Eur. Phys. J. C (2025) 85: 530
https://doi.org/10.1140/epjc/s10052-025-14244-4

Regular Article - Theoretical Physics

Discovery prospects for the light charged Higgs boson decay to an off-shell top quark and a bottom quark at future high-energy colliders

¹ School of Physics, Korea Institute for Advanced Study, 85 Hoegi-ro, Dongdaemun-gu, 02455, Seoul, Republic of Korea
² Department of Physics, Konkuk University, 120 Neungdong-ro, Gwangjin-gu, 05029, Seoul, Republic of Korea
³ Institute of High Energy Physics (HEPHY), Austrian Academy of Sciences (OeAW), Georg-Coch-Platz 2, 1010, Vienna, Austria

^a jhsong@konkuk.ac.kr

Received: 16 December 2024
Accepted: 28 April 2025
Published online: 14 May 2025

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 $H^{\pm }\to {\tau }^{\pm }\nu$ 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 $H^{+}{H}^{-}\to t^{\ast }b\tau \nu \to bbjj\tau \nu$. 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 $5\sigma$ 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.