| 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 \\hefour gas (Phase II) of variable pressure.
The introduction of gas generated a refractive photon mass $m_\\gamma$, thereby achieving the maximum possible
conversion rate for those axion masses \\ma~that match $m_\\gamma$.
With 160 different pressure settings we have scanned
\\ma~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 $\\gag\\lesssim 2.17\\times
10^{-10}\\;{\\rm GeV}^{-1}$ at 95\\% CL for $\\ma \\lesssim
0.4$\\;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 $f_{\\rm a}\\sim10^{7}$\\;GeV.
Currently in the second part of CAST Phase II, we are searching for axions with
masses up to about 1.2\\;eV using \\hethree as a buffer gas. |
