New physics decaying into metastable particles: impact on cosmic neutrinos

TL;DR

This paper explores how decays of hypothetical unstable particles into metastable species in the early universe influence cosmic neutrino properties and cosmological observables, with implications for upcoming precision measurements.

Contribution

It introduces a novel analysis of metastable particle decays affecting neutrino spectra and $N_{eff}$, including a computational tool for further study.

Findings

Metastable particle decays alter neutrino spectral distortions.

Decays impact the effective number of neutrino species, $N_{eff}$.

Implications for Big Bang Nucleosynthesis and CMB observations.

Abstract

We investigate decays of hypothetical unstable new physics particles into metastable species such as muons, pions, or kaons in the Early Universe, when temperatures are in the MeV range, and study how they affect cosmic neutrinos. We demonstrate that the non-trivial dynamics of metastables in the plasma alters the impact of the new physics particles on the neutrino population, including the effective number of neutrino degrees of freedom, $N_{\rm eff}$, modifies neutrino spectral distortions, and may induce asymmetries in neutrino and antineutrino energy distributions. These modifications have important implications for observables such as Big Bang Nucleosynthesis and the Cosmic Microwave Background, especially in light of upcoming CMB observations aiming to reach percent-level precision on $N_{\rm eff}$. We illustrate our findings with a few examples of new physics particles and provide a computational tool available for further exploration.