Lepton-flavour violating decays in theories with dimension 6 operators

TL;DR

This paper uses an effective field theory approach to analyze lepton-flavour violating decays, deriving constraints on new physics operators from experimental limits, and compares the sensitivity of different search methods.

Contribution

It provides a comprehensive, model-independent analysis of dimension 6 operators causing lepton-flavour violation, including next-to-leading order effects and comparison of search sensitivities.

Findings

Stringent limits on Wilson coefficients of lepton-flavour violating operators.

Low-energy experiments impose stronger constraints than high-energy searches for certain decays.

Flavour-violating Z-boson decays are more constrained by low-energy processes.

Abstract

Despite a large experimental effort, so far no evidence for flavour-violating decays of charged leptons such as $l_i\to l_j\gamma$ and $l_i\to l_j l_k l_k$ 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 $Z\to \mu e$ are much more constrained from low-energy experiments $\mu\to e \gamma$ than from the limits of current and future direct searches at high energy.