Sterile Neutrinos with Secret Interactions -- Cosmological Discord?
Xiaoyong Chu🇦🇹 · Basudeb Dasgupta🇮🇳 · Mona Dentler🇩🇪 · Joachim Kopp🇩🇪 · Ninetta Saviano🇩🇪
Several long-standing anomalies from short-baseline neutrino oscillation experiments -- most recently corroborated by new data from MiniBooNE -- have led to the hypothesis that extra, 'sterile', neutrino species might exist. Models of this type face severe cosmological constraints, and several ideas have been proposed to avoid these constraints. Among the most widely discussed ones are models with so-called 'secret interactions' in the neutrino sector. In these models, sterile neutrinos are hypothesized to couple to a new interaction, which dynamically suppresses their production in the early Universe through finite-temperature effects. Recently, it has been argued that the original calculations demonstrating the viability of this scenario need to be refined. Here, we update our earlier results from arXiv:1310.6337 [JCAP 1510 (2015) no.10, 011] accordingly. We confirm that much of the previously open parameter space for secret interactions is in fact ruled out by cosmological constraints on the sum of neutrino masses and on free-streaming of active neutrinos. We then discuss possible modifications of the vanilla scenario that would reconcile sterile neutrinos with cosmology.