Probing eV-scale axions with CAST

Abstract

We have searched for solar axions or other pseudoscalar particles that couple to two photons by using the CERN Axion Solar Telescope (CAST) setup. Whereas we previously have reported results from CAST with evacuated magnet bores (Phase I), setting limits on lower mass axions, here we report results from CAST where the magnet bores were filled with He-4 gas (Phase II) of variable pressure. The introduction of gas generates a refractive photon mass m(gamma), thereby achieving the maximum possible conversion rate for those axion masses m(a) that match m(gamma). With 160 different pressure settings we have scanned m(a) up to about 0.4 eV, taking approximately 2 h of data for each setting. 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) less than or similar to 2.2 x 10(-10) GeV-1 at 95% CL for m(a) less than or similar to 0.4 eV, the exact result depending on the pressure setting. The excluded parameter range covers realistic axion models with a Peccei-Quinn scale in the neighborhood of f(a) similar to 10(7) GeV. Currently in the second part of CAST Phase II, we are searching for axions with masses up to about 1.2 eV using He-3 as a buffer gas.