New solar axion search using the CERN Axion Solar Telescope with ${}^4\mathrm{He}$ filling

The CERN Axion Solar Telescope (CAST) searches for $a\to \gamma$ conversion in the 9 T magnetic field of a refurbished LHC test magnet that can be directed toward the Sun. Two parallel magnet bores can be filled with helium of adjustable pressure to match the x-ray refractive mass $m_{\gamma }$ to the axion search mass $m_a$. After the vacuum phase (2003–2004), which is optimal for $m_a\lesssim 0.02\mathrm{eV}$, we used ${}^4\mathrm{He}$ in 2005–2007 to cover the mass range of 0.02–0.39 eV and ${}^3\mathrm{He}$ in 2009–2011 to scan from 0.39 to 1.17 eV. After improving the detectors and shielding, we returned to ${}^4\mathrm{He}$ in 2012 to investigate a narrow $m_a$ range around 0.2 eV (“candidate setting” of our earlier search) and 0.39–0.42 eV, the upper axion mass range reachable with ${}^4\mathrm{He}$, to “cross the axion line” for the KSVZ model. We have improved the limit on the axion-photon coupling to $g_{a\gamma }<1.47\times {10}^{-10}{\mathrm{GeV}}^{-1}$ (95% C.L.), depending on the pressure settings. Since 2013, we have returned to the vacuum and aim for a significant increase in sensitivity.

Figure 1

Exclusion regions in the $m_a\text{−}{g}_{a\gamma }$ plane achieved by CAST in the vacuum phase [32, 33] and with ${}^4\mathrm{He}$ [34] and ${}^3\mathrm{He}$ [35, 36] filling. We also show constraints from horizontal branch (HB) stars [50, 51] (a similar limit stems from blue-loop suppression in massive stars [52]), and the hot dark matter (HDM) bound [42, 43, 44, 45, 46, 47]. The yellow band represents typical theoretical models with $|E/N-1.95|=0.07–7$. The green solid line corresponds to $E/N=0$ as in the KSVZ model [48, 49], a typical hadronic axion model. In red we show our new limits near $m_a=0.2$ and 0.4 eV from our 2012 data-taking campaign with ${}^4\mathrm{He}$ gas.

Figure 2

Comparison between background (blue bars) and tracking (red bars) spectra of one Micromegas detectors installed in the sunset side.

Figure 3

Expanded view of the limit achieved during the 2012 CAST data-taking campaign, using ${}^4\mathrm{He}$ as buffer gas. The plot on the bottom corresponds to the “candidate setting” at 0.2 eV, while the plot on the top is the excluded region above 0.4 eV. The green line represents the KSVZ benchmark model with $E/N=0$.