The research, carried out by Giorgio Arcadi of the University of Messina and INFN Catania, Abdelhak Djouadi of the University of Granada, and Stefano Profumo of the University of California, Santa Cruz, focuses on a theoretical framework known as the 2HD+a model. This construction extends the Standard Model of particle physics by adding two Higgs doublets, which give rise to two neutral scalar bosons, a pair of charged Higgs particles, and two CP-odd pseudoscalar states. One of these pseudoscalars is a light singlet field that mixes with the heavier doublet component through an angle theta, and it serves as the mediator between dark matter and ordinary matter. The dark matter candidate itself is a fermionic particle coupled to the two CP-odd bosons, and the model has become the primary benchmark that the ATLAS and CMS collaborations use to guide dark matter searches at the Large Hadron Collider.

What makes this framework especially interesting is the behavior of pseudoscalar-mediated interactions at low velocities. When dark matter particles scatter off nuclei in a detector, they move slowly compared with the speed of light, and in this non-relativistic limit, interactions mediated by pseudoscalars are suppressed by the momentum transferred in the collision. At tree level, the scattering is helicity-suppressed, meaning the direct detection cross-section becomes extraordinarily small. The dominant contribution instead arises only at one-loop order, through box and triangle diagrams involving virtual Higgs bosons circulating in quantum loops. This suppression is precisely what can push viable dark matter scenarios below the neutrino fog, where conventional searches lose their clean discovery power but where the particles may still be hiding.

Crucially, the same suppression does not apply to the processes that governed dark matter production in the early Universe. Annihilation of dark matter particles in the hot primordial plasma proceeds at much higher velocities, where s-wave contributions dominate and are not suppressed by the pseudoscalar structure. This means the model can simultaneously satisfy the observed relic density of dark matter, measured precisely by the Planck satellite, while predicting scattering cross-sections far below current experimental sensitivity. The researchers performed extensive parameter scans using the micrOMEGAs package together with purpose-built codes for the loop-induced scattering cross-section, generating roughly ten thousand viable model points in their main scan and about one thousand in a subsidiary scan designed to explore fine-tuned regions of parameter space.

The team organized their analysis into three distinct regimes based on the strength of the mixing between the singlet and doublet pseudoscalar states. In the standard regime with substantial mixing, they found that a significant fraction of parameter space naturally falls inside the neutrino fog, particularly for dark matter masses above roughly one hundred gigaelectronvolts, where annihilation into pairs of Higgs bosons becomes kinematically accessible and governs the freeze-out process. Current limits from the LUX-ZEPLIN experiment and from Fermi-LAT observations of dwarf spheroidal galaxies already constrain much of this territory, but projected sensitivities of the next-generation XLZD observatory and the Cherenkov Telescope Array will probe deeper into the surviving regions.

In the regime of strongly suppressed mixing, where the mixing angle drops below one part in a thousand, the phenomenology changes dramatically. The annihilation channel into fermion pairs becomes negligible, and freeze-out proceeds almost exclusively through annihilation into pairs of Higgs bosons or into heavy CP-even states. The direct detection cross-section becomes entirely loop-induced, sinking comfortably into the neutrino fog for the smallest mixing values. Remarkably, the same suppression that hides dark matter from direct detection also extends the lifetime of the light pseudoscalar to macroscopic scales, producing displaced-vertex signatures at the Large Hadron Collider that are within the acceptance of existing ATLAS searches for long-lived particles. This creates a genuine complementarity between underground dark matter experiments and collider physics, because parameter points that are difficult to reach through conventional dark matter searches may be accessible through displaced hadronic jets at the LHC.

The most extreme case, where the mixing angle vanishes entirely, produces something qualitatively different: a two-component dark matter scenario. In this limit, the light pseudoscalar becomes a pure singlet with no direct coupling to Standard Model fermions, and if its mass falls below half the mass of the fermionic dark matter candidate, it becomes cosmologically stable and constitutes a second dark matter component alongside the fermion. The researchers solved the coupled Boltzmann equations governing the evolution of both components in the early Universe and found that the pseudoscalar component, which scatters off nucleons at tree level through exchange of the heavy CP-even Higgs boson, produces direct detection signals that are already strongly constrained by current data. Most of the viable parameter space in this two-component scenario is disfavored by existing limits alone, making it the most tightly constrained of the three regimes the team explored.

The study also identified ways to broaden the viable parameter space in both single-component and two-component scenarios by imposing specific relations among the model parameters that suppress the coupling between the 125-GeV Higgs boson and pairs of light pseudoscalars. This fine-tuning, which requires adjusting quartic couplings in the scalar potential to precisions of a few percent, opens up regions at lower dark matter masses that would otherwise be excluded. In these tuned regions, the correct relic density is achieved through p-wave suppressed annihilation processes, and the resulting parameter points populate the neutrino fog with a distinctive narrow strip in the plane of dark matter mass versus coupling strength.

Looking forward, the authors argue that their results provide strong motivation to fully exploit future ton-scale detectors such as XLZD, which will extend sensitivity by another order of magnitude beyond current liquid xenon experiments like PandaX-4T, XENONnT, and LUX-ZEPLIN. But they also emphasize that direct detection alone will not be sufficient. The complementarity between direct detection, indirect detection through gamma-ray observations, and collider searches for displaced vertices means that the Cherenkov Telescope Array, rather than the next generation of xenon experiments, may be the instrument that first probes the sub-floor parameter space identified in this work, particularly for dark matter masses in the range of one hundred to several hundred gigaelectronvolts.

The broader lesson emerging from this analysis is that the neutrino fog should not be understood as a barrier marking the end of the WIMP paradigm, but rather as a threshold beyond which the multichannel nature of dark matter phenomenology becomes essential. Neither direct detection alone, nor indirect detection alone, nor collider searches alone can fully characterize the parameter space explored here. The combination of all three approaches is necessary and, taken together, largely sufficient to probe the model across the full mass range from ten gigaelectronvolts to several teraelectronvolts. As experiments push toward the fog, the theoretical framework developed in this study offers a concrete roadmap for interpreting whatever signals, or absences of signals, the coming decade of dark matter searches may reveal.

Subject of Research: Dark matter phenomenology of the 2HD+a model and its detectability below the neutrino fog

Article Title: Charting dark matter down to the neutrino floor/fog in the 2HD+a scenario

Article References: Charting dark matter down to the neutrino floor/fog in the 2HD+a scenario. (n.d.). https://doi.org/10.3389/fphy.2026.1920686

Image Credits: AI Generated

DOI: 10.3389/fphy.2026.1920686

Keywords: dark matter, WIMP, neutrino fog, 2HD+a model, direct detection, indirect detection, LHC, pseudoscalar mediator, Higgs physics, XLZD, Cherenkov Telescope Array, relic density