector will also occur in the other one. The scientists won't have to worry about being fooled by different detector performance. There's no chance for one detector to zig and the other to zag. If one zigs, both will. This will reduce measurement uncertainties.

So that's the plan. Both the SBND and ICARUS detectors are in place. The beam is the same one used by the MiniBooNE and MicroBooNE experiments, so that's ready to go too. The ICARUS detector was originally used in Europe, detecting neutrinos created at CERN. ICARUS is the first neutrino detector that was built using liquid argon as the central technology. After completing operations in Europe, it was moved to Fermilab, where it began US operations in the summer of 2021. SBND is still in the final stages of assembly and shakedown. It's expected to begin data taking in

early 2024. Because it's located close to the point where the neutrinos are made, it experiences the most intense beam conditions. They expect to record at least 20 to 30 times more neutrino/argon interactions than have been recorded to date. With so many neutrino interactions, SBND scientists will also study the data, looking for possible discoveries beyond the core program of looking for sterile neutrinos. Looking at the Fermilab neutrino program at a higher level, having this short baseline neutrino program has already advanced the

DUNE program and will continue to do so. For example, the scientists developing the SBND and ICARUS detectors have learned what works and what doesn't. All of that technical know-how has informed the design of the DUNE detector. In addition, while designing a detector is all well and good, no detector works exactly as designed. There will be unexpected idiosyncrasies in the detector performance. When I talk to my neutrino colleagues, some of them estimate that the experience gained by a successful SBN program will shave a couple of years off the

release of the first DUNE measurement. In addition, particle physics analyses are usually performed by young scientists or faculty, supported by their students and postdoctoral researchers. However, in order to have young faculty when DUNE begins operations, those individuals need to be learning the ropes now. Indeed, today's SBN students and postdocs will be some of the most energetic and impactful analyzers of DUNE data. There is one other potentially huge benefit that the SBN program will bequeath to the DUNE experiment. DUNE is intended to perform a precision measurement of the differences between

the oscillation properties of neutrinos and antimatter neutrinos. If it turns out that the sterile neutrinos exist, getting a handle on the behavior of sterile neutrinos will be a crucial step towards achieving the precision that DUNE is aiming for. So that's what the near-term Fermilab neutrino program will be doing. It has already improved the technical design of the future DUNE detector. It is developing the people who will be future leaders. And, from a scientific point of view, it will provide critical- perhaps definitive- measurements that will tell us whether sterile neutrinos exist or don't. Fermilab's current

short baseline neutrino experimental program is the very foundation on which future neutrino research success depends. (phasing sound) Okay- so neutrinos are pretty cool. They have fooled scientists time and time again, from how they showed in the 1950s that the weak force interacts differently with matter and antimatter, to how they surprised scientists in the 1960s when it became clear that there were different type of neutrinos. Then there were the hints from the 1970s through the 1990s that neutrinos can change their identity. Who knows what

future surprises they hold? That's what we're trying to figure out. If you enjoyed the video, please like it and subscribe to the channel. And come back again and again to hear more similar videos. You'll be a better person for the experience and I'm quite confident that you will come to embrace that fundamental truth of the universe, which is that physics is everything. (outro music)