Constraints on Sub-MeV Dark Matter from Solar Reflection with DAMIC-M

Abstract

The Sun acts as a natural dark matter accelerator. Galactic halo particles scattering in the solar plasma emerge with velocities well beyond the Galactic escape speed, providing a boosted flux that extends the kinematic reach of direct detection experiments into the sub-MeV mass regime. We present constraints on solar-reflected dark matter (SRDM) from the DAMIC-M prototype detector using $\sim$1.3~kg-day of data acquired with silicon skipper charge coupled devices (CCDs) at the Modane Underground Laboratory. Exploiting the spatial diffusion signature of low-energy electron recoils, we derive 90% C.L. upper limits on the DM-electron scattering cross section for both heavy- and ultralight-mediator benchmarks, reaching $\barσ_e\sim 3.16 \cdot 10^{-37} \rm{cm^2}$ at $0.1$ MeV. For the ultralight mediator, our limits are competitive with the world-leading constraints, achieved with an integrated exposure of $\sim1.3$~kg-day. These results probe the parameter space between stellar-cooling bounds and terrestrial limits from standard halo searches, a region inaccessible to direct detection experiments relying solely on the standard halo flux.