A scalar-extended ${\rm U(1)_{L_μ-L_τ}}$ explanation of the LUX-ZEPLIN 248 keV excess

Abstract: The recently observed dark matter (DM) nuclear recoil event at an energy of $$248 \pm 23\,(\text{stat.}) \pm 23\,(\text{syst.})\,\text{keV}$$ at the LUX-ZEPLIN (LZ) experiment, favoring an inelastic scattering interpretation, has motivated various beyond the Standard Model (BSM) scenarios aimed at explaining the observed excess. In particular, this provides an opportunity to probe models that naturally accommodate inelastic DM scattering. In this work, we explore one such possibility by extending the $\rm U(1)<em>{L</em>μ-L_τ}$ model with two complex scalar fields, $Φ$ and $S$, carrying charges $-1$ and $+2$, respectively, under the $\rm U(1)<em>{L</em>μ-L_τ}$ gauge symmetry. These charge assignments allow a trilinear interaction between the two scalar fields, which is crucial for generating the DM mass splitting. Following the spontaneous breaking of the $\rm U(1)<em>{L</em>μ-L_τ}$ symmetry, with $S$ acquiring a vacuum expectation value and generating a mass for the BSM gauge boson $Z_{μτ}$, the real and imaginary components of $Φ$ acquire a mass splitting. This naturally gives rise to an inelastic DM scattering scenario that can accommodate the LZ excess. We perform a likelihood-based assessment of the sensitivity of the model parameters to the observed event while consistently accounting for other relevant phenomenological constraints. Finally, we discuss the prospects for probing the viable parameter space at future experiments.