Lepton-flavour violating decays in theories with dimension 6 operators

Abstract

Abstract. Despite a large experimental effort, so far no evidence for flavour-violating decays of charged leptons such as l i → l j γ and l i → l j l k l 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 of Wilson 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.

The SMEFTsim package, theory and tools

“…Experimental constraints on C ψX rs , with r = s are significant in some cases, but not for all possible flavour transitions, see the recent discussion in Refs. [127][128][129][130]. Furthermore, the separation of flavour diagonal and flavour off diagonal interactions of the dipole operators is a scale dependent distinction, flavour mixing is extensive in the SMEFT RGE (see Refs.…”

Reactor neutrino oscillations as constraints on effective field theory

“…The contribution of dimension-6 operators to CLFV observables arises at O(Λ −4 ), which is the leading order in this case because the SM contributions are absent. The resulting constraints on lepton-flavor off-diagonal SMEFT operators are typically very severe [65][66][67][68]. Using the analytical formula for the µ → e conversion rate in Ref.…”

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“…Limits on the off-diagonal Yukawa couplings from their one-loop contribution to 2 → 3 1 decays are weaker [29].…”