A closer look at the LZ 248 keV event through the lens of cosmic-ray boosted dark matter

Abstract

We investigate whether cosmic ray boosted dark matter (CRDM) can explain the 248 keV nuclear recoil event observed by the LUX-ZEPLIN (LZ) collaboration. We consider spin-dependent interactions, mediated by a pseudoscalar, and incorporate relativistic kinematics in CRDM production, propagation, and detection. We find that spin-dependent interactions can produce sufficient number of nuclear recoils in the observed region of interest, without populating events at lower energies, which have not been observed. However, for a benchmark pseudoscalar interaction corresponding to the non-relativistic effective operator $\mathbb{O}_6$, the coupling required to produce the observed event corresponds to an energy scale comparable to the momentum transfer scales, thus making contact interaction approach challenging. Instead, we find that a pseudoscalar mediator with mass $\sim 300\,$MeV and $O(1)$ couplings can provide a promising solution to this unexplained event.