# Probing the Scale of New Physics in the $ZZ\gamma$ Coupling at $e^+e^-$ Colliders

## TL;DR

This paper investigates how future electron-positron colliders can detect signs of new physics through the $ZZ\gamma$ coupling, which is absent at tree level in the Standard Model but can arise from higher-dimension operators, by analyzing specific reaction channels and angular distributions.

## Contribution

It introduces a method to probe dimension-8 operators affecting the $ZZ\gamma$ coupling at future colliders, enhancing sensitivity with polarization and angular analysis.

## Key findings

- Dimension-8 operators can be probed up to multi-TeV scales.
- Angular distributions help suppress Standard Model backgrounds.
- Beam polarization significantly improves detection sensitivity.

## Abstract

The $ZZ\gamma$ triple neutral gauge couplings are absent in the Standard Model (SM) at the tree level. They receive no contributions from dimension-6 effective operators, but can arise from effective operators of dimension-8. We study the scale of new physics associated with such dimension-8 operators that can be probed by measuring the reaction $e^+e^-\to Z\gamma$, followed by $Z \to \ell\bar{\ell},\nu\bar{\nu}$ decays, at future $e^+e^-$ colliders including the ILC, CEPC, FCC-ee and CLIC. We demonstrate how angular distributions of the final state mono-photon and leptons can play a key role in suppressing SM backgrounds. We further show that using electron/positron beam polarizations can significantly improve the signal sensitivities. We find that the dimension-8 new physics scale can be probed up to the multi-TeV region at such lepton colliders.