Global Analysis of Leptophilic Z' Bosons

TL;DR

This paper performs a comprehensive global analysis of leptophilic Z' bosons, deriving bounds on their couplings to leptons by considering various precision observables, flavor-violating decays, and Z-Z' mixing effects.

Contribution

It introduces a global fit approach to constrain leptophilic Z' models, exploring different coupling scenarios and their implications for anomalies like (g-2)_μ and lepton flavor universality.

Findings

Stringent bounds on Z' couplings from precision measurements.

Certain coupling scenarios can explain (g-2)_μ and lepton flavor universality hints.

Weak correlations in generic models become significant under specific symmetry assumptions.

Abstract

New neutral heavy gauge bosons ($Z^\prime$) are predicted within many extensions of the Standard Model. While in case they couple to quarks the LHC bounds are very stringent, leptophilic $Z^\prime$ bosons (even with sizable couplings) can be much lighter and therefore lead to interesting quantum effects in precision observables (like $(g-2)_\mu$) and generate flavour violating decays of charged leptons. In particular, $\ell\to\ell^\prime\nu\bar\nu$ decays, anomalous magnetic moments of charged leptons, $\ell\to\ell^\prime\gamma$ and $\ell\to3\ell^\prime$ decays place stringent limits on leptophilic $Z^\prime$ bosons. Furthermore, in case of mixing $Z^\prime$ with the SM $Z$, $Z$ pole observables are affected. In light of these many observables we perform a global fit to leptophilic $Z^\prime$ models with the main goal of finding the bounds for the $Z^\prime$ couplings to leptons. To this end we consider a number of scenarios for these couplings. While in generic scenarios correlations are weak, this changes once additional constraints on the couplings are imposed. In particular, if one considers an $L_\mu-L_\tau$ symmetry broken only by left-handed rotations, or considers the case of $\tau-\mu$ couplings only. In the latter setup, on can explain the $(g-2)_\mu$ anomaly and the hint for lepton flavour universality violation in $\tau\to\mu\nu\bar\nu/\tau\to e\nu\bar\nu$ without violating bounds from electroweak precision observables.