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The Sterile Neutrino

Sterile Neutrinos: Beyond the Standard Model

Sterile Neutrinos are proposed additional neutrino flavors theorized to explain certain discrepancies in earlier neutrino experiments. However, as their name would suggest, sterile neutrinos do not interact with other matter through the weak force. This makes them impossible to detect directly, as they can only be observed through their mixing with other neutrino flavors. What causes this mixing of the sterile neutrino with other flavors and also the mixing among the normal three flavors is unknown. Fortunately, IsoDAR is designed to take advantage of the sterile neutrino mixing and make a conclusive judgement on the existence of sterile neutrinos.

Experimental evidence for sterile neutrinos comes from disappearance and appearance studies by LSND, MiniBooNe, and a few other recent neutrino detectors. Anomalies in ν^μ→ ν^e appearance and ν^e→ ν^e disappearance experiments, among others, suggest that mechanisms other than the well understood 3ν oscillation model are at play. One proposed solution is the existence of extra oscillation and mass eigenstates, which would produce different appearance/disappearance probabilities the more closely match experimental observations. Additional mass states with Δm₂~ 1 eV₂ or greater could be added to the current neutrino oscillation model to fit the experimental data. Unfortunately, these additional states cannot explain the anomalies while simultaneously accounting for data sets that do not show such anomalies, which is one of the main arguments against sterile neutrinos.

While additional neutrino flavors would help explain the observed discrepancies, precise measurements of the Z₀ width show that there are only three types of weakly interacting neutrino flavors. Thus any additional neutrino flavors must be sterile, only interacting through oscillation mechanics with the other flavor states and gravity. One possibility is that the sterile neutrinos have right handed chirality, as opposed to all other neutrino flavors which are left-handed. Interestingly, this is one possible explanation for active neutrino masses.

The minimal model for sterile neutrinos is a so called 3+1 model. In this model there is only one additional neutrino flavor and one additional mass state, leading to just one more Δm₂ and 3 more mixing angles. The 3+1 model explains the observed anomalies with some errors, but higher order models such as 3+2 or 3+3 do little better despite the greatly increased number of parameters these models introduce, while also introducing problems of their own. In its five year run, IsoDAR will be able to distinguish between a 3ν, 3+1, or 3+2 model for neutrino oscillations.