Dynamics of metastable Standard Model particles from long-lived particle decays in the MeV primordial plasma

TL;DR

This paper explores how hypothetical long-lived particles decaying in the early universe's MeV plasma influence cosmological phenomena, providing computational tools and analyzing their effects on nucleosynthesis and neutrino properties.

Contribution

It introduces coupled Boltzmann equations for metastable particle dynamics, provides public codes for their evolution, and applies these to specific new physics models.

Findings

Metastable particles can annihilate or interact before decay, reducing their abundance.

Decay suppression affects cosmic neutrino properties and cosmological observables.

Public codes enable detailed modeling of these processes in early universe scenarios.

Abstract

We investigate the cosmological impact of hypothetical unstable new physics particles that decay in the MeV-scale plasma of the Early Universe. Focusing on scenarios where the decays produce metastable species such as muons, pions, and kaons, we systematically analyze the dynamics of these particles using coupled Boltzmann equations governing their abundances. Our results demonstrate that the metastable species can efficiently annihilate or interact with nucleons, often leading to their disappearance before decay. The suppression of decay significantly alters the properties of cosmic neutrinos, impacting cosmological observables like Big Bang nucleosynthesis and the Cosmic Microwave Background. To support further studies, we provide two public codes: the Mathematica code that traces the evolution of these metastable particles, as well as the python-based unintegrated neutrino Boltzmann solver that uses this evolution as an input and may be applied to a broad range of scenarios. We then utilize them for studying a few particular new physics models.