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

We revise cosmological mass bounds on hadronic axions in low-reheating cosmological scenarios, with a reheating temperature TRH ≤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 ma<0.26 eV. However, axions with masses ma≃1 eV, or heavier, would be allowed for reheating temperatures TRH≲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.