Prospects for a flavour violating $$Z^\prime $$ explanation of $$\Delta a_{\mu ,e}$$

Abstract

The apparent tensions emerging from the comparison of experimental data of the anomalous magnetic moments of the muon and electron to the Standard Model predictions ($$\Delta a_{\mu ,e}$$ Δ a μ , e ) could be interpreted as a potential signal of New Physics. Models encompassing a light vector boson have been known to offer a satisfactory explanation to $$\Delta a_{\mu }$$ Δ a μ , albeit subject to stringent experimental constraints. Here we explore a minimal extension of the Standard Model via a leptophilic vector boson $$Z^\prime $$ Z ′ , under the hypothesis of strictly flavour-violating couplings of the latter to leptons. The most constraining observables to this ad-hoc construction emerge from lepton flavour universality violation (in Z and $$\tau $$ τ decays) and from rare charged lepton flavour violating transitions. Once these are accommodated, one can saturate the tensions in $$\Delta a_{\mu }$$ Δ a μ , but $$\Delta a_{e}$$ Δ a e is predicted to be Standard Model-like. We infer prospects for several observables, including leptonic Z decays and several charged lepton flavour violating processes. We also discuss potential signatures of the considered $$Z^\prime $$ Z ′ at a future muon collider, emphasising the role of the $$\mu ^+\mu ^- \rightarrow \tau ^+\tau ^- $$ μ + μ - → τ + τ - forward-backward asymmetry as a key probe of the model.

Anomalous and axial Z′ contributions to g−2

“…After parametrizing the anomalous three-gauge-boson couplings, we evaluate their contribution to the g−2 of the muon, first presented in general terms without using any 45,51,52,[84][85][86][87][88][96][97][98], without including the anomalous coupling. 6 In the presence of more than one Z s this is no longer the case [116].…”

Charge Lepton Flavor Violation in muons: theory overview

“…Likewise, the tau sector should also be included1, even the more involved decay modes (e.g. doubly lepton-flavour violating 𝜏 → 𝜇 ē𝜇 3-body decays), which can sometimes be at the source of the most stringent constraints (see, for instance, the study of [11]).…”

zfit: Scalable pythonic fitting

“…Another approach is to use the EFT formalism, which describes physics up to a given energy scale and absorbs effects originating from higher energy scales into effective coupling constants, the Wilson coefficients (WCs). This integrates out the effect of high energy contributions, making an interaction point-like above the energy scale of interest 3 𝑍′ boson as a mediator [17]. The right diagram contains a leptoquark-like particle that converts leptons to quarks and vice versa [16].…”