Z lepton flavour violation as a probe for new physics at future $e^+e^-$ colliders

Abstract

In this work we assess the potential of discovering new physics by searching for leptonflavour-violating (LFV) decays of the Z boson, Z → i j , at the proposed circular e + e − colliders CEPC and FCC-ee. Both projects plan to run at the Z-pole as a "Tera Z factory", i.e., collecting O 10 12 Z decays. In order to discuss the discovery potential in a modelindependent way, we revisit the LFV Z decays in the context of the Standard Model effective field theory and study the indirect constraints from LFV µ and τ decays on the operators that can induce Z → i j . We find that, while the Z → µe rates are beyond the expected sensitivities, a Tera Z factory is promising for Z → τ decays, probing New Physics at the same level of future low-energy LFV observables.

Renormalization group improved implications of semileptonic operators in SMEFT

“…While finalizing our paper we notice Ref. [66] that appeared on the arXiv. In this paper, a study of the LFV decays in the context of semileptonic decays is presented.…”

Leptonic CP violation from a vector-like lepton

“…Thus u j is real up to rephasing. But then, from the discussion of the first part, u j is also an eigenvector of A 1 similarly to (51) and u j = e 0 up to rephasing.…”

“…The flavor changing Z decays for example currently constrain |X | 10 −3 . In the future Tera Z factories are expected to reach a sensitivity similar of low energy CLFV observables [51] in the channels Z → τ . Lepton flavor universality violation tests are already considered in the fit of Ref.…”

Flavorful vector-like leptons and the lepton-flavor-violating decays Z → lilj

“…Thus, the effects of the VLS models can be probed via electroweak precision tests, the LFV decay and production processes. The LFV Z decays Z → l i l j (i = j) are one kind of processes, which are accessible at high-energy colliders and have been extensively studied in the literature (for example see [12] and its references). Taking into account the constraints on the VLS models, we will investigate the LFV decays Z → l i l j in this paper and see whether the contributions of the VLS models to Z → l i l j can be detected in future e + e − collider experiments, such as CEPC and FCC-ee.…”