Higgs boson decays $h\rightarrow Z γ$ and $h\rightarrow m_V Z$ in the $U(1)_X$VLFM

TL;DR

The paper investigates rare Higgs decays $h\to Zγ$ and $h\to m_V Z$ within the $U(1)_X\mathrm{VLFM}$ model that adds vectorlike fermions and an extra $U(1)_X$ gauge symmetry. It derives the modified CP-even and CP-odd $hγZ$ couplings from loop diagrams involving new states and provides analytic expressions for the decay amplitudes, including both direct and indirect contributions to $h\to m_V Z$ with NRQCD treatment for vector mesons. A thorough numerical analysis, constrained by current experimental bounds on Higgs properties, VLQ/VLL masses, and $Z'$ parameters, shows that NP effects can be sizable: $\Gamma_{NP}(h\to Zγ)/\Gamma_{SM}(h\to Zγ)$ can reach up to $\sim 1.65$, while enhancements in $h\to m_V Z$ are typically larger for light vector mesons (e.g., $1.2$–$1.7$ for $\omega,\rho$ and $\sim 1.1$–$1.3$ for $\phi, J/\psi, \Upsilon$). The results demonstrate that these channels are valuable probes of new physics, with potential observability at the HL-LHC and future colliders, and emphasize the role of vectorlike fermions and kinetic mixing in shaping Higgs interactions.

Abstract

We study the Higgs boson decays $h \to Zγ$ and $h \to m_VZ$ in a model with vectorlike fermions and $U(1)_X$ symmetry ($U(1)_X$VLFM), where $m_V$ is a vector meson ($ρ,\ ω,\ φ,\ J/ψ,\ Υ$). The exotic Yukawa interactions in this model generate mixing between Standard Model (SM) fermions and vectorlike fermions, and this mixing affects the Higgs boson mass and Higgs couplings. The corrections to the CP-even and CP-odd $hγZ$ couplings come from loop diagrams that involve the new particles, and these corrections have a clear impact on the decay rates of $h\to Zγ$ and $h\to m_VZ$. In suitable regions of the parameter space, the model can produce non-negligible deviations in $Γ_{\rm NP}(h\to Zγ)/Γ_{\rm SM}(h\to Zγ)$ and $Γ_{\rm NP}(h\to m_VZ)/Γ_{\rm SM}(h\to m_V Z)$, providing possible signals of new physics (NP) beyond the SM.

Figures (15)

• Figure 1: The diagrams contributing to the decay $h\rightarrow m_V Z$.
• Figure 2: The one loop diagrams with new particles for the decay $h\rightarrow Z \gamma$.
• Figure 3: $R_{\gamma\gamma}$ varying with $Y_{XD}$ for different $Y_{PD}$.
• Figure 4: $R_{VV^*}$ varying with $Y_{XU}$ for different $Y_{PD}$.
• Figure 5: ${\Gamma}_{NP}(h\rightarrow Z \gamma)/{\Gamma}_{SM}(h\rightarrow Z \gamma)$ in $Y_{PU}-Y_{XU}$ plane, and the marking of the scattering points represents: $\textcolor{blue}{\blacktriangle}~({\Gamma}_{NP}(h\rightarrow Z \gamma)/{\Gamma}_{SM}(h\rightarrow Z \gamma)<1.294),~\textcolor{orange}{\blacksquare}~(1.294\leq {\Gamma}_{NP}(h\rightarrow Z \gamma)/{\Gamma}_{SM}(h\rightarrow Z \gamma)<1.3),~ \textcolor{green}{\blacklozenge}~(1.3\leq {\Gamma}_{NP}(h\rightarrow Z \gamma)/{\Gamma}_{SM}(h\rightarrow Z \gamma)<1.35),~ \textcolor{red}{\bullet}~(1.35\leq {\Gamma}_{NP}(h\rightarrow Z \gamma)/{\Gamma}_{SM}(h\rightarrow Z \gamma$).