Probing a scalar singlet-triplet extension of the Standard Model via VBF at the Muon Collider

Abstract: In this article, we investigate the $Y=0$ $SU(2)$ scalar triplet and $Z_2$-odd scalar singlet extension of the Standard Model (SM). Here, the triplet charged Higgs boson decays to $ZW<sup>\pm$, breaking the custodial symmetry at the tree-level, proportional to the triplet vev, while the singlet provides the dark matter (DM) relic. The triplet neutral Higgs ($T<sup>0$) can decay fully invisibly owing to the triplet-singlet portal coupling $\lambda_{st}$. The other SM Higgs portal couplings $\lambda_{ht}, \lambda_{hs}$ are constrained by the Higgs to di-photon observations, and the dark matter relic and direct searches as well as invisible Higgs decay bounds, respectively. For a cleaner signature, we indulge in a futuristic multi-TeV muon collider (MuC) to probe both the triplet scalars ($T^\pm, T⁰$) via vector boson fusion with Forward muon tagging, at the centre-of-mass energies of 3 TeV and 10 TeV. The analysis is comprised of a traditional cut-based approach and a BDT classifier, where the latter is more effective for lower energies. With large missing energy contributions to the final states from combinations of DM mass and $\lambda_{st}$, The 3 TeV MuC is projected to probe triplet scalar masses of 450 GeV with the BDT classifier. The 10 TeV MuC can pinpoint the custodial symmetry breaking $T^\pm \to ZW^\pm \to 3$-lepton decay up to 800 GeV of triplet scalar mass from cut-based analysis, with $\lambda_{st}$ as low as 1.5 being adequate.