Systematic analysis of search strategies for $L_μ-L_τ$ gauge bosons at Belle II

Abstract: Extensions of the Standard Model with masses at or below the GeV scale are motivated by searches for dark matter and precision measurements in the quark and lepton flavour sectors, including that of the muon anomalous magnetic moment. An excellent experimental environment to test such light new physics is given by the Belle II experiment, which foresees to take up to 50 ab$^{-1}$ of data. Here we consider a model with an additional gauged $U(1){L\mu - L_\tau}$ symmetry that introduces a neutral gauge boson, a Dark Photon, with possibly large couplings to muon- and tau-flavored leptons, including neutrinos. Dark Photon mixing with the Standard Model photon is loop induced, allowing it to couple to electrically charged fermions other than muons and taus. We systematically investigate the possible search strategies for Dark Photons with four fermions final states. We identified search channels with muons as the most promising ones, and we analyse the kinematic distributions to obtain cuts that optimise the sensitivity of Belle II searches for the Dark Photon. Summarising the sensitivities from the most promising search channels we provide a comprehensive overview of future searches at Belle II.