Secret neutrino interactions: a pseudoscalar model

Neutrino oscillation experiments point towards the existence of additional mostly sterile neutrino mass eigenstates in the eV mass range. At the same time, such sterile neutrinos are disfavoured by cosmology (Big Bang Nucleosynthesis, Cosmic Microwave Background and Large Scale Structure), unless they can be prevented from being thermalised in the early Universe. To this aim, we introduce a model of sterile neutrino secret interactions mediated by a new light pseudoscalar: The new interactions can accomodate sterile neutrinos in the early Universe, providing a good fit to all the up to date cosmological data.