Phys. Rev. D 84, 071301 (2011)

As the particle physics community struggles to understand the OPERA experiment's seeming detection of superluminal neutrinos, anomalous results from other neutrino experiments are also under scrutiny. The LSND experiment, which ran at Los Alamos National Laboratory in the 1990s, found evidence of neutrino oscillations that didn't tally with the standard model of three neutrino species, or with other experimental data. And MiniBooNE, at Fermilab, has seen excess events that could support a particular explanation of the LSND anomaly — that there is an additional heavy neutrino (or even more than one), which is some kind of admixture of the usual three neutrino species but 'sterile'.

Many authors have considered where definitive evidence of the sterile neutrino might be found. Assuming, unusually, that the sterile neutrino might have a dominant radiative decay mode, Claudio Dib and colleagues investigate the possibilities of kaon decay (to a muon, a neutrino and a photon) and tau-lepton decay (to a muon, two neutrinos and a photon). Existing data on these processes are, they point out, not conclusive, limited by the experimental precision. But the authors suggest that kaon decay to four leptons (muon, neutrino and two electrons) could hold a useful clue — if experiments could push to be sensitive to low enough invariant masses of the electron pair.