Rare Higgs Decay into a Photon and a Z Boson in Radiatively-Driven Natural Supersymmetry

Abstract: In this article, we study the rare decay process in which a Higgs boson decays into a $Z$ boson and a photon. In the first part of the paper, we analyze the Standard Model (SM) contributions to the corresponding decay width, including the full leading-order result, two-loop $O(\alpha_s)$ QCD corrections, and the recently reported two-loop $O(\alpha)$ electroweak corrections, evaluated under four different $\alpha$ renormalization schemes. The dependence on the Higgs boson mass is studied within the experimentally allowed range reported by the LHC. In the considered schemes, a non-negligible variation of about $4\%$ is found when the mass is varied within its current experimental uncertainty. In the second part of the paper, we analyze the leading-order contributions to the same process within the Minimal Supersymmetric Standard Model (MSSM). The spectrum of soft SUSY-breaking parameters and SUSY particle masses at the electroweak scale, which enter the computation of the one-loop amplitudes contributing to the decay width, is obtained by evolving the GUT-scale parameters of a Radiatively-Driven Natural Supersymmetry (RNS) model with non-universal Higgs boson masses. Variations of the RNS parameters can enhance the average SM prediction by up to $\sim20\%$, reaching a value of $\sim7.5~$keV, while still satisfying the Higgs boson mass constraint. However, this comes at the cost of allowing a moderately large fine-tuning parameter, with values exceeding $100$, thereby placing the model outside its most natural parameter region. The predicted decay width in the RNS scenario is closer to the recent ATLAS RUN 2 + 3 combined measurement than the SM expectation.