Probing Light Thermal Dark-Matter With a Higgs Portal Mediator (1512.04119v2)

Abstract: We systematically study light (< few GeV) Dark Matter (DM) models that thermalize with visible matter through the Higgs portal and identify the remaining gaps in the viable parameter space. Such models require a comparably light scalar mediator that mixes with the Higgs to avoid DM overproduction and can be classified according to whether this mediator decays (in)visibly. In a representative benchmark model with Dirac fermion DM, we find that, even with conservative assumptions about the DM-mediator coupling and mass ratio, the regime in which the mediator is heavier than the DM is fully ruled out by a combination of collider, rare meson decay, and direct detection limits; future and planned experiments including NA62 can further improve sensitivity to scenarios in which the Higgs portal interaction does not determine the DM abundance. The opposite, regime in which the mediator is lighter than the DM and the latter annihilates to pairs of visibly-decaying mediators is still viable, but much of the parameter space is covered by rare meson decay, supernova cooling, beam dump, and direct detection constraints. Nearly all of these conclusions apply broadly to the simplest variations (e.g. scalar or asymmetric DM). Future experiments including SHiP, NEWS, and Super-CDMS SNOLAB can greatly improve coverage to this class of models.

• The paper presents a detailed analysis of light thermal dark matter models mediated by the Higgs boson, clearly distinguishing between visible and invisible mediator decay regimes.
• It applies data from collider experiments, rare meson decays, and direct detection to rigorously constrain the parameter space of the DM-mediator mass relationships.
• The study highlights future prospects by proposing refined experimental strategies, such as enhanced meson decay and direct detection measurements, to further probe thermal dark matter.

Overview of Light Thermal Dark Matter Mediation through Higgs Portal

The paper entitled "Probing Light Thermal Dark-Matter With a Higgs Portal Mediator" authored by Gordan Krnjaic, provides an in-depth analysis of light dark matter (DM) models operating through the Higgs portal. The paper focuses on dark matter candidates with masses below a few GeV, utilizing a scalar mediator that interacts with visible matter via the Higgs boson. Such setups necessitate specific mediator conditions to prevent dark matter overproduction.

Model Formulation and Parameter Space Analysis

The research classifies models based on whether the mediator decays invisibly or visibly. A Dirac fermion DM benchmark model is primarily employed for illustrative purposes. In scenarios where the mediator surpasses the DM in mass, data from collider experiments, rare meson decay studies, and direct detection constraints are comprehensively analyzed. The investigations reveal that with stringent assumptions regarding coupling and mass ratios, the region with a heavier mediator is decisively excluded. Conversely, the alternative regime, where the mediator is lighter than DM with the DM annihilating to pairs of visibly decaying mediators, retains some viable parameter space.

The paper elaborates on the methodology for confronting these light DM models with current experimental data. It leverages collider data, precision measurements in rare meson decays (e.g., $B^+ \rightarrow K^+ \nu \bar{\nu}$ and $K^+ \rightarrow \pi^+ \nu \bar{\nu}$), state-of-the-art direct detection experiments (like LUX and Super-CDMS), and cosmological observations including the Cosmic Microwave Background (CMB) constraints. The analysis is particularly nuanced in its approach to reconcile theoretical predictions with experimental constraints, employing effective field theory metrics to encapsulate potential mediator mixings with the Higgs field.

Implications and Future Prospects

The paper discusses the broader implications of its findings in the field of particle physics and cosmology. It signifies potential advancements and tighter constraints that future experiments might place on such DM models. Proposed future facilities, such as NA62 for meson decay studies and planned enhancements in direct detection experiments like SHiP and NEWS, are poised to further inspect and potentially narrow down the remaining viable parameter space for light DM models addressed in the paper.

The research acknowledges the challenge posed by the broad mass range viable for DM candidates and highlights how preferentially focusing on thermal dark matter can meaningfully constrain the parameter space. This pursuit aligns with the ongoing "broad DM discovery effort," addressing thermal equilibrium arguments in the context of Higgs portal interactions.

Conclusions

In conclusion, the paper presents a rigorous, parameter-driven evaluation of light DM through the Higgs portal, delineating excluded regions of parameter space under a variety of model-specific assumptions. This endeavor not only complements the current understanding of the interplay between dark and visible sectors but also informs prospective experimental strategies crucial for uncovering the nature of DM. By refining theoretical models and advancing experimental techniques, the research underlines an ongoing commitment to resolving one of the most compelling enigmas in modern physics—the true identity and characteristics of dark matter.