A Freeze-In Interpretation of the LZ High-Energy Nuclear Recoil Event

Abstract

The recent LUX-ZEPLIN (LZ) experiment analysis reported a high energy nuclear recoil candidate at $E_R \simeq 248~{\rm keV}$, motivating interpretations in terms of inelastic dark matter. We study this event in a pseudo-Dirac fermion model with a vector boson mediator $Z'$, assuming a low reheating temperature and dark matter production through freeze-in at stronger coupling. In contrast to the thermal freeze-out case, the reheating temperature provides an additional parameter controlling the relic abundance and breaks the one-to-one relation between the dark matter mass and direct-detection cross section. As a result, the LZ candidate can be reproduced over a continuous region of parameter space, including mass splitting below the thermal benchmark.