Maximal flavor violating $U(1)_{L_{\mu}-L_{\tau}}$ model with singly-charged scalar

Abstract

The $U(1)_{L_\mu-L_\tau}$ $Z'$ model has emerged as a promising candidate to address the longstanding muon $(g-2)_\mu$ anomaly. Flavor-conserving $Z'$ interactions are subject to stringent constraints from the neutrino trident process and NA64$\mu$ experiments, which limit the $Z'$ mass to $m_{Z'}<40$ MeV. To circumvent these constraints, flavor-conserving $Z'$ interactions can be converted into maximal flavor-violating interactions through a discrete exchange symmetry. Maximal flavor-violating $Z'$ interactions contribute to $\tau\to\mu\nu\bar\nu$ via the neutral current, yet the parameter space for this process conflicts with the $(g-2)_\mu$ allowed regions. To resolve this conflict, we propose introducing a singly-charged scalar that mediates via charged current interactions. This scalar is anticipated to produce a negative contribution to the lepton $(g-2)_l$ while concurrently inducing the tau decay $\tau\to\mu\nu\bar\nu$. Three distinct scenarios arise from the introduction of singly-charged scalars: the $\mathcal{O}_{LL}$ operator, driven by weak singlet and triplet, and the $\mathcal{O}_{LR}$ operator, driven by weak doublet. Our analysis of the phenomenology in these three cases reveals that the tension between the muon $(g-2)_\mu$ anomaly and $\tau\to\mu\nu\bar\nu$ for large $Z'$ masses can be effectively alleviated only by the singly-charged scalars from the weak triplet, whereas the singlet and doublet scenarios fall short. Furthermore, the singly-charged scalars from the weak triplet offer an additional explanation for the electron $(g-2)_e$ anomaly. Our findings indicate that weak triplets could play a crucial role in $Z'$ models, potentially providing valuable insights for future research into $U(1)$ frameworks.