When feebly interacting massive particles decay into neutrinos: The <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>N</mml:mi><mml:mi>eff</mml:mi></mml:msub></mml:math> story
Alexey Boyarsky, Maksym Ovchynnikov, Nashwan Sabti, Vsevolod Syvolap
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.