Probing an extra Higgs boson at future linear $e^+ e^-$ colliders

TL;DR

This work tests the existence of an extra CP-even Higgs $H$ in the General 2HDM (G2HDM) and its relevance to electroweak baryogenesis by examining the process $e^+e^- \to H\nu\bar{\nu}$ with $H\to W^+W^-$, which directly probes the Higgs mixing $c_\gamma=\cos\gamma$. It combines current LHC constraints on $c_\gamma$ and the top-Yukawa coupling $\rho_{tt}$ with a detector-level collider study for future linear colliders (CLIC) at $\sqrt{s}=1.5$ and 3 TeV, showing that an $H$ in the $200$–$400$ GeV range can be probed through WW fusion. The analysis finds that HL-LHC can reach sensitivities down to $|c_\gamma|,|\rho_{tt}| \sim 0.1$, while CLIC can achieve significant discrimination for the target mass window, providing a direct measurement of $c_\gamma$ and testing the EWBG-favored $\rho_{tt}$ scenario. Overall, the paper demonstrates a complementary path to probing extended Higgs sectors and their baryogenesis implications at future $e^+e^-$ colliders.

Abstract

We investigate the possibility of probing an extra Higgs boson at future linear $e^+ e^-$ colliders. We consider the production process $e^+e^- \to Hν\barν$, followed by the decay $H \to W^+W^-$, where $H$ is the extra $\textit{CP}$-even Higgs boson of the general two Higgs doublet model (G2HDM). This process is governed by the $\textit{CP}$-even Higgs mixing angle, $\cosγ$, offering direct access to this parameter. We discuss constraints on $\cosγ$ using existing LHC data and test the viability of the G2HDM top-quark-driven scenario for electroweak baryogenesis. We perform a full Monte Carlo simulation of the signal and background, and show that an extra Higgs boson in the mass range $200 \leq m_H \leq 400$ GeV could be probed at high energy linear $e^+ e^-$ colliders. Promising results are found for CLIC running at 1.5 and 3 TeV collision energies.

Figures (5)

• Figure 1: The $2\sigma$ allowed parameter region by SM Higgs signal strength measurements.
• Figure 2: Exclusion bounds in the $m_H$-$c_\gamma$ plane for $\rho_{tt} = -0.1$ (left) and in the $m_H$-$\rho_{tt}$ plane for $c_\gamma = 0.2$ (right) from vacuum stability requirement (light gray), electroweak precision constraints (orange), SM Higgs signal strength measurements (hatched), as well as $H \to ZZ$ (cyan), $A \to Zh$ (magenta), and $H^+ \to t\bar{b}$ (green) searches. The expected HL-LHC $H \to ZZ$ (dashed cyan) and $A \to Zh$ (dashed magenta) search limits are also shown.
• Figure 3: Cross sections vs. $\sqrt{s}$ for $m_{H} = 200$ GeV, $m_A = m_{H^+} = 500$ GeV, and $c_\gamma = 0.2$. Cross sections are calculated at leading order using MadGraph5_aMC@NLOAlwall:2014hca.
• Figure 4: Normalized distributions for signal and background processes at 1.5 TeV CLIC.
• Figure 5: Significance as a function of $m_H$ for both energy stages of CLIC.