Measuring the properties of hydrogen doped liquid xenon for current and future liquid xenon observatories

Despite decades of progress, the particle nature of dark matter (DM) remains still unknown. Dual-phase liquid xenon (LXe) time projection chambers (TPCs) excluded large regions of GeV-scale DM parameter space thanks to their excellent self-shielding against external backgrounds, low energy thresholds, and high energy resolution. The same properties make LXe TPCs sensitive to other rare-event phenomena, like neutrinoless double beta decay (0νββ) of 136Xe or solar neutrinos. Building on this success, the XLZD collaboration aims to construct a next-generation LXe observatory with a target mass of 60–80 t , to probe this parameter space down to the neutrino fog.With no confirmed DM signal, attention has extended also to MeV–GeV scale DM candidates motivated by theories beyond the Standard Model, such as asymmetric DM. However, the large nuclear mass of xenon limits sensitivity in this regime due to unfavorable recoil kinematics. The HydroX collaboration proposes percent-level doping of LXe with H2 introducing light scattering targets while preserving the background rejection power and scalability of LXe TPCs. In addition to enabling sensitivity to sub-GeV DM, H2-doping is expected to enhance event topology discrimination by increasing electron drift velocity and reducing longitudinal diffusion, offering potential benefits for 0νββ and neutrino searches.Given that no published measurements of H-doped LXe exists, XLZD-NixeH aims to systematically investigate the impact of H2-doping on LXe by leading the UK effort of HydroX. Key objectives are: (O1) characterization of electron transport and charge/light yield modifications; (O2) measurement of proton recoil responses relative to xenon nuclear recoils; and (O3) development of machine-learning-based reconstruction techniques for high-energy electron tracks with and without H2 doping. These studies will significantly contribute to HydroX and the feasibility of H2-doping of current and future LXe observatories.