Probing eV-scale axions with CAST

TL;DR

This paper reports on CAST's search for solar axions using a gas-filled magnet to match photon and axion masses, setting new limits on axion-photon coupling for masses up to 0.4 eV.

Contribution

It introduces a novel gas-pressure scanning method in CAST to probe axion masses up to 0.4 eV, improving constraints on axion properties.

Findings

Set an upper limit on axion-photon coupling of ~2.17×10^{-10} GeV^{-1} for masses up to 0.4 eV.

Scanned 160 pressure settings, each for about 2 hours, to cover a range of axion masses.

Excluded parameter space relevant to realistic axion models with Peccei-Quinn scale around 10^7 GeV.

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 \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.