Improved cosmological bound on the thermal axion mass

Relic thermal axions could play the role of an extra hot dark matter component in cosmological structure formation theories. By combining the most recent observational data we improve previous cosmological bounds on the axion mass mₐ in the so-called hadronic axion window. We obtain a limit on the axion mass mₐ<0.42 eV at the 95% C.L. (mₐ<0.72 eV at the 99% C.L.). A novel aspect of the analysis presented here is the inclusion of massive neutrinos and how they may affect the bound on the axion mass. If neutrino masses belong to an inverted hierarchy scheme, for example, the above constraint is improved to mₐ<0.38 eV at the 95% C.L. (mₐ<0.67 eV at the 99% C.L.). Future data from experiments as CAST will provide a direct test of the cosmological bound.