Probing the Scalar Sector: Discovery Reach for Heavy Higgs Pairs at a $\sqrt{s} = 6$ TeV Muon Collider in the 2HDM Alignment Limit

TL;DR

This work evaluates the discovery potential for heavy Higgs-pair production ($HH,HA,AA,H^+H^-$) at a $\sqrt{s}=6$ TeV Muon Collider within the 2HDM Type-I alignment limit. Using two benchmarks with degenerate masses $m_\Phi=1$ and $2$ TeV, the study demonstrates that the resulting high-multiplicity hadronic final states (8 jets for $H^+H^-$ and 12 jets for $HA/AA$) provide extremely efficient background suppression, leading to exceptional statistical significance at feasible luminosities, e.g., $S/\sqrt{B}$ of $\sim 10^5$ for $H^+H^-$ and $\sim 3\times10^3$ for $HA$ at $L=10\,\mathrm{ab}^{-1}$ (BP1). The analysis shows cross-sections follow the expected $1/s$ scaling above threshold with a prominent threshold effect at $2m_\Phi$, and selection efficiencies improve as scalar masses increase (BP2). Comparisons with literature and internal consistency checks corroborate the discovery reach, while the clean muon-collider environment and robust $b$-jet tagging drive strong signal-to-background ratios. The results suggest that a 6 TeV Muon Collider offers a definitive facility to probe and characterize the extended scalar sector of the 2HDM, with future work needed to incorporate detector-level BIB and NLO corrections.

Abstract

This study provides a comprehensive phenomenological investigation into the discovery potential of heavy Higgs boson pairs ($HH, HA, AA, H^+H^-$) at a $\sqrt{s}=6$~TeV Muon Collider. Utilizing the Two-Higgs-Doublet Model (2HDM) Type-I within the alignment limit ($\sin(β-α) \approx 1$), we evaluate two primary benchmarks with degenerate scalar masses of 1000~GeV (BP1) and 2000~GeV (BP2). Theoretical calculations performed reveal that Type-I branching fractions to third-generation fermions remain uniquely independent of $\tanβ$, providing a stable signal across the investigated parameter space. We demonstrate that the Muon Collider environment allows for the precise identification of high-multiplicity hadronic final states. A key finding of this research is that the signal processes yield distinctive topological signatures: an 8-jet state ($4j+4b$) for charged pairs and a highly complex 12-jet state ($8j+4b$) for neutral pairs ($HA/AA$). These signatures, combined with hard transverse momentum distributions and central pseudorapidity ($|η| \le 3$), allow for nearly absolute suppression of Standard Model backgrounds like $t\bar{t}$, $W^+W^-Z$, and $ZZZ$. At an integrated luminosity of 10~ab$^{-1}$, we report a staggering statistical significance of 104,000 for the $H^+H^-$ channel and 3343 for the $HA$ channel in the BP1 scenario. Furthermore, total selection efficiencies were found to increase from approximately 20\% at BP1 to 47\% at BP2, suggesting that the decay products of heavier scalars are kinematically easier to resolve. We conclude that a 6~TeV Muon Collider offers an unparalleled discovery reach for the extended scalar sector, providing a definitive facility for probing physics beyond the Standard Model.

Figures (12)

• Figure 1: Branching ratio vs $\tan\beta$ for $H/A \to b\bar{b}, t\bar{t}, \tau^+\tau^-$ at $m_\phi = 2$ TeV.
• Figure 2: Branching ratio vs $\tan\beta$ for $H^+ \to t\bar{b}$ and $H^+ \to \tau^+ \nu_\tau$ at $m_\phi = 2$ TeV.
• Figure 3: Representative Feynman diagrams for the signal processes $\mu^+\mu^- \to H^+H^-$ (left) and $\mu^+\mu^- \to AA$ (right) at a multi-TeV Muon Collider. These diagrams illustrate the complex decay chains through top quarks and gauge bosons, which result in the high-multiplicity hadronic final states ($N_{jets} \ge 8$ for neutral pairs) utilized for effective background suppression.
• Figure 4: Full leading-order Feynman diagram for the signal process $\mu^+\mu^- \to H^+H^-$, illustrating the decay chain into an 8-jet final state ($4j+4b$).
• Figure 5: Feynman diagram for the CP-odd neutral Higgs pair production $\mu^+\mu^- \to AA$, demonstrating the high-multiplicity 12-jet final state ($8j+4b$) resulting from four top quarks.