Probing the minimal $U(1)_X$ model at future electron-positron colliders via the fermion pair-production channel

TL;DR

This paper explores how future electron-positron colliders can detect a minimal $U(1)_X$ extension of the Standard Model through fermion pair-production, analyzing interference effects, asymmetries, and collider constraints on new gauge bosons.

Contribution

It provides a detailed analysis of $Z'$ effects on fermion pair-production and proposes collider observables to probe the $U(1)_X$ model beyond current LHC limits.

Findings

Significant deviations in asymmetries from the SM predictions due to $Z'$ effects.

Collider constraints limit the $U(1)_X$ coupling and $Z'$ mass.

Future colliders can effectively probe multi-TeV $Z'$ bosons.

Abstract

The minimal $U(1)_X$ extension of the Standard Model (SM) is a well-motivated new physics scenario, where anomaly cancellation dictates new neutral gauge boson ($Z^\prime$) couplings with the SM fermions in terms of the $U(1)_X$ charges of the new scalar fields. We investigate the SM charged fermion pair-production process for different values of these $U(1)_X$ charges at future $e^-e^+$ colliders: $e^+e^-\to f\bar f$. Apart from the standard $\gamma$ and $Z$-mediated processes, this model features additional $s$-channel (or both $s$ and $t$-channel when $f=e^-$) $Z^\prime$ exchange which interferes with the SM processes. We first estimate the bounds on the $U(1)_X$ coupling $(g^\prime)$ and the $Z^\prime$ mass $(M_{Z^\prime})$ considering the latest dilepton and dijet constraints from the heavy resonance searches at the LHC. Then using the allowed values of $g^\prime$, we study the angular distributions, forward-backward $(\mathcal{A}_{\rm{FB}})$, left-right $(\mathcal{A}_{\rm{LR}})$ and left-right forward-backward $(\mathcal{A}_{\rm{LR, FB}})$ asymmetries of the $f\bar{f}$ final states. We find that these observables can show substantial deviations from the SM results in the $U(1)_X$ model, thus providing a powerful probe of the multi-TeV $Z^\prime$ bosons at future $e^+e^-$ colliders.