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High-luminosity fixed target experiments provide impressive sensitivity to new light weakly coupled degrees of freedom. We revisit the minimal case of a scalar singlet $S$ coupled to the Standard Model through the Higgs portal, that decays visibly to leptons for scalar masses below the di-pion threshold. The dataset from the LSND experiment is found to impose the leading constraints within two mass windows between $m_S sim 100$ and 350 MeV. In the process, we analyze a number of scalar production channels in the target, finding that proton bremsstrahlung provides the dominant channel at LSND beam energies.
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Light scalar fields that couple to matter through the Higgs portal mediate long range fifth forces. We show how the mixing of a light scalar with the Higgs field can lead to this fifth force being screened around macroscopic objects. This behaviour can only be seen by considering both scalar fields as dynamical, and is missed if the mixing between the Higgs field and the scalar field is not taken into account. We explain under which conditions the naive integrating out procedure fails, i.e. when the mass matrix of the Higgs-scalars system has a nearly vanishing mass eigenvalue. The resulting flat direction in field space can be lifted at the quadratic order in the presence of matter and the resulting fifth force mediated by the Higgs portal can be screened either when the gravitating objects are large enough or their surface Newton potential exceeds a threshold. Finally we discuss the implications of these results for nearly massless relaxion models.
Higgs portal interactions provide a simple mechanism for addressing two open problems in cosmology: dark matter and the baryon asymmetry. In the latter instance, Higgs portal interactions may contain the ingredients for a strong first order electroweak phase transition as well as new CP-violating interactions as needed for electroweak baryogenesis. These interactions may also allow for a viable dark matter candidate. We survey the opportunities for probing the Higgs portal as it relates to these questions in cosmology at the LHC and possible future colliders.
We study the impact on leptogenesis of Higgs portal couplings to a new scalar singlet. These couplings open up additional $CP$-violating decay channels for the higher mass singlet neutrinos $N_2$ and $N_3$. We analyze the simplest case of two-level $N_1-N_2$ leptogenesis, including significant mass hierarchies, in which the $CP$ asymmetry is generated in part by singlet-mediated decays of $N_2$. For these models, provided the lightest singlet neutrino $N_1$ is sufficiently weakly coupled to avoid excessive washout, its mass scale is not directly constrained by the Davidson-Ibarra bound.
We review scenarios in which the particles that account for the Dark Matter (DM) in the Universe interact only through their couplings with the Higgs sector of the theory, the so-called Higgs-portal models. In a first step, we use a general and model-independent approach in which the DM particles are singlets with spin $0,frac12$ or $1$, and assume a minimal Higgs sector with the presence of only the Standard Model (SM) Higgs particle observed at the LHC. In a second step, we discuss non-minimal scenarios in which the spin-$frac12$ DM particle is accompanied by additional lepton partners and consider several possibilities like sequential, singlet-doublet and vector-like leptons. In a third step, we examine the case in which it is the Higgs sector of the theory which is enlarged either by a singlet scalar or pseudoscalar field, an additional two Higgs doublet field or by both; in this case, the matter content is also extended in several ways. Finally, we investigate the case of supersymmetric extensions of the SM with neutralino DM, focusing on the possibility that the latter couples mainly to the neutral Higgs particles of the model which then serve as the main portals for DM phenomenology. In all these scenarios, we summarize and update the present constraints and future prospects from the collider physics perspective, namely from the determination of the SM Higgs properties at the LHC and the search for its invisible decays into DM, and the search for heavier Higgs bosons and the DM companion particles at high-energy colliders. We then compare these results with the constraints and prospects obtained from the cosmological relic abundance as well as from direct and indirect DM searches in astroparticle physics experiments. The complementarity of collider and astroparticle DM searches is investigated in all the considered models.
A coupling of a scalar, charged under an unbroken global U(1) symmetry, to the Standard Model via the Higgs portal is one of the simplest gateways to a dark sector. Yet, for masses $m_{S}geq m_{H}/2$ there are few probes of such an interaction. In this note we evaluate the sensitivity to the Higgs portal coupling of di-Higgs boson production at the LHC as well as at a future high energy hadron collider, FCC-hh, taking into account the full momentum dependence of the process. This significantly impacts the sensitivity compared to estimates of changes in the Higgs-coupling based on the effective potential. We also compare our findings to precision single Higgs boson probes such as the cross section for vector boson associated Higgs production at a future lepton collider, e.g. FCC-ee, as well as searches for missing energy based signatures.
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