arXiv : Global Analysis of Leptophilic $Z^\prime$ Bosons

Buras, Andrzej J.; Crivellin, Andreas; Kirk, Fiona; Manzari, Claudio Andrea; Montull, Marc

doi:10.1007/JHEP06(2021)068

Article
Report number	arXiv:2104.07680 ; AJB-21-3 ; CERN-56 ; PSI-PR-21-06 ; ZU-TH 15/20
Title	Global Analysis of Leptophilic $Z^\prime$ Bosons
Related title	Global analysis of leptophilic Z′ bosons
Author(s)	Buras, Andrzej J. (TUM-IAS, Munich) ; Crivellin, Andreas (CERN ; Zurich U. ; PSI, SLS) ; Kirk, Fiona (Zurich U. ; PSI, SLS) ; Manzari, Claudio Andrea (Zurich U. ; PSI, SLS) ; Montull, Marc (Zurich U. ; PSI, SLS)
Publication	2021-06-09
Imprint	2021-04-15
Number of pages	48
Note	48 pages, 17 figures
In:	JHEP 2106 (2021) 068
DOI	10.1007/JHEP06(2021)068
Subject category	nucl-th ; Nuclear Physics - Theory ; nucl-ex ; Nuclear Physics - Experiment ; hep-ex ; Particle Physics - Experiment ; hep-ph ; Particle Physics - Phenomenology
Accelerator/Facility, Experiment	CERN LHC
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.
Copyright/License	preprint: (License: CC-BY-4.0) publication: 2021 (License: CC-BY-4.0), sponsored by SCOAP³

$Feynman diagrams illustrating the leading $Z'$-contributions to the process $\tau\to \mu \nu_\tau \bar\nu_\mu$. In the presence of flavour off-diagonal $Z'$-couplings, the process arises already at tree-level (see left diagram), whereas in the case of flavour-diagonal couplings, corrections to $\tau\to \mu \nu_\tau \bar\nu_\mu$ are induced by four boxes, one of which is shown on the right.$ $Feynman diagrams illustrating the leading $Z'$-contributions to the process $\tau\to \mu \nu_\tau \bar\nu_\mu$. In the presence of flavour off-diagonal $Z'$-couplings, the process arises already at tree-level (see left diagram), whereas in the case of flavour-diagonal couplings, corrections to $\tau\to \mu \nu_\tau \bar\nu_\mu$ are induced by four boxes, one of which is shown on the right.$