gauge - boson production from LFV τ decays at Belle II

proposed that such a light gauge boson can be detected at the Belle II experiment with a luminosity of 50 ab−1 by the e+e− → γ + / E process through the kinetic mixing with the photon, where the missing energy / E is from the Z ′ → ν̄ν decays. We study the phenomenological implications when a pair of singlet vector-like leptons carrying different Lμ −Lτ charges are included, and a complex singlet scalar (φS) is introduced to accomplish the spontaneous U(1)Lμ−Lτ symmetry breaking. It is found that the extension leads to several phenomena of interest, and they include: (i) branching ratio (BR) for h → μτ can be of the order of 10−3; (ii) φS-mediated muon g − 2 can be of the order of 10 × 10−10; (iii) BR for τ → μφS → μZ ′Z ′ can be 10−8; and (iv) kinetic mixing between the Z ′ boson and photon is sensitive to the relative heavy lepton masses. The predicted BRs for τ → (3μ+ / E, 5μ) through the leptonic Z ′ decays can reach a level of 10−9, in which the results fall