CAST search for sub-eV mass solar axions with 3He buffer gas

TL;DR

The CAST experiment used 3He buffer gas to extend solar axion searches to higher masses, setting new limits on axion-photon coupling and excluding KSVZ axions in a specific mass range.

Contribution

This work introduces the use of 3He buffer gas in CAST to probe higher axion masses, achieving the first exclusion of KSVZ axions at these masses.

Findings

Excluded KSVZ axions at the upper end of the mass range

Set an upper limit on axion-photon coupling of g_ag < 2.3 x 10^{-10} GeV^{-1}

Scanned axion mass range 0.39 eV to 0.64 eV

Abstract

The CERN Axion Solar Telescope (CAST) has extended its search for solar axions by using 3He as a buffer gas. At T=1.8 K this allows for larger pressure settings and hence sensitivity to higher axion masses than our previous measurements with 4He. With about 1 h of data taking at each of 252 different pressure settings we have scanned the axion mass range 0.39 eV < m_a < 0.64 eV. From the absence of excess X-rays when the magnet was pointing to the Sun we set a typical upper limit on the axion-photon coupling of g_ag < 2.3 x 10^{-10} GeV^{-1} at 95% CL, the exact value depending on the pressure setting. KSVZ axions are excluded at the upper end of our mass range, the first time ever for any solar axion search. In future we will extend our search to m_a < 1.15 eV, comfortably overlapping with cosmological hot dark matter bounds.