Low-energy threshold demonstration for dark matter searches in TREX-DM with an ${}^{37}$Ar source produced at CNA HiSPANoS

We report on the successful implementation of an ${}^{37}$Ar calibration source in the TREX-DM detector, a high-pressure time projection chamber designed for low-mass dark matter searches. The ${}^{37}$Ar source was produced through fast neutron activation of CaO powder at the HiSPANoS facility of Centro Nacional de Aceleradores (CNA) in Spain, yielding $O(1)$ kBq of activity. Using a novel combined GEM-Micromegas readout system, we successfully detected both characteristic emissions from ${}^{37}$Ar decay (2.82 keV and 270 eV) and achieved unprecedented energy threshold performance in TREX-DM, approaching the single-electron ionization energy of argon.