A viable Le-Lμ model with μ→e violation

We extend the Standard Model gauge group by U ( 1 ) L e - L μ and introduce two scalars, a doublet and a singlet, that are charged under this new group and have lepton flavour violating couplings. Since in this model μ → e processes can only be mediated by μ → τ × τ → e interactions, bounds from μ → e transitions can be avoided while allowing for accessible new physics. We consider the case of a Z ' boson with a mass of M Z ' ≃ 10 GeV and a gauge coupling g ' ≃ 10 - 4 , which is in reach of Belle-II, and a long-lived Z ' boson with a mass of MeV ≲ M Z ' ≲ m μ - m e which can be probed by searching for ℓ → ℓ ' + inv . . Neutrino masses and mixing angles can also be accounted for if sterile neutrinos are added to the spectrum.

Introduction to Charged Lepton Flavor Violation

Neutrino masses are evidence of lepton flavor violation, but no violation in the interactions among the charged leptons has been observed yet. Many models of Physics Beyond the Standard Model (BSM) predict Charged Lepton Flavor Violation (CLFV) in a wide spectrum of processes with rates in reach of upcoming experiments. The experimental searches that provide the current best limits on the CLFV searches are reviewed, with a particular emphasis on the muon-based experiments that give the most stringent constraints on the BSM parameter space. The next generation of muon-based experiments (MEG-II, Mu2e, COMET, Mu3e) aims to reach improvements by many orders of magnitude with respect to the current best limits, thanks to several technological advancements. We review popular heavy BSM theories, and we present the calculations of the predicted CLFV branching ratios, focusing on the more sensitive μ→e sector.