The muon abundance in the primordial Universe

TL;DR

This paper calculates the temperature-dependent production and decay rates of muons in the early Universe, pinpointing when muons vanish as the Universe cools below approximately 4.135 MeV, revealing their abundance relation to baryons.

Contribution

It provides a detailed evaluation of muon production and decay rates in the primordial plasma and identifies the precise temperature at which muons disappear from the Universe.

Findings

Muons vanish when the Universe cools below ~4.135 MeV.

At disappearance, muon abundance is nearly equal to baryon abundance.

Muons decay faster than they are produced below this temperature.

Abstract

Muon abundance is required for the understanding of several fundamental questions regarding properties of the primordial Universe. In this paper we evaluate the production and decay rates of muons in the cosmic plasma as a function of temperature. This allows us to determine when exactly the muon abundance disappears. When the Universe cools below the temperature $kT_\mathrm{disappear}\approx 4.135$ MeV the muon decay rate overwhelms production rates and muons vanish quasi-instantaneously from the Universe. Interestingly, we show that at $T_\mathrm{disappear}$ the muon number is nearly equal to baryon abundance.