Precise Relic WIMP Abundance and its Impact on Searches for Dark Matter Annihilation (1204.3622v3)

Abstract: If dark matter (DM) is a weakly interacting massive particle (WIMP) that is a thermal relic of the early Universe, then its total self-annihilation cross section is revealed by its present-day mass density. The canonical thermally averaged cross section for a generic WIMP is usually stated as 3*10^-26 cm^3s^-1, with unspecified uncertainty, and taken to be independent of WIMP mass. Recent searches for annihilation products of DM annihilation have just reached the sensitivity to exclude this canonical cross section for 100% branching ratio to certain final states and small WIMP masses. The ultimate goal is to probe all kinematically allowed final states as a function of mass and, if all states are adequately excluded, set a lower limit to the WIMP mass. Probing the low-mass region is further motivated due to recent hints for a light WIMP in direct and indirect searches. We revisit the thermal relic abundance calculation for a generic WIMP and show that the required cross section can be calculated precisely. It varies significantly with mass at masses below 10 GeV, reaching a maximum of 5.2*10^-26 cm^3s^-1 at masses around 0.3 GeV, and is 2.2*10^-26 cm^3s^-1 with feeble mass-dependence for masses above 10 GeV. These results, which differ significantly from the canonical value and have not been taken into account in searches for annihilation products from generic WIMPs, have a noticeable impact on the interpretation of present limits from Fermi-LAT and WMAP+ACT.

• The paper revises thermal relic calculations, showing that the WIMP annihilation cross section varies significantly with particle mass, especially below 10 GeV.
• It employs both numerical integration and refined analytical methods to update early Universe thermal dynamics and account for evolving relativistic degrees of freedom.
• The findings prompt a recalibration of indirect detection strategies, particularly for low-mass WIMPs, to yield more accurate dark matter constraints.

Overview of Precise Relic WIMP Abundance and its Impact on Searches for Dark Matter Annihilation

The paper "Precise Relic WIMP Abundance and its Impact on Searches for Dark Matter Annihilation" revisits the thermal relic abundance calculation for Weakly Interacting Massive Particles (WIMPs) to provide a more nuanced understanding of their annihilation cross section. This work is directed towards enhancing the interpretation of contemporary indirect dark matter detection experiments, particularly those probing astrophysical annihilation signatures.

The authors present a detailed reappraisal of the thermal relic abundance calculation. They assert that the WIMP annihilation cross section, commonly taken to be constant at ∼3 × 10⁻²⁶ cm³ s⁻¹, should in fact be regarded as varying significantly with respect to the particle mass, especially at masses below 10 GeV. The computed required cross section is 5.2 × 10⁻²⁶ cm³ s⁻¹ at a WIMP mass of 0.3 GeV, contrasting sharply with the canonical value. At larger masses, above 10 GeV, this cross section stabilizes to 2.2 × 10⁻²⁶ cm³ s⁻¹.

Detailed Findings

1. Numerical and Analytical Approach: The authors employ both numerical integration and an improved analytical method to examine the relic abundance, considering updates to the coefficients and inputs associated with the Universe's evolution during the early thermal era. Additionally, they account for changes in relativistic degrees of freedom at temperatures near the quark-hadron transition.
2. Mass-Dependent Annihilation Cross Section: The traditional assumption of a fixed annihilation cross section irrespective of mass is challenged by the authors. They present evidence that for WIMP masses less than 10 GeV, the required cross section varies significantly, peaking at a very low mass. This indicates that previous search strategies may have understated the annihilation rate for low-mass WIMPs.
3. Experimental Implications: The paper's revelations have substantial implications for the interpretation of data from indirect detection experiments, such as those based on gamma-ray observations (Fermi-LAT) and cosmic microwave background measurements (WMAP+ACT). These revised cross sections affect the inferred constraints on WIMP properties, notably showing that the expected annihilation signals at low masses can be larger than previously assumed.
4. Cosmological Constraints: The research further implicates cosmological observations as potential means of ruling out certain low-mass WIMP mass ranges. The larger cross sections suggested for lower mass WIMPs reinforce constraints derived from CMB observations, although these may be relaxed under certain model-specific assumptions involving annihilation predominantly into neutrinos.

Theoretical and Practical Implications

This paper highlights crucial theoretical issues regarding the treatment of WIMP dark matter models and their thermally averaged annihilation cross sections. It underlines the necessity of recalibrating experimental analyses to align with a more mass-sensitive understanding of WIMP physics. Furthermore, acknowledging mass-dependent variations could lead to altered strategies in the setup of indirect detection experiments.

In terms of practical impact, these findings urge experimenters to revisit and expand their analyses concerning low-mass WIMPs, perhaps incorporating additional decay channels or revisiting lower energy thresholds where possible. This nuanced treatment of relic WIMP abundance potentially modifies the parameter space of interest for both direct and indirect dark matter searches.

Future Prospects and Developments

The paper paves the way for more refined explorations of dark matter models, potentially influencing the design and interpretative frameworks of future experiments. As indirect detection experiments refine their methods, the elaboration developed here anticipates more reliable constraints on the permissible mass and interaction cross-sections of dark matter candidates.

Further inquiries might explore a broader range of model assumptions, such as non-standard cosmologies or varying initial conditions, to test the robustness of these findings. This work exemplifies ongoing progress in the search for a fuller understanding of dark matter, reiterating the complexity and dynamism inherent in astroparticle physics research.