Probing the Scale of New Physics in the $ZZ{\hat{I}}^3$ Coupling at $e^{+}{e}^{-}$ Colliders

Abstract

The $ZZ{\hat{I}}^3$ 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 $e^{+}{e}^{-}\to Z{\hat{I}}^3$, followed by $Z\to \ell \bar{\ell },\hat{I}½\bar{\hat{I}}½$ 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.

Final Journal Version, 53 pages [12 Figs (30 plots), 7 Tables]. Extended version, all conclusions and results unchanged, clarifications & references added