A new cosmological mass limit on thermal relic axions

Observations of the cosmological large-scale structure provide well-established neutrino mass limits. We extend this argument to thermal relic axions. We calculate the axion thermal freeze-out temperature and thus their cosmological abundance on the basis of their interaction with pions. For hadronic axions we find a new mass limit mₐ<1.05 eV (95% CL), corresponding to a limit on the axion decay constant of fₐ>5.7\× 10⁶ GeV. For other models this constraint is significantly weakened only if the axion-pion coupling is strongly suppressed. For comparison we note that the same approach leads to \∑ m_\ν<0.65 eV (95% CL) for neutrinos.