Constraints on 2ℓ2q operators from μ↔e flavor-changing meson decays

Abstract

We study lepton flavour violating two-and three-body decays of pseudoscalar mesons in Effective Field Theory (EFT). We give analytic formulae for the decay rates in the presence of a complete basis of QED×QCD-invariant operators. The constraints are obtained at the experimental scale, then translated to the weak scale via one-loop RGEs. The large RGmixing between tensor and (pseudo)scalar operators weakens the constraints on scalar and pseudoscalar operators at the weak scale.

What is Leading Order for LFV in SMEFT?

“…We first consider µ → e processes because the most restrictive experimental constraints on LFV arise in this sector, and upcoming experiments aim to improve the sensitivities by several orders of magnitude (see table 3; indeed, there are plans to reach a conversion ratio 10 −18 for µA → eA [43]). The Branching Ratios can be expressed (see eg [3,10,42]) in terms of the coefficients, evaluated at the experimental scale, of operators which contribute at tree level. For instance, the low-energy operators…”

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.…”

Non-perturbative effects in μ → eγ

“…Detailed analyses of these RGEs have been presented in refs. [10,11,13,19,[42][43][44]. We evaluate the bounds from µ → eγ and µ → e conversion by turning on single operators at a time at the scale µ = M W .…”

“…The constraints of lepton-flavor-violating processes such as µ → eγ and µ → 3e on the Wilson coefficients of both SMEFT and LEFT have attracted a lot of attention in connection with perturbative effects [8][9][10][11], while the non-perturbative effects due to leptonflavor-violating quark operators have been studied mainly in reactions involving hadrons as external states [12][13][14][15][16][17][18][19]. In our analysis, we consider non-perturbative contributions to µ → eγ, a decay that has no strongly interacting external particles.…”