Discovery Prospects for the Light Charged Higgs Boson Decay to an Off-Shell Top Quark and a Bottom Quark at Future High-Energy Colliders

Abstract

The charged Higgs boson ($H^{\pm }$) with a mass below the top quark mass remains a viable possibility within the Type-I two-Higgs-doublet model under current constraints. While previous LHC searches have primarily focused on the $H^{\pm }\to \ddot{I}{„}^{\pm }\hat{I}½$ decay mode, the decay channel into an off-shell top quark and a bottom quark, $H^{\pm }\to t^{\ast }b$, is leading or subleading for $H^{\pm }$ masses between 130 and 170 GeV. This study investigates the discovery potential of future colliders for this off-shell decay mode through pair-produced charged Higgs bosons decaying via $H^{+}{H}^{-}\to t^{\ast }b\ddot{I}„\hat{I}½\to bbjj\ddot{I}„\hat{I}½$. We perform signal-to-background analyses at the HL-LHC and a prospective 100 TeV proton-proton collider, employing cut-flow strategies and the Boosted Decision Tree method. However, due to the softness of the $b$ jets, signal significances fall below detection thresholds at these facilities. Extending our study to a multi-TeV muon collider (MuC), we demonstrate that a 3 TeV MuC achieves high signal significance, surpassing the $5\ddot{I}ƒ$ threshold with an integrated luminosity of 1 ab${}^{-1}$ and a 10\% background uncertainty. Specifically, for $M_{H^{\pm }}=130$, 150, and 170 GeV, the significances are 13.7, 13.5, and 6.06, respectively. In contrast, a 10 TeV MuC requires 10 ab${}^{-1}$ to achieve similar results. Our findings highlight the critical role of the MuC in probing the new signal channel $H^{\pm }\to t^{\ast }b$, offering a promising avenue for future charged Higgs boson searches involving off-shell top quarks.

22 pages in two columns with 9 figures and 4 tables