Cosmological constraints on heavy sterile neutrinos

TL;DR

This paper investigates how heavy sterile neutrinos with masses between 10 MeV and 135 MeV affect cosmological observations like the CMB and Big Bang Nucleosynthesis, constraining their properties using current and future data.

Contribution

It provides a detailed Boltzmann equation analysis of heavy sterile neutrino effects on cosmological parameters, constraining their properties with current Planck data and forecasting future CMB-S4 sensitivity.

Findings

Current Planck data constrains sterile neutrino parameter space.

Future CMB-S4 observations will probe additional parameter regions.

Heavy sterile neutrinos can significantly alter $N_{\rm eff}$ and ${}^4$He production.

Abstract

We consider heavy sterile neutrinos $\nu_s$, with mass in the range 10 MeV $\lesssim m_s \lesssim m_{\pi} \sim 135$ MeV, thermally produced in the early universe and freezing out after the QCD phase transition. The existence of these neutrinos may alter the value of the effective number of neutrino species $N_{\rm eff}$, measured by the cosmic microwave background (CMB), and the ${}^4$He production during the BBN. We provide a detailed account of the solution of the relevant Boltzmann equations and we identify the parameter space constrained by current Planck satellite data and forecast the parameter space probed by future Stage-4 ground-based CMB (CMB-S4) observations.