Probing new physics in dimension-8 neutral gauge couplings at e+e− colliders

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 ZγZ* and Zγγ* vertices with rapid energy dependence ∝ E 5 , which can be probed sensitively via the reaction e + e − → Zγ . We demonstrate that measuring the nTGCs via the reaction e + e − → Zγ followed by Z → q 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 s = ( 0.25 − 1 ) TeV, and up to the range (10–16) TeV at CLIC with s = ( 3 − 5 ) TeV, assuming in each case an integrated luminosity of 5 ab −1 . 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 − Zγ vertex.