New solar axion search in CAST with $^4$He filling

TL;DR

This paper reports on the CAST experiment's search for solar axions using helium-filled magnet bores to match the axion mass, improving limits on axion-photon coupling and exploring a range of axion masses.

Contribution

The study introduces a novel approach of filling CAST's magnet bores with helium to scan specific axion mass ranges, enhancing sensitivity compared to previous vacuum searches.

Findings

Set a new upper limit on axion-photon coupling: $g_{a ext{ extminus} ext{} ext{}} < 1.47 imes 10^{-10} ext{ GeV}^{-1}$.

Successfully scanned axion masses from 0.02 to 0.42 eV using helium fillings.

Improved detector sensitivity and shielding contributed to more stringent constraints.

Abstract

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 $m_a\lesssim0.02$ eV, we used $^4$He in 2005--2007 to cover the mass range of 0.02--0.39 eV and $^3$He in 2009--2011 to scan from 0.39--1.17 eV. After improving the detectors and shielding, we returned to $^4$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 $^4$He, to "cross the axion line" for the KSVZ model. We have improved the limit on the axion-photon coupling to $g_{a\gamma}< 1.47\times10^{-10} {\rm GeV}^{-1}$ (95% C.L.), depending on the pressure settings. Since 2013, we have returned to vacuum and aim for a significant increase in sensitivity.