Phenomenology of a Kinetic Higgs Portal

TL;DR

The paper investigates a non-minimal, momentum-dependent Higgs portal to a hidden sector, framed within an EFT that couples a ${\mathbb{Z}}_2$-symmetric scalar to the SM via derivative and non-derivative portal terms. By combining direct invisible-Higgs-decay searches with indirect probes of Higgs propagation, it shows that four-top production and precision Higgs measurements provide perturbatively reliable constraints, even in regions where $H\to SS$ decays are suppressed. The study demonstrates that the kinetic portal yields HEFT-like modifications to the Higgs sector, with potential to alter the electroweak phase transition and dark matter phenomenology, including relic abundance and direct-detection considerations. It further analyzes the cosmological implications, illustrating that while certain regions can reconcile relic abundance with constraints, achieving a strong first-order EWPT within perturbativity is delicate, making future lepton colliders essential to fully explore the parameter space.

Abstract

We explore the phenomenological consequences of non-minimal hidden sector interactions on observable correlations in the Higgs sector, mediated through the $\mathbb{Z}_2$-symmetric Higgs portal. Particular attention is given to non-standard momentum dependencies of the hidden sector scalar, which arise naturally in an effective field theory (EFT) framework. We demonstrate that perturbatively reliable constraints can be derived from four-top quark production data and precision measurements of Higgs couplings. These constraints are especially relevant in parameter regions where destructive interference suppresses invisible Higgs decays to light exotic scalars, keeping them within experimentally allowed limits. Finally, we discuss the implications of such hidden sector interactions for the thermal history of the universe. We show that non-standard momentum dependencies open up the Higgs portal to account for the observed dark matter relic abundance whilst evading current direct detection constraints. They can also be probed at the (HL-)LHC and, ultimately, at future lepton colliders such as a FCC-ee.

Figures (8)

• Figure 1: Generic non-minimal ${\mathbb{Z}}_2$-symmetric Higgs portal interactions coupling two emergent scalar $S$ bosons to the SM Higgs doublet $\Phi$ via interactions ${\cal{O}}_S$ at the lowest multiplicity order.
• Figure 2: Constraints from a representative invisible Higgs search at ATLAS ATLAS:2023tkt projected to the HL-LHC phase for off-shell production $m_S>m_H/2$. These direct search constraints on $\eta_{KS}$ (assuming $\Lambda=1~\text{TeV}$) start to probe the perturbativity limit of the model for $m_S\gtrsim 200~\text{GeV}$.
• Figure 3: Relative Higgs cross section constraints $\delta\sigma / \sigma$ understood as universal Higgs coupling corrections for different colliders and the model discussed in the text. We show $M_S=30~\text{GeV}$ for parameter choices that remove invisible branching ratio constraints (dot-dashed) through a choice of $\eta_S$. Also shown is $M_S=120~\text{GeV}$ for $\eta_{S}=0$ (dashed). The green bands represent the scale uncertainty, which is obtained from varying the renormalisation scale in $\mu \in [0.5m_H,2m_H]$ for a central choice $\mu=m_H$. Throughout, we choose $\Lambda=1~\text{TeV}$. An optimistic target for the HL-LHC is a 2% determination of universally rescaled SM Higgs couplings Cepeda:2019klc, which can be improved by a 0.31% measurement of Higgs strahlung Selvaggi:2025kmd at a future Higgs machine (here represented by FCC-ee). Other concepts such as the ILC Bambade:2019fyw, CLIC CLICdp:2018cto, CEPC CEPCStudyGroup:2023quu, or LCF LinearColliderVision:2025hltLinearCollider:2025lya can obtain quantitatively similar constraints.
• Figure 4: Constraints from four top quark production on the kinetic Higgs portal with $\Lambda=1~\text{TeV}$. In (a), we also show constraints from invisible Higgs constraints imposed by 125 GeV Higgs boson signal strength measurements for $\eta_S=0$ (black, dashed). These are extended by WBF constraints in the off-shell regime (black solid). (b) shows the constraints on $\eta_{KS}$ when $\eta_S=\eta_S(\eta_{KS})$ is chosen to remove the invisible Higgs decay according to Eq. \ref{['eq:decay']}. The different coloured bands correspond to the allowed ranges for different choices of the renormalisation scale $\mu$.
• Figure 5: Cross section deviations of $gg\to ZZ$ from the SM expectation through its dependence on the modified Higgs propagator, for $\eta_S=0$.