A Light Lepton-flavor-violating Flavon: the Messenger of Neutrino Mixing and Muon $g-2$

Abstract

In neutrino physics, a class of models with local or global family symmetries may be invoked, and then a flavon field is needed to realize the full neutrino mixing. This flavon may shed light on the long-standing muon $g-2$ puzzle. In this work, we explore this idea in the $({B-L})_{13}$ gauge extension to the standard model (SM), in which realistic neutrino mixing requires both a SM singlet flavon $s$ and a vector-like lepton (VLL) doublet. The dominant coupling between the flavon and leptons is in the manner of lepton-flavor-violation (LFV). Through an analytical analysis of the SM lepton-VLL mixing matrix, we find that the parameter space of the $s\bar\mu e$-type flavon to explain the muon $g-2$ has been completely excluded by the specific LFV process, muonium-antimuonium oscillation. But the $s\bar\mu \tau$-type flavon still has the opportunity; however, it confronts the strong constraint from $\tau\to \mu$ conservation and, in particular, the lepton flavor universality test of $Z$ boson decay, which arises due to our way to realize the LFV flavon. The surviving flavon is highly predictable, with mass in the narrow window $m_\tau\lesssim m_s\lesssim 1.5~ m_\tau$ and LFV coupling strength $\sim 10^{-2}$. Besides, it leaves a TeV scale VLL with a multi-lepton signature at the LHC.