Production of charged Higgs bosons associated with CP-even Higgs bosons at future multi--TeV muon colliders

Abstract

We report the first results for charged Higgs boson production associated with CP-even Higgs bosons at future multi--TeV muon colliders within the scenario of the Type-Y Two-Higgs-Doublet Model (Type-Y THDM). The viable parameter regions of the Type-Y THDM are updated in this work. Based on the allowed parameter space, all two-body decay channels of the charged Higgs boson are then calculated with the help of the computer tool {\tt H-COUP}. The production cross sections of $\mu^- \mu^+ \to H^\pm H^\mp h/H \to t\bar{t} b\bar{b} h/H$ are evaluated by scanning over the constrained parameter space of the considered model. We find that the production cross sections for $\mu^- \mu^+ \to H^\pm H^\mp h \to t\bar{t} b\bar{b} h$ ($t\bar{t} b\bar{b}H$) can reach approximately $500$ ab ($10$ ab) in specific regions of the parameter space. The results for the first production channel provide an opportunity to probe charged Higgs boson production at future multi--TeV muon colliders. Furthermore, we evaluate the signal significances for $\mu^- \mu^+ \to H^\pm H^\mp h \to t\bar{t} b\bar{b} h$ with respect to the corresponding Standard Model (SM) backgrounds. The signal significances are also estimated by including the leptonic decay modes of the top quarks, the decay $h \to b\bar{b}$ and together with $b$-tagging effects. Thanks to the high integrated luminosities expected at future multi--TeV muon colliders, we find that the signal significances can exceed $\sim 3\sigma$ at several benchmark points selected in the viable parameter space of the Type-Y THDM.