Dark Matter Interpretation of the Super-Kamiokande Antineutrino Excess in $\mathrm{U}(1)_{L_μ-L_τ}$ model

Abstract

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 $2.3σ$ with electron-like antineutrino events at $E_{\barν_e} \simeq 20\,\mathrm{MeV}$. We investigate whether this excess can be explained by MeV-scale dark matter annihilation into neutrinos in a $\mathrm{U}(1)_{L_μ- L_τ}$ model. The dark matter is a Dirac fermion with $m_χ\simeq 22\,\mathrm{MeV}$ that annihilates via a light $Z'$ mediator into $ν_μ\barν_μ$ and $ν_τ\barν_τ$, which are partly converted into $\barν_e$ 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-$μ$, Borexino, and the contribution to $ΔN_\mathrm{eff}$.