Probing the Weak Interaction with Intermediate Mass Sterile Neutrinos in the Early Universe

Neutrino oscillation experiments suggest that the active (left-handed) neutrino species have non-zero masses, hinting at the existence of a fourth, sterile (right-handed) neutrino species. Unlike active neutrino species, sterile neutrinos do not interact via the weak nuclear force except through a sub-weak interaction arising from active-sterile neutrino mixing, making them extremely difficult to probe in Earth-based laboratories. We simulate the effects of intermediate mass sterile neutrinos that are thermally populated in the early universe and that decay into Standard Model particles around the time of Big Bang Nucleosynthesis. As the sterile neutrinos decay, they deposit a significant amount of entropy into the plasma which is tractable in observations of anisotropies in the CMB. Our simulation can be leveraged alongside CMB stage 4 observations of cosmological observables such as the effective number of active-neutrino species and primordial abundances to permit improved constraints on, or indications of, Beyond Standard Model (BSM) physics.