The Global Electroweak Fit and its Implication to Z-prime

TL;DR

This paper explores how a Z' boson with specific properties can reconcile discrepancies in Z-pole observables with the Standard Model, suggesting new physics beyond the SM.

Contribution

It demonstrates that a Z' boson with generation-dependent, anomaly-free charges can reduce deviations in key electroweak observables and improve the global fit.

Findings

Z' boson reduces deviations from 2.7σ to 1.2σ

Favors no Z' coupling to second-generation quarks and certain leptons

Supports a negative contribution to the S parameter from Z'

Abstract

Among the $Z$-pole observables, $A_{FB}^{(0,b)}$ and $A_e$ suffer moderately-large standard deviations from the Standard Model predictions. Fine-tuning of the unknown Higgs mass only reduces the deviation of one of them at the expense of increasing the deviation of the other observable. If we take this fact seriously, the result can be interpreted as independent experimental evidence of existing new physics beyond SM, even if a 125GeV Higgs on the LHC is finally confirmed to be SM Higgs. We show in this paper that the existence of a $Z'$ boson mixing with $Z$ and $\gamma$ and with generation-dependent anomaly-free charge assignments, helps to suppress $A_{FB}^{(0,b)}$ and $A_e$ at $Z$-pole simultaneously and dose reduce the largest deviation from $2.7\sigma$ in SM predictions to $1.2\sigma$ in our scenario. The global electroweak fit does not prefer $Z'$ coupling to the second-generation quarks and the first-generation left-handed lepton and right-handed electron. The fitting result also supports a negative contribution to the $S$ parameter from the $Z'$ boson.