Lepton-flavour violating decays in theories with dimension 6 operators

¹ Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
² Physik-Institut, Universität Zürich, CH-8057 Zürich Switzerland

^a e-mail: Giovanni-Marco.Pruna@psi.ch
^b e-mail: Adrian.Signer@psi.ch

Published online: 26 April 2016

Abstract

Despite a large experimental effort, so far no evidence for flavour-violating decays of charged leptons such as l_i → l_jγ and l_i → l_jl_kl_k has been found. The absence of a signal puts very severe constraints on many extensions of the Standard Model. Here we apply a model independent approach by studying such decays in the Standard Model effective field theory. Going beyond leading order in the Standard Model couplings and considering all dimension 6 operators that might lead to lepton-flavour violation, we are able to extract limits on a large number ofWilson coefficients of such operators. We are also able to compare the impact of particular searches and find, for example, that flavour-violating decays of the Z-boson Z→ µe are much more constrained from low-energy experiments µ → eγ than from the limits of current and future direct searches at high energy.