Dark Matter Candidates (0903.4849v4)

Abstract: An overview is given of various dark matter candidates. Among the many suggestions given in the literature, axions, inert Higgs doublet, sterile neutrinos, supersymmetric particles and Kaluza-Klein particles are discussed. The situation has recently become very interesting with new results on antimatter in the cosmic rays having dark matter as one of the leading possible explanations. Problems of this explanation and possible solutions are discussed, and the importance of new measurements is emphasized. If the explanation is indeed dark matter, a whole new field of physics, with unusual although not impossible mass and interaction properties may soon open itself to discovery.

• The paper systematically categorizes dark matter candidates, detailing axions, inert Higgs models, sterile neutrinos, neutralinos, and Kaluza-Klein particles along with their theoretical bases.
• It emphasizes the role of the cold dark matter paradigm and links experimental anomalies like gamma-ray signatures and cosmic ray excesses to dark matter detection.
• The review calls for enhanced theoretical modeling and experimental efforts, spotlighting upcoming probes such as Fermi-LAT and advanced direct detection techniques.

Dark Matter Candidates: A Review of Particle Physics Prospects and Astrophysical Implications

The detection and understanding of dark matter remain among the foremost challenges in contemporary cosmology and particle physics. Lars Bergström's review, "Dark Matter Candidates," provides an analytical overview of potential particle dark matter candidates, their theoretical bases, and their implications in astrophysical observations. This paper situates itself within the historical and ongoing endeavors to elucidate the dark matter conundrum through both well-known and speculative approaches.

Theoretical Background and Dark Matter Paradigms

Historically, dark matter emerged as a critical concept to explain discrepancies in galactic rotation curves and velocity dispersions in galaxy clusters. The most widely accepted framework, the Cold Dark Matter (CDM) paradigm, serves as the backbone for cosmological structure formation models. This framework demands particles that are non-relativistic (or "cold") at the epoch of structure formation, enhancing the density contrasts necessary for galaxy formation.

Inventory of Dark Matter Candidates

Bergström categorizes dark matter candidates based on their theoretical origins and properties:

1. Axions: These pseudo-scalar particles originally emerged as a solution to the strong CP problem in particle physics. Despite their non-thermal production mechanism and the implicit fine-tuning requirement for their mass, axions remain a persistent candidate for dark matter, particularly in light of their potential to address cold dark matter density contributions without overclosing the universe.
2. Inert Higgs Doublet: This minimal extension of the Standard Model introduces electroweak symmetry-breaking particles that can act as dark matter. These models are characterized by an inert particle stable under discrete symmetries, offering viable channels for indirect detection through gamma-ray signatures.
3. Sterile Neutrinos: Positioned as warm dark matter candidates, sterile neutrinos, though not interacting via the known weak force, could bridge certain small-scale structure gaps found in CDM models. However, constraints on their mass and mixing angles remain stringent, and their role would likely constitute only a fraction of the total dark matter.
4. Supersymmetric Particles (Neutralinos): Arising from supersymmetric extensions of the Standard Model, neutralinos are stable, neutral, and naturally weakly interacting. They embody the WIMP (Weakly Interacting Massive Particles) hypothesis, holding promise for both direct and indirect detection channels given their cross-section properties at the electroweak scale.
5. Kaluza-Klein Particles in Universal Extra Dimensions: This class derives from theories positing additional spatial dimensions wherein all fields propagate. Here, the stability of the lightest Kaluza-Klein mode, somewhat analogous to neutralinos in supersymmetry, is ensured by a discrete symmetry, making them compelling candidates for dark matter searches.

Addressing Observational Signatures and Anomalies

Recent experimental results, notably from the PAMELA and ATIC collaborations, highlight anomalous cosmic ray excesses that could potentially be linked to dark matter annihilations or decays, necessitating enhanced annihilation rates through mechanisms like Sommerfeld enhancement. Such phenomena demand models where either local density clumping or velocity-dependent forces significantly amplify annihilation rates without violating existing gamma-ray and CMB constraints.

Future Prospects and Experimental Considerations

The implications of Bergström's review emphasize the necessity for continued investments in both theoretical model development and experimental infrastructure to probe dark matter's particle nature. Upcoming results from the Fermi-LAT and advancements in direct detection techniques are expected to either validate or constrain existing hypotheses. Additionally, the exploration of non-WIMP dark matter, such as asymmetric dark matter models or derivative approaches like self-interacting dark matter, may offer alternative insights.

In summary, Bergström's review delineates a comprehensive assessment of the diverse particle candidates for dark matter. The synthesis of particle physics, astrophysical observations, and cosmological modeling continues to be paramount in the search for a theory that elegantly reconciles dark matter's elusive nature with empirical reality. This review serves as a foundational reference for researchers aiming to navigate the complex landscape of dark matter research.