First Results from the CERN Axion Solar Telescope

Hypothetical axionlike particles with a two-photon interaction would be produced in the sun by the Primakoff process. In a laboratory magnetic field (“axion helioscope”), they would be transformed into x-rays with energies of a few keV. Using a decommissioned Large Hadron Collider test magnet, the CERN Axion Solar Telescope ran for about 6 months during 2003. The first results from the analysis of these data are presented here. No signal above background was observed, implying an upper limit to the axion-photon coupling $g_{a\gamma }<1.16\times {10}^{-10}{\mathrm{GeV}}^{-1}$ at 95% C.L. for $m_a\lesssim 0.02\mathrm{eV}$. This limit, assumption-free, is comparable to the limit from stellar energy-loss arguments and considerably more restrictive than any previous experiment over a broad range of axion masses.

Figure 1

Panels (a) and (b) show, respectively, the experimental subtracted spectrum of the TPC data set and MM data set A, together with the expectation for the best fit $g_{a\gamma }$ (lower curve) and for the 95% C.L. limit on $g_{a\gamma }$. For (a) the vertical dashed lines indicate the fitting window. The structure at 3 keV in the expected spectrum reflects the change in the efficiency curves due to the Ar $K$ edge of the detector gas mixtures. Panel (c) shows both the tracking (dots) and background (dashed line) spectra of the CCD data set, together with the expectation (background plus signal) for $g_{a\gamma }$ at its 95% C.L. limit, in units of total counts in the restricted CCD area ($54.3{\mathrm{mm}}^2$) in the tracking exposure time (121.3 h).

Figure 2

Exclusion limit (95% C.L.) from the CAST 2003 data compared with other constraints discussed in the introduction. The shaded band represents typical theoretical models. Also shown is the future CAST sensitivity as foreseen in the experiment proposal.