Axion–nucleon sensitivity using ultra-low background Micromegas detectors for the International Axion Observatory

The International Axion Observatory (IAXO) aims to detect ultra-light bosonic dark matter candidates by combining strong magnetic fields, X-ray optics, and ultra-low background detectors. This work investigates the efficiency and sensitivity of various Micromegas detector configurations within IAXO, with a particular focus on the challenging axion-nucleon coupling channel. Through detailed simulations, we evaluate the intrinsic efficiency of the IAXO-D1 prototype, exploring both conventional argon-isobutane gas mixtures and a custom blend composed of 48.85% xenon, 48.85% neon, and 2.3% isobutane, under different pressure conditions. Detector responses are studied across a range of axion spectra. To support this analysis, a dedicated simulation framework was developed by integrating the Rare Event Searches Toolkit (REST) with Garfield++, Magboltz, and Geant4. The results highlight that Micromegas detectors operated with the xenon–neon-based gas mixture exhibit a marked improvement in performance compared to the nominal IAXO-D1 baseline configuration, with a signal-to-noise ratio enhancement of up to a factor of 3.7 in the case of axion–nucleon coupling sensitivity.