Abstract
We consider relatively heavy neutrinos $\nu_H$, mostly contributing to a sterile state $\nu_s$, with mass in the range 10 MeV $\lesssim m_s \lesssim m_{\pi} \sim 135$ MeV, which are thermally produced in the early universe in collisional processes involving active neutrinos, and freezing out after the QCD phase transition. If these neutrinos decay after the active neutrino decoupling, they generate extra neutrino radiation, but also contribute to entropy production. Thus, they alter the value of the effective number of neutrino species $N_{\rm eff}$ as for instance measured by the cosmic microwave background (CMB), as well as affect primordial nucleosynthesis (BBN), notably ${}^4$He production. We provide a detailed account of the solution of the relevant Boltzmann equations. We also identify the parameter space allowed by current Planck satellite data and forecast the parameter space probed by future Stage-4 ground-based CMB observations, expected to match or surpass BBN sensitivity.
Highlights
Interacting particles characterized by extremely suppressed interactions with the Standard Model particles have received growing interest in the past decade
We identify the parameter space allowed by current Planck satellite data and forecast the parameter space probed by future stage-4 ground-based cosmic microwave background (CMB) observations, expected to match or surpass BBN sensitivity
The parameter space of a fourth neutrino in this mass range is strongly constrained by collider and beam-dump experiments for a dominant mixing with either νe and νμ [6,7], but it is significantly less constrained if mixed with ντ, with bounds at high masses coming from searches of decays of D mesons and τ leptons [8] and SuperKamiokande data [9]
Summary
Interacting particles characterized by extremely suppressed interactions with the Standard Model particles have received growing interest in the past decade (see [1] for a recent review). The nonthermal νe and νe spectra enter weak interactions, altering—together with Neff—the neutron-to-proton ratio which rules the abundance of the primordial yields [10,11,15,16,17,18], affecting, in particular, the 4He abundance encoded in the primordial helium mass fraction parameter Yp. The aim of our paper is to perform a detailed calculation of heavy sterile neutrino decoupling in the early Universe, in particular, computing the effects on Neff and Yp and assessing the impact of the approximations presented in the seminal works of Refs. Appendix C is devoted to a comparison of our results with others previously reported in the literature
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