Search for solar axions produced through the axion-electron coupling $g_{ae}$ using a new GridPix detector at CAST

Abstract

We present a search for solar axions produced through the axion-electron coupling $(g_{ae})$ using data from a novel 7-GridPix detector installed at the CERN Axion Solar Telescope (CAST). The detector, featuring ultra-thin silicon nitride windows and multiple veto systems, collected approximately 160 hours of solar tracking data between 2017-2018. Using machine learning techniques and the veto systems, we achieved a background rate of $1.06\times 10^{-5}{\text{keV}}^{-1}{\text{cm}}^{-2}{\text{s}}^{-1}$ at a signal efficiency of about $80\%$ in the $0.2$-$8\text{keV}$ range. Analysis of the data yielded no significant excess above background, allowing us to set a new upper limit on the product of the axion-electron and axion-photon couplings of $g_{ae}\cdot g_{a{\hat{I}}^3}<7.35\times 10^{-23}{\text{GeV}}^{-1}$ at $95\%$ confidence level. This result improves upon the previous best helioscope limit and demonstrates the potential of GridPix technology for rare event searches. Additionally, we derived a limit on the axion-photon coupling of $g_{a{\hat{I}}^3}<9.0\times 10^{-11}{\text{GeV}}^{-1}$ at $95\%$ CL, which, while not surpassing CAST's best limit, provides complementary constraints on axion models.