Axions and saxions from the primordial supersymmetric plasma and extra radiation signatures

TL;DR

This paper calculates the thermal production rates of axions and saxions in the early universe's supersymmetric plasma, analyzing their potential contribution to extra radiation and implications for cosmological observations.

Contribution

It provides a systematic, gauge-invariant calculation of axion and saxion production rates using thermal field theory, and explores their impact on cosmological extra radiation constraints.

Findings

Thermal saxion decays into axions can explain observed extra radiation.

Updated limits on axion and saxion properties from primordial helium and CMB data.

Upcoming Planck data will significantly test supersymmetric axion models.

Abstract

We calculate the rate for thermal production of axions and saxions via scattering of quarks, gluons, squarks, and gluinos in the primordial supersymmetric plasma. Systematic field theoretical methods such as hard thermal loop resummation are applied to obtain a finite result in a gauge-invariant way that is consistent to leading order in the strong gauge coupling. We calculate the thermally produced yield and the decoupling temperature for both axions and saxions. For the generic case in which saxion decays into axions are possible, the emitted axions can constitute extra radiation already prior to big bang nucleosynthesis and well thereafter. We update associated limits imposed by recent studies of the primordial helium-4 abundance and by precision cosmology of the cosmic microwave background and large scale structure. We show that the trend towards extra radiation seen in those studies can be explained by late decays of thermal saxions into axions and that upcoming Planck results will probe supersymmetric axion models with unprecedented sensitivity.