Probing the scale of new physics in the <i>ZZγ</i> coupling at <i>e</i><sup>+</sup><i>e</i><sup>−</sup> colliders
John Ellis, Shao-Feng Ge, Hong-Jian He, Rui-Qing Xiao
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.