Probing anomalous $hZγ$ couplings in single Higgs production at future muon colliders

Abstract

Future multi-TeV muon colliders offer an ideal environment to probe physics beyond the Standard Model (SM) through high-precision measurements. In this work, we investigate the sensitivity to anomalous $hZγ$ couplings within the dimension-six Standard Model Effective Field Theory (SMEFT) framework at $\sqrt{s} = 3\text{ TeV}$ and $10\text{ TeV}$ muon colliders. Focusing on single Higgs boson production---particularly through vector boson fusion---followed by the rare $h \to Zγ$ decay channel, we perform a comprehensive simulation using \textsc{MadGraph5\_aMC@NLO}, \textsc{Pythia}, and \textsc{Delphes} for fast detector response. To maximize signal sensitivity, we contrast a traditional cut-and-count methodology with a multi-bin Boosted Decision Tree (BDT) shape analysis, evaluated within a multi-bin $χ^2$ statistical framework. Our most stringent projections are achieved using the multivariate BDT analysis at $10\text{ TeV}$. Assuming an integrated luminosity of $10\text{ ab}^{-1}$ and a $5\%$ systematic uncertainty, we derive expected $95\%$ confidence level intervals of $c_{HW} \in [-0.005, +0.003]$ and $c_{HB} \in [-0.003, +0.005]$. Notably, the bound on the $c_γ$ coefficient is slightly tighter due to its larger cross-section enhancement, reaching $c_γ \in [-0.002, 0.001]$. These results underscore the unique capability of high-energy muon colliders to constrain anomalous Higgs-gauge couplings.