Light Dark Matter: Models and Constraints (1709.07882v2)

Abstract: We study the direct detection prospects for a representative set of simplified models of sub-GeV dark matter (DM), accounting for existing terrestrial, astrophysical and cosmological constraints. We focus on dark matter lighter than an MeV, where these constraints are most stringent, and find three scenarios with accessible direct detection cross sections: (i) DM interacting via an ultralight kinetically mixed dark photon, (ii) a DM sub-component interacting with nucleons or electrons through a light scalar or vector mediator, and (iii) DM coupled with nucleons via a mediator heavier than ~ 100 keV.

• The paper presents comprehensive analyses of sub-GeV dark matter, exploring interactions via dark photons, light scalar/vector mediators, and heavier mediators above 100 keV.
• It employs astrophysical, cosmological, and laboratory constraints to rigorously define the viable parameter spaces for each proposed model.
• The study proposes innovative detection techniques, such as Dirac materials and superfluid helium detectors, to probe elusive dark matter in new experimental regimes.

An Examination of Light Dark Matter Models and Constraints

The paper "Light Dark Matter: Models and Constraints" presents a comprehensive analysis of the prospects for directly detecting sub-GeV dark matter, specifically focusing on the challenges imposed by current terrestrial, astrophysical, and cosmological constraints. The authors consider dark matter candidates lighter than an MeV, a mass range where constraints are particularly stringent. Three notable models are explored:

1. Dark matter interacting through an ultralight kinetically mixed dark photon.
2. A dark matter sub-component interacting with nucleons or electrons via a light scalar or vector mediator.
3. Dark matter coupled to nucleons via a mediator heavier than approximately 100 keV.

The introduction of novel techniques for sub-GeV dark matter detection has been driven by the absence of signals in traditional WIMP searches. These candidates could communicate with the Standard Model (SM) through various mediators, including ultralight dark photons or scalar/vector mediators, with proposed detection experiments targeting both nuclear recoils and electron scattering.

Review of Models Considered

Ultralight Kinetically Mixed Dark Photon:

In this scenario, sub-GeV dark matter interacts via a dark photon with kinetic mixing to the SM photon. This model facilitates exploring the direct detection potential across a wide dark matter mass range due to the smallness of the dark photon mass, mitigating many of the stellar cooling constraints typical for vector mediators.

Light Scalar or Vector Mediator:

The authors also outline models where dark matter scatters off nucleons or electrons through a light mediator. Here, astrophysical constraints such as stellar cooling and BBN limits play significant roles, especially as they limit the parameter space for lighter mediators. This requires careful balancing of mediator masses and couplings to maintain agreement with observational data.

Heavier Mediators:

For mediators above 100 keV, the primary challenge comes from thermal and cosmological considerations that influence the dark matter relic abundance and contribute to the number of relativistic degrees of freedom $N_{\text{eff}}$. The parameter space remains influenced by the interplay between self-interaction bounds and cosmological history requirements that trace back to the early universe.

Key Constraints

Stellar Cooling:

Stars can impose strict limits on light dark matter via energy loss mechanisms, particularly in HB and RG stars, leading to constraints on the mass and couplings of the mediators involved.

Self-Interactions:

The analysis includes stringent self-interaction constraints, especially relevant for light mediators, where such interactions might modify galactic halos observable profiles or leave signatures in colliding galaxy clusters.

Cosmology and $N_{\text{eff}}$:

The paper highlights the tight BBN and CMB constraints on $N_{\text{eff}}$, introducing bounds on how many degrees of freedom are available in the dark sector. These provide fundamental limitations on light scalar or vector models unless the model parameters allow dark sector particles to efficiently decouple from thermal equilibrium with the SM.

Conclusions and Prospective Developments

The paper delineates the complex landscape of theoretical boundaries for light dark matter, providing several avenues for potential discovery or further constraints using future experiments. The authors emphasize the importance of upcoming technologies—like Dirac materials and superfluid helium detectors—which hold potential to probe the sub-MeV regime, depending on the assumed dark sector interactions.

Despite the challenging constraints, the research clarifies that possibilities remain open for models that abide by astrophysical and cosmological limits. The expansion of detection methods could uncover new interactions or refine our understanding of these little-explored dark matter candidates, offering insight critical for the wider theoretical framework encompassing new physics beyond the Standard Model. The results call for further exploration at both theoretical levels and with observational innovations to test the advocated limits and proposals, paving the way for a deeper understanding of light dark matter under current observational paradigms.