Nearly Degenerate Majorana Dark Matter and Its Self-Interactions in a Gauged $U(1)_{L_\mu - L_\tau}$ Model

Abstract

We propose a model of nearly degenerate Majorana dark matter (DM) in a gauged $U(1)_{L_\mu - L_\tau}$ extension of the Standard Model. Spontaneous symmetry breaking generates a dominant Majorana mass via a strong Yukawa coupling, splitting the dark fermion into two nearly degenerate states. The lighter state ($\chi_-$) is the DM candidate, with an allowed mass of $\sim 10$ GeV to several hundred GeV. Crucially, within the $10 \sim 75$ GeV mass range and a scalar mediator at the tens of MeV scale, this strong coupling dictates the thermal relic abundance and induces significant elastic self-interactions. These self-interactions naturally resolve small-scale structure anomalies, such as the core-cusp problem, while satisfying massive cluster constraints. Furthermore, we place stringent joint constraints on the scalar mass and the Higgs mixing angle $\alpha$ using the latest LZ 2025 direct detection data. The model's gauge boson $Z^\prime$ maintains early-Universe thermal equilibrium and accommodates the muon anomalous magnetic moment $(g-2)_\mu$, remaining consistent with updated cosmological and experimental bounds.