$H\rightarrow Z\gamma$ decay and $CP$ violation

TL;DR

This paper investigates the effects of CP violation on the Higgs decay to Z gamma, constraining form factors, revisiting Standard Model contributions, and exploring new physics scenarios that could explain experimental excesses.

Contribution

It provides new constraints on CP-violating form factors in Higgs decay and explores potential new physics contributions that could account for observed anomalies.

Findings

Constraints on CP-violating form factors are less than 1.15 GeV.

FCNC contributions to form factors are negligible (~10^{-5} GeV).

New quark models could produce form factors of order 0.1 GeV, explaining experimental excess.

Abstract

This study examines the impact of $CP$-violation on the signal strength $\mu^{Z\gamma}$, which was reported as $2.2\pm 0.7$ by the LHC. We obtain constraints on the real and absorptive parts of the $CP$-violating form factor $h_3^{Z\gamma}$ and find that they are less than 1.15 GeV. Additionally, we revisit the leading order Standard Model contributions to the $H\rightarrow Z\gamma$ decay and calculate contributions to $h_3^{Z\gamma}$ from FCNC complex couplings mediated by the $Z$ and $H$ bosons. By using the current bounds on such couplings, we find that the FCNC contribution to $h_3^{Z\gamma}$ with top and charm quarks in the loop is of order $10^{-5}$ GeV. While in a model with new quarks that preserves the SM predictions on Higgs decays, the $CP$-violating form factor $h_3^{Z\gamma}$ can be of order $10^{-1}$ GeV and could explain the excess on the signal strength $\mu^{Z\gamma}$.