Limits on Threshold and "Sommerfeld" Enhancements in Dark Matter Annihilation

Abstract

We find model-independent upper limits on rates of dark matter annihilation in galactic halos. The Born approximation generally fails, while exotic threshold enhancements akin to "Sommerfeld factors" also turn out to be baseless The most efficient annihilation mechanism involves perturbatively small decay widths that have largely been ignored. Widths that are very small compared to TeV mass scales suffice to effectively saturate unitarity bounds on annihilation rates. Bound state formation in weakly coupled theories produces small effects due to wave function normalizations. Unitarity shows the Sommerfeld factor cannot produce large changes in cross sections, and serves to identify where those approximations break down.

8 pages, 3 figures. Accepted for publication in Phys.Rev.D. No significant changes from V2

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• Effects of Velocity-Dependent Dark Matter Annihilation on the Energy Spectrum of the Extragalactic Gamma-ray Background
• Exploration of decaying dark matter in a left-right symmetric model
• Potentially Large One-loop Corrections to WIMP Annihilation
• Extragalactic and galactic gamma-rays and neutrinos from annihilating dark matter
• Extracting the size of the cosmic electron-positron anomaly
• Gauge-Independent Approach to Resonant Dark Matter Annihilation
• Dark Resonance