Probing Neutral Triple Gauge Couplings via $\boldsymbol{Z\gamma\,(\ell^+\ell^-\gamma)}$ Production at $\boldsymbol{e^+e^-}$ Colliders

TL;DR

This paper investigates how future electron-positron colliders can detect new physics effects from dimension-8 operators affecting neutral triple gauge couplings, using detailed simulations of Z gamma production.

Contribution

The study provides the first detailed collider sensitivity analysis of dimension-8 nTGC operators at the CEPC, including detector-level simulations and form factor constraints.

Findings

Sensitivity limits on nTGC form factors are established.

Probing new physics scales up to several TeV is feasible.

The analysis demonstrates the potential of CEPC for beyond Standard Model physics.

Abstract

Neutral triple gauge couplings (nTGCs) are absent in the Standard Model (SM) and at the dimension-6 level in the Standard Model Effective Field Theory (SMEFT), arising first from dimension-8 operators. As such, they provide a unique window for probing new physics beyond the SM. These dimension-8 operators can be mapped to nTGC form factors whose structure is consistent with the spontaneously-broken electroweak gauge symmetry of the SM. In this work, we study the probes of nTGCs in the reaction $e^+e^-\to Z\gamma$ with $Z\to\ell^+\ell^-\,(\ell =e,\mu)$ at an $e^+e^-$ collider. We perform a detector-level simulation and analysis of this reaction at the Circular Electron Positron Collider (CEPC) with collision energy $\sqrt{s} = 240$ GeV and an integrated luminosity of 20 ab$^{-1}$. We present the sensitivity limits on probing the new physics scales of dimension-8 nTGC operators via measurements of the corresponding nTGC form factors.