As a young graduate student, I loved the Standard Model of particle physics. It tamed the chaos of the subatomic world: organizing its particles into neat groups, like chemistry’s periodic table, only simpler. Its conservation laws, symmetries and forces determined which interactions were allowed, forbidden or merely suppressed. Its rules for calculating seemed infallible, and while they could quickly become intractable, I believed that with infinite computing one could, in theory, compute the probability of any event, including whether I’d have eggs for breakfast next Tuesday.
Eventually, I learned that my senior colleagues did not share this affection. Instead they viewed the Standard Model as a tyrant: conforming all measurements to its will. There are no particles to discover that were not predicted by it. Which is bad, because if that last statement is true there are no particles left to discover at all, since the last Standard Model particle, the Higgs boson, was discovered in 2012. Each new or refined measurement of a fundamental parameter, like a particle’s mass or the strength of its interactions, further hems in all future measurements, until, as was the case with the Higgs boson, an open search becomes a binary question: either the predicted particle is right where the Standard Model says it will be or it’s not—and it always is.
So, that which gave me comfort as a student was then, and is still today, a great source of frustration, particularly to an experimental community that still includes some who can remember a time when the turn-on of any new particle accelerator was accompanied by a burst of discoveries.
To be fair, the Standard Model of particle physics is a remarkable scientific achievement; the crown jewel of the physics revolution that dominated the 20th century, but in the 21st century its apparent infallibility saps the vitality of the field. That’s why today nearly all of particle physics is focused on finding a crack, any crack, in its relentless edifice.
For example, there are dozens of experiments trying to make a direct detection of particle dark matter, long known to cosmology but unknown to particle physics; there are searches for other particles beyond the Standard Model particle with names like axions and magnetic monopoles; a third of the papers coming out of CERN’s Large Hadron Collider are direct searches for non–Standard Model particles and effects, while another third are precision tests of the Standard Model; and there are tens of thousands of theory papers on hypothetical physics models beyond the Standard Model.
In 1998, we thought we had found it with the discovery of neutrino oscillations, a phenomenon in which neutrinos morph between the three known types—the three types predicted by the Standard Model—in a repeating, oscillatory pattern. According to the rules of quantum mechanics, this should only happen if neutrinos have mass, and in the Standard Model, as it existed at that time, neutrinos were assumed to be massless. It turns out that neutrino mass could be hacked onto the Standard Model without a complete overhaul, although we don’t have a great explanation for why the neutrino masses are so small. Nevertheless, neutrinos asserted themselves as a promising frontier in the battle to take down the Standard Model.
Fast forward to earlier this month, with over 800 neutrino physicists preparing to gather in Heidelberg, Germany for the week-long 28th biennial International Conference on Neutrino Physics and Astrophysics, the premier neutrino physics meeting, when the MiniBooNE experiment got a jump on the excitement by posting a preprint claiming a statistically significant confirmation of a 25-year-old anomaly. Starting in 1993, the Liquid Scintillator Neutrino Detector (or LSND) experiment reported seeing a statistically suggestive excess of electron neutrinos in a beam of muon neutrinos, which persisted through the end of the experiment in 1998. If interpreted as a type of neutrino oscillation, it suggested the existence of a new fourth type of neutrino, which would be well outside the bounds of the Standard Model. The properties of this hypothetical particle were tightly constrained by well-established measurements. Where the three regular neutrinos are notoriously shy, interacting only very rarely with other particles, this fourth neutrino did not interact at all, earning it the title the “sterile” neutrino.
As the conference opened, the MiniBooNE report was a hot topic of informal discussions. The sterile neutrino sessions opened with reports from five active short-baseline reactor neutrino experiments. These experiments are designed to look for a distinct oscillatory pattern in the disappearance of electron neutrinos (technically antineutrinos) from nuclear reactors.
This is a direct test of the hypothesis that the reactor antineutrino anomaly, in which all past and current reactor neutrino experiments consistently see fewer neutrino interactions than predicted, is due to oscillations involving the sterile neutrino. Interestingly, the apparent oscillation frequency of the reactor anomaly deficit overlaps with the frequency of LSND/MiniBooNE excess. Indeed, that’s exactly what you would expect to happen, since the rules of neutrino oscillation require that electron neutrinos should disappear at the same periodic frequency that they appear.
The results of the reactor experiments were mixed. The two longest running experiments showed restrictive limits on the allowed parameter space but also displayed tantalizing wiggles at the limits of their sensitivity. One went as far as to fit these wiggles as an oscillation, finding a suggestive hint for this interpretation. The newer experiments don’t yet match the leading sensitivity, but they did show that they will be complementary, with their sensitivities optimized for the regions preferred by the funny wiggles.
MiniBooNE was up next. The excess at the heart of their claim appears at low energies, which corresponds to a slightly lower frequency oscillation than observed by the earlier LSND Experiment. So it’s not exactly where it was expected. The central question from an instrumental point of view is whether these excess events truly come from electron neutrino interactions, as MiniBooNE claims, or might they come from a class of muon neutrino interactions that produce energetic photons. In the MiniBooNE detector, photons would look just like the electrons in electron neutrino interactions.
The next speaker discussed a running experiment that will directly test this photon/electron ambiguity. The MicroBooNE experiment, which is currently running in the MiniBooNE beam line at the Fermi National Accelerator Laboratory, uses a detector that should be able to differentiate between the two. So, if they observe the MiniBooNE excess, we’ll know if it’s truly from electron neutrinos.
In the next talk, theorist Michele Maltoni of Madrid’s Instituto de Física Teoríca, explained another nagging problem with the MiniBooNE claim. Just as muon to electron neutrino appearance would be accompanied by electron neutrino disappearance, it also requires the disappearance of muon neutrinos. Many experiments have looked for this effect to no avail, setting tight upper limits on its rate. These three phenomena should be closely linked and in the current global fits are incompatible.
So where do we stand? Running experiments may soon give us clarity, but at this time there is no resolution to the sterile neutrino question. In the final talk of the session, I highlighted some ideas for truly definitive searches. If the sterile neutrino is still around in 2020 for next neutrino conference, we should commit to one or more of these projects and get it done.
In the conference sidelines a graduate student told me, “MiniBooNE can’t be right, there’s no sterile neutrino, it doesn’t fit the Standard Model.”
I smiled and politely replied, “You’re probably right, but I root for the downtrodden and the oppressed, because, in case you don’t yet know, the Standard Model is a tyrant.”