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, → ν ν decays, anomalous magnetic moments of charged leptons, → γ and → 3 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 L µ − L τ 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 τ → µν ν/τ → eν ν without violating bounds from electroweak precision observables.

Muon ${g-2}$ discrepancy within D-brane string compactifications

“…Since the couplings of the anomalous U (1) bosons are in general much larger than the SM coupling (especially when their masses are large), we can compare the constraints on µ α with LHC limits on the generalized sequential model [65], containing the sequential SM boson gauge boson that has SM-like couplings to SM fermions [66]. We can see that the discrepancy δa µ reported by Fermilab cannot be accommodated within the 1σ contours without being in tension with LHC data [63,64], unless the U (1) gauge boson is leptophilic [52]. If we require µ α 5 TeV and µ β 5 TeV, then δa µ 3 × 10 −11 , with g c ∼ 1.…”

“…IV. Before proceeding we note that the latest Fermilab data already lead to several new physics interpretations with connections to other fundamental problems in particle physics, astrophysics, and cosmology [44][45][46][47][48][49][50][51][52].…”

Muon $g-2$, $B\to K^{(*)}μ^+ μ^-$ anomalies, and leptophilic dark matter in $U(1)_{μ-τ}$ gauge symmetry

“…In fact, there is a wide variety of recent applications of U (1) µ−τ models, which are distinguished by additional particle contents to address flavor anomalies; Refs [42][43][44][45][46][47] for neutrino oscillations to control lepton flavor structure, Refs [28,29,[48][49][50][51][52][53][54][55][56][57][58][59] for muon g− [60,61,66] while flavor dependent charge assignment in the SM quark sector is considered in ref. [62].…”

Leptophilic U(1) massive vector bosons from large extra dimensions

“…The second anomalous U(1) should be much heavier to avoid the LHC bound and its contribution to a µ is negligible. To accommodate the Fermilab data one can advocate the violation of lepton flavour universality [23]. Alternatively, as we have shown in Fig.…”

“…The zero-mode of the anomalous U(1), hereafter Z , gains a mass via the GS mechanism by absorbing an axionic field from the R-R (Ramond) closed string sector. To get as much contribution to a µ as possible without violating the LHC bounds [21,22], it is natural to consider a leptophilic (in our case meaning large g L ≡ g c ) Z [23]. Next, we compare with the LHC data considering the resonant production cross section of σ(pp → Z → ).…”