Impact of eV-mass sterile neutrinos on neutrino-driven supernova outflows

Abstract

Motivated by recent hints for sterile neutrinos from the reactor anomaly, we study active-sterile conversions in a three-flavor scenario (2 active + 1 sterile families) for three different representative times during the neutrino-cooling evolution of the proto-neutron star born in an electron-capture supernova. In our ``early model'' (0.5 s post bounce), the νe-νs MSW effect driven by Δm2 = 2.35eV2 is dominated by ordinary matter and leads to a complete νe-νs swap with little or no trace of collective flavor oscillations. In our ``intermediate'' (2.9 s p.b.) and ``late models'' (6.5 s p.b.), neutrinos themselves significantly modify the νe-νs matter effect, and, in particular in the late model, νν refraction strongly reduces the matter effect, largely suppressing the overall νe-νs MSW conversion. This phenomenon has not been reported in previous studies of active-sterile supernova neutrino oscillations. We always include the feedback effect on the electron fraction Ye due to neutrino oscillations. In all examples, Ye is reduced and therefore the presence of sterile neutrinos can affect the conditions for heavy-element formation in the supernova ejecta, even if probably not enabling the r-process in the investigated outflows of an electron-capture supernova. The impact of neutrino-neutrino refraction is strong but complicated, leaving open the possibility that with a more complete treatment, or for other supernova models, active-sterile neutrino oscillations could generate conditions suitable for the r-process.