In all the fuss about how amazing the LHC is going to be, we often forget that there are things it won’t be able to do. One of the most glaring holes in the LHC’s research program is how little work it plans to do on neutrino physics, one of the most exciting and promising fields in the quest to go beyond the Standard Model. Neutrinos are elementary particles that are nearly massless and have no charge. They rarely interact with other particles, so you need to make a lot of them to have the faintest hope of detecting just a few in experiments. In other words, you don’t need very high energy protons to produce neutrinos, but you do need a lot of lower energy ones.
It wouldn’t really make sense to devote much of the LHC’s particle yield to experiments that don’t need anything approaching its high energies, especially during the early years of the experiment. So Fermilab, somewhat presciently, is stepping in to fill the gap. As our tour guide and gracious host Kurt Riesselmann told us, “Fermilab is moving from the energy frontier to the intensity frontier” — meaning that instead of producing a small number of the highest possible energy particles, the lab is figuring out how to make as many lower energy particles as possible.
A particularly exciting area of neutrino physics is the question of neutrino oscillations. Neutrinos come in three “flavors:” the electron neutrino, the muon neutrino, and the tau neutrino. The discovery that a single neutrino can transform into all three types — and back again — was the first clue that neutrinos have mass, and that the masses of each flavor must be different. That was a surprise, to say the least, because the Standard Model predicts that neutrinos are massless. In our search to discover gaps in the Standard Model, the questions of neutrino mass and oscillations have taken center stage.
We saw got a closer look at two neutrino experiments during our visit: MINOS and MiniBooNE. The Main Injector Neutrino Oscillation Search (MINOS) studies neutrino oscillation by sending a stream of the particles through the earth to a detector 450 miles away in an old iron mine in Soudan, Minnesota. Since neutrinos have no charge and barely any mass, they have no problem passing through the earth’s crust — so, unlike many current physics experiments, MINOS didn’t involve building a tunnel. It did, however, involve digging a 350-ft. hole.
The experiment uses a coarse detector, since its main purpose is to simply count neutrinos, rather than identify their exact positions. “When we count how many neutrinos we see here [in Fermilab’s near detector], we’re able to project how many you expect to see up in Minnesota,” explained MINOS physicist Catherine James. “Do you see see that many? The answer is no.” The disappearing muon neutrinos are thought to have oscillated to become electron and tau neutrinos. (NOvA, a future experiment in a mine even further north along the beam than Soudan, will attempt to observe some of the missing muon neutrinos reappearing as electron neutrinos.)
Whereas the Tevatron experiments, CDF and DZero, have to wade through piles of useless data to find the interesting stuff, MINOS and all other neutrino experiments must struggle to find any data at all. A proton pulse from Fermilab’s Main Injector accelerator hits an underground target and produces a blast of billions of muon neutrinos every two seconds. But as Catherine told us, “We see maybe two, three, or four of those neutrinos in our tons of tons of steel. Our far detector [in the Soudan Mine] sees one beam neutrino a day.” Those numbers help put Fermilab’s idea of the intensity frontier in perspective. The more protons MINOS starts with, the more neutrinos they will be able to detect at every step of the experiment.
Another neutrino experiment at Fermilab is the Mini Booster Neutrino Experiment (MiniBooNE). While it also looks for muon neutrinos oscillating to become electron neutrinos, it was specifically designed to confirm or refute the results of the Liquid Scintillator Neutrino Detector (LSND) experiment at Los Alamos, which found evidence implying the existence of a fourth type of neutrino, called a sterile neutrino. In 2007, the MiniBooNE team released results which showed no evidence for sterile neutrinos, but it did detect what it called a “low energy excess.” They are now planning for the next generation experiment, MicroBooNE, to investigate what exactly is causing that excess.
The MiniBooNE detector is remarkably simple: a sphere filled with 800 tons of baby oil and lined with over 1000 photodetectors. The photodetectors pick up the signals produced when a neutrino collides with the nucleus of an atom, which happens about once every 20 seconds. MicroBooNE’s detector will be even more finely tuned and be able to distinguish between different types of signals. As MiniBooNE’s Georgia Karagiorgi explained to us, “A lot of background [in MiniBooNE] comes from interactions that produce a single photon. If it was a true signal, it would produce a single electron. It [MicroBooNE] can really tells us if the low energy excess is being caused by electron neutrinos interacting in the detector or some other interaction that we did not account for somehow — or is a new interaction that takes place that we didn’t know about before.”
If it weren’t buried underground (to avoid receiving even more background radiation), the MiniBooNE detector would be up there with the bubble chamber among the weirdest and most beautiful things at Fermilab. Check out a picture here. As it were, we could only see the room above the detector.
Georgia and our other MiniBooNE tour guide, Christina Ignarra, are both graduate students at MIT working with Janet Conrad, perhaps the greatest spokesperson neutrinos could ever hope to have. As Georgia said, “She makes you so excited about physics.” I got to know Janet at Columbia, where she taught before MIT, and her enthusiasm for the field is indeed infectious. You can read all about MiniBooNE and Janet’s cult of personality in this New Yorker profile, written by Los Angeles Times science writer (and fellow Barnard graduate) K.C. Cole.
I interviewed Janet about neutrino physics and Fermilab’s future in 2007. Back then she had recently started working on Project X, which she described as an upgrade that “provides a lot of protons, and it’s done with the technology you would use for the ILC” — the International Linear Collider, the proposed sister accelerator to the LHC (stay tuned for more on this exciting topic!). But when scientists at Fermilab first started thinking about the idea, the intensity frontier wasn’t part of the lab’s official narrative. “Project X was a code name because you couldn’t talk about anything but the ILC,” Janet explained. “And the code name stuck.”
Needless to say, the days of neutrino physics sneaking around behind the backs of higher energy experiments are over. Experiments like MINOS and MiniBooNE have, as Georgia said, already “proven that the Standard Model is inadequate to describe particle properties” — something that is notoriously difficult for experiments to do. Fermilab is clearly excited about its strong neutrino physics program, and I am excited to see the lab taking a new direction. Flexibility and a willingness to change direction aren’t often features of Big Science (or DOE programs), but Fermilab is investing in its own future precisely by being open to many different possibilities. I think Robert Wilson would be proud.
-Lizzie (photos and captions by Nick)
Update: Lizzie’s uncle, Don Wildman, also visited MINOS as the host of the History Channel’s show Cities of the Underworld. Read more about his visit here.