Global analysis of leptophilic Z′ bosons

Abstract

New neutral heavy gauge bosons (Z′) are predicted within many extensions of the Standard Model. While in case they couple to quarks the LHC bounds are very stringent, leptophilic Z′ bosons (even with sizable couplings) can be much lighter and therefore lead to interesting quantum effects in precision observables (like (g − 2)μ) and generate flavour violating decays of charged leptons. In particular, $$ \mathrm{\ell}\to \mathrm{\ell}^{\prime }v\overline{v} $$ ℓ → ℓ ′ v v ¯ decays, anomalous magnetic moments of charged leptons, ℓ → ℓ′γ and ℓ → 3ℓ′ decays place stringent limits on leptophilic Z′ bosons. Furthermore, in case of mixing Z′ with the SM Z, Z pole observables are affected. In light of these many observables we perform a global fit to leptophilic Z′ models with the main goal of finding the bounds for the Z′ 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 Lμ− Lτ symmetry broken only by left-handed rotations, or considers the case of τ − μ couplings only. In the latter setup, on can explain the (g − 2)μ anomaly and the hint for lepton flavour universality violation in $$ \tau \to \mu v\overline{v}/\tau \to ev\overline{v} $$ τ → μv v ¯ / τ → ev v ¯ without violating bounds from electroweak precision observables.

Consequences of chirally enhanced explanations of (g − 2)μ for h → μμ and Z → μμ

“…Next, leptoquark (LQ) models can even display an enhancement factor of m t /m µ ≈ 1700 [65][66][67][68][69][70][71][72][73], allowing for a TeV-scale explanation with perturbative couplings that evades direct LHC searches and might even explain the other anomalies pointing towards lepton flavor universality violation. Other models in which chiral enhancement may be present include composite or extra-dimensional models [74][75][76], models with two Higgs doublets [77][78][79][80], or Z models with τ µ couplings [81,82].…”

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

“…In this article we will employ the SMEFT framework to study low-energy constraints on LFVZD. This type of decays have been also studied within several UV-complete models, such as heavy sterile neutrinos [39][40][41][42][43][44][45], supersymmetry [22,46], leptoquarks [47], or in scenarios with extended gauge sectors [48][49][50][51]. They have also been previously explored in the context of SMEFT in [16,24,25,28,33].…”

First-generation new physics in simplified models: from low-energy parity violation to the LHC

“…In principle, it would lead to too large effects in D 0 -D0 mixing, but these bounds can be avoided by tuning against the (unknown) SM contribution or other NP effects 5. For a detailed analysis of the SU(2)L singlet coupling only to leptons see ref [219]…”