Abstract
Light sterile neutrinos, first proposed after the results of the LSND experiment, have been a polemical topic for the last decades after seemingly contradictory data appeared from different experiments. In this overview, I review the experimental hints that point towards sterile neutrinos as well as their statistical compatibility. Even though the muon neutrino disappearance experiments strongly rule out vanilla sterile neutrinos, each oscillation channel remains internally mostly consistent. In any case, in the near future a series of independent and precise experiments should finally settle down this issue.
Highlights
Neutrinos are an essential part of the Standard Model of particle physics (SM)
There are several hints from neutrino appearance and disappearance experiments at short baselines that point towards new physics in the leptonic sector
These hints come from leptonic flavour violation, a clear signal for physics beyond the Standard Model, with both neutrinos and antineutrinos, different sources, and different detection techniques
Summary
Neutrinos are an essential part of the Standard Model of particle physics (SM). Originally, they were introduced minimally, just to explain their interactions: due to the chiral structure of the SM, only left-handed neutrinos were required. The so-called 3 × 3 paradigm (3 leptonic flavours and 3 light neutrino mass eigenstates) to explain the observed neutrino flavour transitions is well established. This excess, shown, was interpreted as due to mass-induced neutrino flavour oscillations If this were the case, the typical LSND L/E ratio requires, from Eq 2, a squared-mass splitting ∆m2 ∼ O(eV2). This splitting is several orders of magnitude larger than the other two well-established ones, and explaining LSND through massive neutrino oscillations requires a fourth light neutrino mass eigenstate Due to unitarity, this means there must exist a fourth neutrino flavour eigenstate that, due to LEP constraints [6], cannot couple to the Z boson. The LSND results are, rather polemical: an independent reanalysis reevaluated the neutrino fluxes, backgrounds and systematics; lowering the significance of the excess to 2.3σ [11, 12]
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