A new evaluation of the $HZZ$ coupling: direct bounds on anomalous contributions and $CP$ violating effects via a new asymmetry

Abstract

The standard model (SM) one-loop contributions to the most general $H^*Z^*Z^*$ coupling are obtained via the background field method in terms of Passarino-Veltman scalar functions, from which the contributions to the $H^*ZZ$ and $HZZ^*$ couplings are obtained in terms of two $CP$-conserving $h_{1,2}^V$ and one $CP$-violating $h_3^V$ form factors ($V=H, Z$). The current CMS constraints on the $HZZ$ coupling ratios are then used to obtain bounds on the real and absorptive parts of the anomalous $HZZ$ couplings. The former are up to two orders of magnitude tighter than previous ones, whereas the latter are the first one of this kind. The effects of the absorptive parts of the $HZZ$ anomalous couplings, which have been overlooked in the past, are analyzed via the partial decay width $\Gamma_{H^\ast\rightarrow ZZ}$, and a significant deviation from the SM tree-level contribution is observed at low energies, though it becomes negligible at high energies. We also explore the possibility that polarized $Z$ gauge bosons are used for the study of non-SM $HZZ$ contributions via a new left-right asymmetry $\mathcal{A}_{LR}$, which is sensitive to $CP$-violating complex form factors and can be as large as the unity at most, though in a more conservative scenario it is four to five orders of magnitude larger than the SM prediction arising up to the three-loop level. The partial decay widths $\Gamma_{H^\ast\rightarrow Z_LZ_L}$ and $\Gamma_{H^\ast\rightarrow Z_RZ_R}$ are also studied in several scenarios and it is observed that the deviations from the SM can be large at high energies and increases as the energy increases. Thus, the use of polarized $Z$ gauge bosons could give hints of $CP$ violation. The Mathematica code for our analytical results and the numerical evaluation is available in our GitLab site.