Search for Sub-eV Mass Solar Axions by the CERN Axion Solar Telescope with ${}^3\mathrm{He}$ Buffer Gas

The CERN Axion Solar Telescope (CAST) has extended its search for solar axions by using ${}^3\mathrm{He}$ as a buffer gas. At $T=1.8\mathrm{K}$ this allows for larger pressure settings and hence sensitivity to higher axion masses than our previous measurements with ${}^4\mathrm{He}$. With about 1 h of data taking at each of 252 different pressure settings we have scanned the axion mass range $0.39\mathrm{eV}\lesssim m_a\lesssim 0.64\mathrm{eV}$. 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 $g_{a\gamma }\lesssim 2.3\times {10}^{-10}{\mathrm{GeV}}^{-1}$ at 95% C.L., the exact value depending on the pressure setting. Kim-Shifman-Vainshtein-Zakharov axions are excluded at the upper end of our mass range, the first time ever for any solar axion search. In the future we will extend our search to $m_a\lesssim 1.15\mathrm{eV}$, comfortably overlapping with cosmological hot dark matter bounds.

Figure 1

Exclusion regions in the $m_a-g_{a\gamma }$ plane achieved by CAST in the vacuum [20, 21], ${}^4\mathrm{He}$ [22], and ${}^3\mathrm{He}$ phase. We also show constraints from the Tokyo helioscope [17, 18, 19], horizontal branch (HB) stars [11], and the hot dark matter (HDM) bound [6]. The yellow band labeled “Axion models” represents typical theoretical models with $|E/N-1.95|=0.07–7$. The green solid line inside the band corresponds to $E/N=0$ (KSVZ model).

Figure 2

Expanded view of the limit achieved in the ${}^3\mathrm{He}$ CAST phase for axion mass range between 0.39 and 0.64 eV.