Fourth neutrino or systematic error?

Measuring neutrinos over short distances might be the best way to find out whether the particles come in flavors other than electron, muon, and tau, the three accounted for by the standard model of particle physics. In the mid 1990s, researchers with the Liquid Scintillator Neutrino Detector (LSND) experiment measured an unexpectedly high probability for low-energy muon neutrinos traveling 30 meters to convert into electron neutrinos. The simplest theoretical explanation was that some muon neutrinos were oscillating into a novel variety, known as sterile neutrinos, which in turn were morphing into the electron neutrinos that were ultimately observed (see Physics Today, August 1995, page 20 ). Now an experiment designed to double-check LSND has largely confirmed the findings of its predecessor, a development that raises the hopes of new physics but also challenges the results of other sterile-neutrino searches.

At Fermilab, physicists with the MiniBooNE experiment track the appearance of electron neutrinos in a beam initially composed solely of muon neutrinos. The beam travels about 500 meters before passing through a spherical detector fitted with photomultiplier tubes, as shown in the photo, and filled with 800 tons of mineral oil. An electron neutrino passing through the detector can collide with a neutron to produce a proton and also an electron that generates a characteristic pattern of Cherenkov light. The beam energy and the distance to the detector are each a factor of 17 higher than in LSND, which means that the probability of a muon neutrino oscillating into an electron neutrino should be roughly the same in both experiments (see Physics Today, June 2007, page 18 ). After analyzing more than 15 years of data, MiniBooNE physicists reported 2437 electron neutrinos in the 200–1250 MeV energy range, compared with an expected 1880–2070 due to the predicted oscillation rate plus the decay of background particles. The results constitute an excess of 4.8 σ.

Despite the affirmation of LSND’s 20-year-old results, physicists have not concluded that there is a fourth neutrino species. The findings directly contradict those by a diverse set of experiments, including the Main Injector Neutrino Oscillation Search, the Daya Bay Reactor Neutrino Experiment, and the IceCube Neutrino Observatory (see Physics Today, October 2016, page 15 ). The conflict suggests that LSND and MiniBooNE physicists might have overlooked a source of background events. Fortunately, the mission of the MicroBooNE experiment, also at Fermilab, is to investigate the sources of MiniBooNE’s events. That collaboration has released some results but not yet anything that chimes in on the measured excess. (E.-C. Huang, “Excess of electron-like events in MiniBooNE,” Neutrino 2018 conference presentation .)