Charged-lepton decays from soft flavour violation

E.H Aeikens,W Grimus,L Lavoura

doi:10.1016/j.physletb.2017.03.012

Abstract

We consider a two-Higgs-doublet extension of the Standard Model, with three right-handed neutrino singlets and the seesaw mechanism, wherein all the Yukawa-coupling matrices are lepton flavour-diagonal and lepton flavour violation is soft, originating solely in the non-flavour-diagonal Majorana mass matrix of the right-handed neutrinos. We consider the limit mR→∞ of this model, where mR is the seesaw scale. We demonstrate that there is a region in parameter space where the branching ratios of all five charged-lepton decays ℓ1−→ℓ2−ℓ3+ℓ3− are close to their experimental upper bounds, while the radiative decays ℓ1−→ℓ2−γ are invisible because their branching ratios are suppressed by mR−4. We also consider the anomalous magnetic moment of the muon and show that in our model the contributions from the extra scalars, both charged and neutral, can remove the discrepancy between its experimental and theoretical values.

Highlights

In this paper we resume an old idea of two of us [1]: in a multi-Higgs-doublet model furnished with three right-handed neutrino singlets and the seesaw mechanism [2], lepton flavour may be conserved in the Yukawa couplings of all the Higgs doublets and violated solely in the Majorana mass terms of the right-handed neutrinos ν R ( = e, μ, τ ), viz. in (M R ) C ν T R + H.c., (1)1, 2 where C is the charge-conjugation matrix in Dirac space and M R is a non-singular symmetric matrix in flavour space
Lepton flavour-violating processes were explicitly investigated at one-loop order in ref. [3] and the following property of our framework was discovered
In this paper we have demonstrated, through an explicit numerical example, that there is a class of models where the radiative

Summary

Introduction

The amplitudes of the lepton flavour-violating processes involving gauge bosons, like μ− → e−γ and Z 0 → e−μ+, scale down as 1 m2R when mR → ∞; this holds even when in those processes the gauge bosons γ and Z 0 are virtual, i.e. they are off-mass shell; ii. The amplitudes of the box diagrams for lepton flavour-violating processes like τ − → μ−μ−e+ and τ − → e−e−μ+ scale down as 1 m2R for a large seesaw scale; iii. If n ≥ 2, the amplitudes for lepton flavour-violating processes. Sb0 ∗ → happens e+e−, to the is unsuppressed in the amplitudes of the four τ limit mR − decays followed by. +− 23 3 are unsuppressed when mR → ∞ It is the purpose of this paper to investigate those decays numerically in the framework of ref. MR is much larger than the masses of the scalars in the model, which we assume to be in between one and a few TeV

Two new experiments are planned in search for lepton flavour-violation at the

The effective lepton flavour-violating interaction

Two Higgs doublets

The anomalous magnetic moment of the muon

Numerics

Conclusions