We study a simple Dirac fermion dark matter model in U(1)Lμ−Lτ theory. The new light gauge boson X plays important roles in both dark matter physics and the explanation for the muon g− 2 anomaly. The observed dark matter relic density is realized by a large U(1)Lμ−Lτ charge without introducing a resonance effect of the X boson. As a by-product of the model, characteristic neutrino signatures from sub-GeV dark matter ψ are predicted depending on the mass spectrum. We formulate the analysis of ψψ¯ → νν¯ , and of ψψ¯ → XX followed by X→ νν¯ in a model independent way. The energy spectrum of neutrinos in the former process is monochromatic while in the latter process is bowl-shape. We also evaluate sensitivity at Super-Kamiokande and future Hyper-Kamiokande detectors. The analysis is finally applied to the U(1)Lμ−Lτ dark matter model.
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