An improved cosmological bound on the thermal axion mass

TL;DR

This paper refines the cosmological upper limits on thermal axion mass using recent observational data, considering neutrino mass hierarchies, and discusses future experimental tests like CAST.

Contribution

It provides the most stringent cosmological bounds on hadronic axion mass to date, including the effects of neutrino mass hierarchies.

Findings

Axion mass < 0.42 eV at 95% confidence level

Inverted neutrino hierarchy tightens the bound to < 0.38 eV

Future experiments like CAST can test these bounds.

Abstract

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 m_a in the so-called hadronic axion window. We obtain a limit on the axion mass m_a < 0.42eV at the 95% c.l. (m_a < 0.72eV 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 m_a < 0.38eV at the 95% c.l. (m_a < 0.67eV at the 99% c.l.). Future data from experiments as CAST will provide a direct test of the cosmological bound.