The Higgs Portal Above Threshold

TL;DR

This work confronts the challenge of heavy Higgs-Portal states that couple to the SM via $c_\phi |H|^2\phi^2$ and are produced through an off-shell Higgs when $m_\phi>m_h/2$. Using LO collider simulations with off-shell Higgs propagation, it analyzes VBF, ggH+jet, and $t\bar t$ associated production at $\sqrt{s}=14$ and $100$ TeV to derive exclusion and discovery reaches for $m_\phi$ and $c_\phi$. The results show that a 100 TeV collider can robustly probe Higgs-Portal scenarios motivated by electroweak baryogenesis, Higgs-Portal dark matter, and neutral naturalness, with VBF driving the reach and monojet channels providing complementary sensitivity, while 14 TeV provides limited but non-negligible coverage. The findings illustrate the potential of next-generation hadron colliders to directly test Higgs-Portal explanations of beyond-Standard-Model phenomena and to complement precision Higgs coupling measurements. These insights inform the physics case and search strategies for future collider programs targeting Higgs-Portal dynamics.

Abstract

The discovery of the Higgs boson opens the door to new physics interacting via the Higgs Portal, including motivated scenarios relating to baryogenesis, dark matter, and electroweak naturalness. We systematically explore the collider signatures of singlet scalars produced via the Higgs Portal at the 14 TeV LHC and a prospective 100 TeV hadron collider. We focus on the challenging regime where the scalars are too heavy to be produced in the decays of an on-shell Higgs boson, and instead are produced primarily via an off-shell Higgs. Assuming these scalars escape the detector, promising channels include missing energy in association with vector boson fusion, monojets, and top pairs. We forecast the sensitivity of searches in these channels at $\sqrt{s} = 14$ & 100 TeV and compare collider reach to the motivated parameter space of singlet-assisted electroweak baryogenesis, Higgs Portal dark matter, and neutral naturalness.

Figures (10)

• Figure 1: Values of $v_c/T_c$ as a function of $m_\phi$ and $c_\phi$ for singlet-assisted electroweak baryogenesis. The shaded purple region indicates a runaway for the Higgs Portal model corresponding to $M^2 < 0$, but the runaway can be stabilized by adding a quartic coupling of the form $\lambda \phi^4$. A perturbative two-step phase transition may then proceed in a $\sim 50$ GeV-wide penumbra at the edge of the shaded purple region, while a two-step transition deeper in the purple region cannot be ruled out but requires nonperturbatively large values of $\lambda$ to ensure the EWSB vacuum is deeper than the singlet vacuum Curtin:2014jma.
• Figure 2: Contours of relic DM density from freeze-out through the Higgs Portal. Constraints on the parameter space from the LUX direct detection experiment Akerib:2013tjd are shown in dotdashed red (labelled $\text{LUX}_{\text{NT}}$) where an additional assumption is made that in regions where thermal freeze-out over- or under-produces DM, additional fields and couplings lead to late-time DM dilution or production to realize the observed density. On the other hand, the solid red line (labelled $\text{LUX}_{\text{TH}}$) and shaded region show the parameter space which is excluded if one only takes the Lagrangian of Eq. (\ref{['eq:portal']}) and the assumption of a standard thermal history. In this case $\Omega_\phi \propto c_\phi^{-2}$ and $\sigma_n \propto c_\phi^{2}$ thus the exclusion is almost independent of the coupling, and largely depends only on the mass.
• Figure 3: The DM-nucleon scattering cross section weighted by the fractional relic density predicted by a standard thermal history $R_{DD} = \sigma_n \times \rho_{TH}/\rho_0$ as a function of the Higgs portal coupling $c_\phi$ for a variety of masses from $100 \text{ GeV} < m_\phi < 500 \text{ GeV}$ in steps of $25$ GeV. Masses $m_\phi < 2 m_h$ are shown in solid black and $m_\phi > 2 m_h$ in dashed red. Regions where DM is over-produced are not shown. This demonstrates that direct detection predictions for the Higgs portal with a standard thermal history only depend very weakly on the coupling and exclusions stronger than $\sigma_n < \mathcal{O}(1 \times 10^{-45} \text{cm}^2)$ typically exclude the Higgs portal with a standard thermal history independent of the coupling. The suppression with large coupling is shown whenever the $\phi + \phi \to h + h$ starts to dominate when kinematically accessible.
• Figure 4: An example of the loop processes contributing to the $E\!\!\!/_T +j$ signal from gluon associated production at hadron colliders.
• Figure 5: The ratio of the differential cross section for $j+E\!\!\!/_T$ for the full one-loop result relative to the HEFT result at 14 and 100 TeV for a variety of singlet scalar masses. When $p_T \sim 2 m_t$ there can be an $\mathcal{O}(1)$ enhancement due to the top mass threshold in the loop. At higher $p_T > 2 m_T$ the HEFT calculation may overestimate the signal rate significantly.