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

TL;DR

This paper explores how short-lived feebly interacting particles that decay into neutrinos influence the early Universe's effective relativistic species count, $N_ ext{eff}$, revealing complex effects including potential decreases in $N_ ext{eff}$ and implications for cosmological tensions.

Contribution

It provides a detailed analysis of how decaying FIMPs, especially Heavy Neutral Leptons, affect $N_ ext{eff}$ and derives bounds on their lifetimes from CMB data.

Findings

Decaying FIMPs can decrease $N_ ext{eff}$ despite injecting neutrino energy.

Neutrino spectral distortions are key to understanding $N_ ext{eff}$ changes.

HNLs may help alleviate the Hubble tension.

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.