Search for solar axions produced through the axion-electron coupling g_ae using a new GridPix detector at CAST

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 × 10⁻⁵ keV⁻¹ cm⁻² s⁻¹ at a signal efficiency of about 80% in the 0.2 to 8 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 · g_aγ < 7.35 × 10⁻²³ GeV⁻¹ at 95% confidence level for axion masses below 10 meV. 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_aγ < 9.0 × 10⁻¹¹ GeV⁻¹ at 95% CL, which, while not surpassing CAST’s best limit, provides complementary constraints on axion models.