Dark Matter Self-Interactions and Light Force Carriers (0911.3898v2)

Abstract: Recent observations from PAMELA, FERMI, and ATIC point to a new source of high energy cosmic rays. If these signals are due to annihilating dark matter, then the annihilation cross section in the present day must be substantially larger than that necessary for thermal freeze-out in the early universe. A new force, mediated by a particle of mass O(100 MeV), leading to a velocity dependent annihilation cross section - a `Sommerfeld enhancement' - has been proposed as a possible explanation. We point out that such models necessarily increase the dark matter (DM) self-scattering cross section, and use observational bounds on the amount of DM-DM scattering allowed in various astrophysical systems to place constraints on the mass and couplings of the light mediator.

• The paper demonstrates how a light boson mediates dark matter self-interactions, enhancing annihilation rates via the Sommerfeld effect.
• It employs astrophysical observations from galaxy clusters, dwarf galaxies, and the Bullet Cluster to constrain mediator mass and coupling constants.
• The study underscores the need for further N-body simulations to refine predictions of velocity-dependent dark matter scattering.

Overview of Dark Matter Self-Interactions and Light Force Carriers

The paper "Dark Matter Self-Interactions and Light Force Carriers," authored by Matthew R. Buckley and Patrick J. Fox, addresses the intriguing hypothesis that dark matter (DM) may be interacting through a new force mediated by a light particle. This concept arises in light of recent astrophysical observations suggesting anomalies in high-energy cosmic ray data, potentially attributed to DM phenomena. Specifically, the paper explores the ramifications of this hypothesis on DM self-interactions and the associated constraints from astrophysical systems.

Context and Motivations

The prevailing understanding from cosmic observations indicates that dark matter constitutes approximately 24% of the universe's energy density. While the precise nature of dark matter remains elusive, Weakly Interacting Massive Particles (WIMPs) have been a favored candidate due to their establishment through the WIMP miracle—a coincidence between the weak-scale mass particle and the required annihilation cross-section during thermal freeze-out at the early universe, resulting in the dark matter abundance we observe today.

Recent results from PAMELA, FERMI, and ATIC have unveiled unexpected cosmic ray excesses, which could be attributed to non-standard DM dynamics, necessitating a larger annihilation cross-section today compared to the early universe. This has led to speculation around dark forces mediated by a light boson, enhancing annihilation through the velocity-dependent Sommerfeld effect.

Implications and Numerical Analysis

The authors explore the consequences of introducing a light force carrier (of mass around 100 MeV) that exerts a Yukawa potential between DM particles, affecting both annihilation and scattering. This mediator could provide a velocity-dependent enhancement in annihilation rates, paralleling the observed phenomena.

However, such force carriers also naturally heighten DM self-scattering cross-sections. The paper utilizes constraints from astrophysical observations—specifically the permissible DM-DM scattering within systems like galaxy clusters, dwarf galaxies, and the Bullet Cluster—to confine the parameters of this light mediator. Notably, the significant Sommerfeld enhancement corresponds with increased scattering cross-sections but occurs at distinct points in parameter space.

Observational Constraints and Theoretical Implications

Astrophysical systems exhibit diverse characteristic velocities, influencing the scattering cross-section implications. For systems such as dwarf galaxies with lower velocities (~10 km/s), stringent bounds emerge. The authors calculate the velocity-averaged transfer cross-section, using it to set limits on the mediator mass and coupling constants that align with DM halo observations.

The researchers highlight that among the different astrophysical constraints, dwarf galaxies play a pivotal role due to their lower velocity dispersion, yielding a constraint on the mediator mass that, for certain couplings, must exceed approximately 40 MeV. The implications are substantial, as this constraint restricts the parameter space within which these dark forces can operate, while still allowing for the Sommerfeld-enhanced DM self-annihilation required to explain cosmic ray observations.

Conclusion and Future Research Directions

In sum, this work elucidates how potential DM interactions via a light mediator influence both theoretical understanding and phenomenology of DM self-interactions. It provides rigorous constraints based on astrophysical observations, offering insight into the viable parameter space for such DM models. The paper advocates for additional studies, particularly $N$-body simulations, to accommodate velocity-dependent cross-sections typical of light mediators.

Future theoretical developments and observational strategies may pivot around refining these constraints, exploring alternative detection methodologies for light mediators, and further delineating the actuality of dark forces in DM physics. This paper importantly bridges the gap between intriguing high-energy cosmic observations and fundamental DM characteristics, contributing cohesively to the overarching discourse on understanding dark matter's profound role in the universe.