Probing New Physics in Dimension-8 Neutral Gauge Couplings at $e^{+}{e}^{-}$ Colliders

Abstract

Neutral triple gauge couplings (nTGCs) are absent in the standard model effective theory up to dimension-6 operators, but could arise from dimension-8 effective operators. In this work, we study the pure gauge operators of dimension-8 that contribute to nTGCs and are independent of the dimension-8 operator involving the Higgs doublet. We show that the pure gauge operators generate both $Z\gamma Z^{\ast }$ and $Z\gamma {\gamma }^{\ast }$ vertices with rapid energy dependence $\propto E^5$, which can be probed sensitively via the reaction $e^{+}{e}^{-}\to Z\gamma$. We demonstrate that measuring the nTGCs via the reaction $e^{+}{e}^{-}\to Z\gamma$ followed by $Z\to q\bar{q}$ decays can probe the new physics scales of dimension-8 pure gauge operators up to the range $(1-5)$TeV at the CEPC, FCC-ee and ILC colliders with $\sqrt{s}=(0.25-1)$TeV, and up to the range $(10-16)$TeV at CLIC with $\sqrt{s}=(3-5)$TeV, assuming in each case an integrated luminosity of 5/ab. We compare these sensitivities with the corresponding probes of the dimension-8 nTGC operators involving Higgs doublets and the dimension-8 fermionic contact operators that contribute to the $e^{+}{e}^{-}Z\gamma$ vertex.

43pp, 9 Figs (32 plots) and 4 Tables. v4: refined Fig.9 by adding results of 5/ab integrated luminosity. v3: Only minor refinements to match online version in Science China, selected as "Editor's Focus" and "Cover Article"