Probing Standard Model-like di-Higgs Production at Photon-Photon Colliders in the I(1+2)HDM Type-I

TL;DR

This work investigates SM-like di-Higgs production via $\gamma\gamma$ collisions at future $e^+e^-$ colliders within the I(1+2)HDM, a two-Higgs-doublet extension that includes an inert doublet and yields both active and inert charged scalars entering loops at the same perturbative order as the SM contributions. By scanning the model parameter space under theoretical and experimental constraints, the authors show that the partonic cross section $\sigma(\gamma\gamma\to hh)$ can be enhanced by up to a factor of about $50$ relative to the SM, driven by inert charged scalars $\chi^{\pm}$ and active $H^{\pm}$ and by modified trilinear and quartic Higgs self-couplings. They identify pronounced threshold effects at $2m_{\chi^{\pm}}$ and $2m_{H^{\pm}}$, and demonstrate that, with controlled photon kinematics and beam polarization, one can extract the charged-scalar masses and couplings from the di-Higgs signal. The results highlight the potential of $\gamma\gamma$ colliders to probe the scalar potential and charged spectrum of extended Higgs sectors, offering complementary information to HL-LHC Higgs measurements and direct searches.

Abstract

In this paper, pair production of Standard Model (SM)-like Higgs bosons, $hh$, is studied through $γγ$ scattering at future electron-positron colliders, in the framework of the Inert Doublet Model with two Active Doublets, i.e., the I(1+2)HDM for short. The relevance of the process $γγ\to hh$ for such a Beyond the SM (BSM) scenario stems from the fact that it is a one-loop process at lowest order, wherein inert charged states $χ^\pm$ contribute alongside with $W^\pm$, $H^\pm$ and heavy fermions (primarily, bottom and top quarks), crucially, at the same perturbative order. {Given that $χ^\pm/H^\pm$ masses and $hS^+S^-$ ($S^\pm=χ^\pm, H^\pm$) couplings are very mildly constrained,} there exist regions of the parameter space of the I(1+2)HDM where the former can be rather light and the latter rather large. After imposing up-to-date theoretical and experimental constraints on the I(1+2)HDM, it is found that the production rates of such process at future $γγ$ machines can be enhanced up to a factor of $\approx$ $50$ with respect to the SM, significantly exceeding typical yields of conventional 2-Higgs Doublet Models (2HDMs). Further, thanks to the level of control that one can attain at such facilities on the photon kinematics, leading to excellent invariant mass resolution of the incoming photon pairs, we show how it is possible to extract from this process the value of the $χ^\pm$ mass (along that of the active $H^\pm$ states) with high precision, whichever the decays of the $hh$ pair, both with and without beam polarization.

Figures (9)

• Figure 1: Generic Feynman diagrams for di-Higgs production, in the unitary gauge, at a photon-photon collider within the SM. The incoming wavy lines correspond to photons while outgoing dashed lines denote (SM-like) Higgs bosons. The loops are mediated either by fermions $F$ (top, bottom quarks and FP ghosts, solid lines) or gauge bosons $V$ ($W^\pm$, wavy internal lines). Herein, we distinguish the diagrams in terms of their propagators, as $s$-channels ones ($s_{1-3}$) and others ($b_{1-5}$), hereafter, denoted by 'Others (in plots)'. Black and brown blobs refer here to the reduced couplings $\kappa_{V,F,\lambda}$ or $\kappa_{2V}$.
• Figure 2: Left: cross section for $\gamma\gamma \to hh$ in the SM as a function of the CoM energy. The total rate (solid red) is decomposed into two contributions: $s$-channel loops (solid orange) and other loops (solid green). The contribution from the $s$-channel loops is further decomposed into the ones from top quarks (solid blue) and from $W^\pm$ bosons (solid purple). Right: total cross section for $\gamma\gamma \to hh$ as a function of $\kappa_\lambda$ for several CM energies. The horizontal lines are the SM cross sections ($\kappa_\lambda=1$).
• Figure 3: Generic additional Feynman diagrams for di-Higgs production at a photon-photon collider within the I(1+2)HDM with respect to the SM. Here, we adopt similar graph structures as those introduced for the SM: the $s$-channel ones ($S_{1-5}$), where the internal black(red)[blue] dash lines represent $H$($S_i=h,H$)[$S_j=\chi^\pm,H^\pm$] plus other contributions ($B_{1-4}$).
• Figure 4: Allowed parameter space in the I(1+2)HDM: left in the $(m_H,m_{A})$ plane and right in the $(m_{\chi},m_{\eta})$ plane. We also show the reduced couplings $(hhh)^{\rm I(1+2)HDM}/(hhh)^{\rm SM}$ and $(hH^+ H^-)^{\rm I(1+2)HDM}/(hhh)^{\rm SM}$.
• Figure 5: Left: SM and I(1+2)HDM total cross section $\sigma(\gamma\gamma \to hh)$ as a function of the collision energy $\sqrt{s}$ for unpolarized beams for each BP. Right: I(1+2)HDM cross section $\sigma(\gamma\gamma \to hh)$ as a function of the collision energy $\sqrt{s}$ for unpolarized beams for BP4 decomposed in the two subchannels described in the text.