Signature for charged Higgs pair production in 2HDM+a model at CLIC

TL;DR

The paper investigates charged-Higgs pair production in the 2HDM+a model at a 1.5 TeV e+e- collider (CLIC), focusing on the channel where one charged Higgs decays to tb and the other to aW, with the pseudoscalar mediator a decaying invisibly. It combines detailed model setup (alignment limit, Type-I Yukawa structure, and a five-parameter effective space) with a full Monte Carlo analysis (MG5_aMC@NLO, Pythia8, Delphes) and a cut-based event selection to isolate the $4j+2b+E_T^{miss}$ final state from SM backgrounds. The study shows that for representative benchmarks, the signal can reach $5\sigma$ significance in parts of the $(m_{H^\pm}, m_a)$ plane for $m_{H^\pm}$ in the 400–650 GeV range at ${\cal L}=2500\,$fb$^{-1}$, and provides 95% CL exclusions that complement existing LHC constraints. These results highlight CLIC’s enhanced sensitivity to this region of parameter space and demonstrate the potential of 2HDM+a to yield observable charged-Higgs signatures at future lepton colliders.

Abstract

In this work, we study the search for charged Higgs bosons in the Two-Higgs-Doublet Model plus an additional pseudo-scalar (2HDM+a) at the Compact Linear Collider (CLIC). Focusing on the pair production of charged Higgs bosons, followed by the decays $H^\pm \to a W $ and $ H^\mp \to t b $, we analyze the signal channel of $4j+2b+E_T^{miss}$. Given the center-of-mass energy of CLIC as $ \sqrt{s} = 1500$ GeV, charged Higgs bosons with masses $ m_{H^\pm} < \sqrt{s}/2$ are explored. Simulation results indicate that the signal significance of 400 GeV and 600 GeV charged Higgs bosons can reach $5σ$ in specific parameter spaces, and the $2σ$ exclusion limits in the $m_{H^\pm}- m_a$ plane are presented for $m_{H^\pm}$ in the range of [400, 650] GeV.

Figures (5)

• Figure 1: Charged Higgs branching ratios as a function of its mass $m_{H^{\pm}}$ with different values of $m_a$ (solid line for $m_a$=150, dashed line for $m_a=250$). Here, the red curves represent the branching ratio of the $H^{\pm}\to aW$, while the green curves are relevant to $H^{\pm}\to tb$ channel. Two typical value of $\sin\theta=0.35$ and $\sin\theta=1/\sqrt{2}$ are shown in left and right, respectively. In both subfigures, $m_A=m_H=1.5\text{TeV}$ and $\tan\beta=1$ are fixed.
• Figure 2: The cross section times branching ratio of $e^+e^- \to H^+H^- \to aW^+aW^-$/$t\bar{b}aW^-$ as a function in $(m_{H^\pm}, m_a)$ plane in CLIC. Other parameters are set as: $m_A = m_H = 1.5\,\text{TeV}$, $y_\chi = 1$, $\lambda_1 = \lambda_2 = \lambda_3 = 3$, $\tan\beta = 1$, $\sin\theta = 1/\sqrt{2}$, $\sin(\beta - \alpha) = 1$, $m_\chi = 100\,\text{GeV}$.
• Figure 3: The normalized distributions for jet ($p_T$) for the signal and backgrounds. Four benchmark points are considered and labeled as BPs.
• Figure 4: The signal significances obtained from the cut based analysis for a set of points with an integrated luminosity of 2500 fb$^{-1}$
• Figure 5: The observed exclusion contours as a function of $(m_a, m_{H^\pm})$ plane. The figure shows a comparison between the ATLAS results and the results of our work. The red solid line in the figure is the result of our work.