Stealthier than a neutrino

MiniBooNE observes excesses of 78.4 ±20.0 (stat) ±20.3 (syst) and 162.0 ±28.1 (stat) ±38.7 (syst) candidate electron neutrino events in antineutrino (top) and neutrino (bottom) modes, respectively. Here they are given as a function of reconstructed neutrino energy.

The search for sterile neutrinos has reached a new milestone. After collecting data for the past decade in both neutrino and antineutrino modes, the MiniBooNE experiment reports in a paper accepted for publication in Physical Review Letters an excess of events that suggests there may be additional neutrinos to the known three. MiniBooNE observed a combined excess of these events with 3.8 sigma significance.

It took 25 years to observe the electron neutrino after it was predicted to exist, so it is not surprising that it could take even longer to observe the proposed sterile partners of neutrinos. A sterile neutrino, unlike the neutrinos of the Standard Model, would not interact through the weak force. The existence of such neutrinos would be a sign of physics beyond the Standard Model.

The excess that MiniBooNE sees is consistent with neutrino oscillations at the approximately 1 eV2 mass scale, which implies these additional neutrinos are sterile. It is also consistent with neutrino oscillation evidence from the Los Alamos National Laboratory LSND experiment, which hinted at the existence of these as yet unobserved particles. In the antineutrino mode, the model with one sterile neutrino fits MiniBooNE data reasonably well, with 66 percent probability. In the neutrino mode, on the other hand, agreement between data and the fitted model is low—only 6 percent probability. However, expanded models with two or more sterile neutrinos improve these so-called best-fit probabilities, as well as agreement with LSND data, by allowing different oscillation patterns for neutrinos and antineutrinos (CP violation).

Future experiments at Fermilab and CERN will be able to test the LSND and MiniBooNE results and determine whether short-baseline oscillations and sterile neutrinos exist. These experiments include MINOS+, MicroBooNE, ICARUS-NESSIE at CERN and nuSTORM. MINOS+ will start taking data later this year with a more intense neutrino beam and will have better sensitivity for muon neutrino disappearance, which is a hallmark of all sterile-neutrino models. MicroBooNE will begin taking data in 2014 and will test whether the excesses observed by MiniBooNE are due to outgoing electrons from neutrino events, as expected by sterile-neutrino models, or to outgoing photons, which could point to other, unexplored phenomena. ICARUS-NESSIE plans to search for sterile neutrinos beginning in 2016. Finally, nuSTORM is a future muon storage ring experiment in which the neutrinos that arise from muon decay are well understood. By building two detectors at different distances from the storage ring, nuSTORM will be able to make a definitive test of short-baseline oscillations and sterile neutrinos.

If sterile neutrinos are proven to exist, then they will have a big impact on particle physics, nuclear physics, astrophysics, and cosmology.

—Zarko Pavlovic

The oscillation allowed regions as a function of Δm2 and sin22θ for antineutrino mode (left) and neutrino mode (right). Also shown are the allowed regions from the LSND experiment, which took data with antineutrinos, as well as limits from the KARMEN and ICARUS experiments.