Cosmic ray electron boosted light dark matter: Implications of LZ 2025 data

Abstract

Current multi-ton detectors put stringent constraints on the GeV-scale galactic dark matter, pushing the allowed cross-section almost towards the neutrino fog, yet remain mostly insensitive to the light dark matter. Cosmic rays can upscatter the non-relativistic halo dark matter particles, making a sub-population of them gain sufficient kinetic energy to be discernible in current direct search experiments. In this work, we explore this alternate strategy to probe sub-MeV electrophilic dark matter boosted by cosmic rays with the latest data of LZ 2025 (WS2024 run) and improve the constraint on the MeV scale dark matter by almost $\sim\mathcal{O}(1)$ compared to the previous XENONnT limit for energy-independent cross-section. Using realistic energy-dependent cross-sections, we also analyse such a scenario, where the associated mediator mass plays a crucial role in governing the event rate and hence the expected limits too. With energy-dependent cross-sections, our obtained limits also remain stronger than the existing constraints from current direct detection experiments. Even compared to the limits from the neutrino detectors with a larger target size, LZ 2025 can put stringent constraints in certain parameter space of the mediator, excluding the previously unexplored regions.