Thermal axions with multi-eV masses are possible in low-reheating scenarios

TL;DR

This paper revises cosmological bounds on hadronic axions in low-reheating scenarios, showing that multi-eV axions are possible and could evade current detection methods, thus expanding the viable axion mass range.

Contribution

It provides updated cosmological mass bounds on hadronic axions considering low-reheating temperatures, highlighting the possibility of multi-eV axions being consistent with observations.

Findings

Standard axion mass bound is less than 0.26 eV.

Axions around 1 eV or heavier are allowed if reheating temperature is below 80 MeV.

Multi-eV axions are beyond current experimental detection capabilities.

Abstract

We revise cosmological mass bounds on hadronic axions in low-reheating cosmological scenarios, with a reheating temperature $T_{\rm RH}~\le 100$ MeV, in light of the latest cosmological observations. In this situation, the neutrino decoupling would be unaffected, while the thermal axion relic abundance is suppressed. Moreover, axions are colder in low-reheating temperature scenarios, so that bounds on their abundance are possibly loosened. As a consequence of these two facts, cosmological mass limits on axions are relaxed. Using state-of-the-art cosmological data and characterizing axion-pion interactions at the leading order in chiral perturbation theory, we find in the standard case an axion mass bound $m_a < 0.26$ eV. However, axions with masses $m_a \simeq 1$ eV, or heavier, would be allowed for reheating temperatures $T_{\rm RH} \lesssim 80$ MeV. Multi-eV axions would be outside the mass sensitivity of current and planned solar axion helioscopes and would demand new experimental approaches to be detected.