Search

Upcoming experiments will improve the sensitivity to μ → e processes by several orders of magnitude, and could observe lepton flavour-changing contact interactions for the first time. In this paper, we investigate what could be learned about New Physics from the measurements of these μ → e observables, using a bottom-up effective field theory (EFT) approach and focusing on three popular models with new particles around the TeV scale (the type II seesaw, the inverse seesaw and a scalar leptoquark). We showed in a previous publication that μ → e observables have the ability to rule out these models because none can fill the whole experimentally accessible parameter space. In this work we give more details on our EFT formalism and present more complete results. We discuss the impact of some observables complementary to μ → e transitions (such as the neutrino mass scale and ordering, and LFV τ decays) and draw attention to the interesting appearance of Jarlskog-like invariants in our expressions for the low-energy Wilson coefficients. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]