When FIMPs Decay into Neutrinos: The $N_{\mathrm{eff}}$ Story

Abstract

The existence of feebly interacting massive particles (FIMPs) could have significant implications on the effective number of relativistic species $N_{\mathrm{eff}}$ in the early Universe. In this work, we investigate in detail how short-lived FIMPs that can decay into neutrinos affect $N_{\mathrm{eff}}$ and highlight the relevant effects that govern its evolution. We show that even if unstable FIMPs inject most of their energy into neutrinos, they may still decrease $N_{\mathrm{eff}}$, and identify neutrino spectral distortions as the driving power behind this effect. As a case study, we consider Heavy Neutral Leptons (HNLs) and indicate which regions of their parameter space increase or decrease $N_{\mathrm{eff}}$. Moreover, we derive bounds on the HNL lifetime from the Cosmic Microwave Background and comment on the possible role that HNLs could play in alleviating the Hubble tension.

arXiv comment13 pages, 7 figures - v2: journal version

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