Dark Matter Interpretation of the Super-Kamiokande Antineutrino Excess in U(1)_(L_μ-L_τ) model

Recent Super-Kamiokande analyses of the diffuse supernova neutrino background, based on data across all SK phases, indicate a mild preference over the zero-DSNB hypothesis at the level of about $2.3\sigma$ with electron-like antineutrino events at $E_{\bar{\nu}_e} \simeq 20\,\mathrm{MeV}$. We investigate whether this excess can be explained by MeV-scale dark matter annihilation into neutrinos in a $\mathrm{U}(1)_{L_\mu - L_\tau}$ model. The dark matter is a Dirac fermion with $m_\chi \simeq 22\,\mathrm{MeV}$ that annihilates via a light $Z'$ mediator into $\nu_\mu \bar{\nu}_\mu$ and $\nu_\tau \bar{\nu}_\tau$, which are partly converted into $\bar{\nu}_e$ through flavor oscillations. We find that this scenario simultaneously accounts for the excess and the observed relic abundance via thermal freeze-out. We further discuss the relevant laboratory and cosmological constraints, including neutrino trident production, NA64-$\mu$, Borexino, and the contribution to $\Delta N_\mathrm{eff}$.