Non-diagonal contributions to $Z\gamma V^\ast$ vertex, polarizations and bounds on $Z\overline{t}q$ couplings

Abstract

We study the flavor-changing neutral current contributions to the trilinear neutral gauge boson couplings $Z\gamma V^\ast$ ($V=Z$, $\gamma$) at the one-loop level. Only the $CP$-conserving form factor $h_3^Z$ is induced, with the relevant contributions arising from top quark couplings. To our numerical analysis, constraints on the $Z\overline{t}q$ ($q=c$, $u$) couplings are obtained from current LHC data, leading to bounds of $|g_{\text{V}}^{tu}|\text{,}|g_{\text{A}}^{tu}|\leqslant 0.007$ and $|g_{\text{V}}^{tc}|\text{,}|g_{\text{A}}^{tc}|\leqslant 0.0095$ for the vector and axial couplings. The new contributions to the $h_3^Z$ form factor are of order $10^{-7}$. Furthermore, we analyze the polarized partial decay widths of the $V^\ast\rightarrow Z\gamma$ process, and a new type of left-right asymmetry is also discussed. We find that in the presence of new physics, the polarized $\Gamma(V^\ast\rightarrow Z\gamma)$ exhibits significant deviations from the SM prediction, resulting in a non-zero asymmetry.