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 m(a) < 1.05 eV (95% CL), corresponding to a limit on the axion decay constant of f(a) > 5.7 x 10(6) 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 Sigma m(nu) < 0.65 eV ( 95% CL) for neutrinos.