CMB constraints on Dark Matter models with large annihilation cross-section

TL;DR

The paper investigates how dark matter self-annihilation around recombination could modify the ionization history and imprint on the CMB, focusing on models with large Sommerfeld-enhanced cross-sections. By incorporating an energy-deposition parameter $p_{ann}=f\langle\sigma v\rangle/m_\chi$ into recombination and CMB calculations (via CAMB/RECFAST), the authors constrain DM annihilation using WMAP5 data and forecast Planck and cosmic-variance-limited experiments. They derive an upper limit of $p_{ann}<2.0\times10^{-6}$ m$^3$/s/kg (95% c.l.) with WMAP5 and show Planck could improve this to $p_{ann}<1.5\times10^{-7}$, with a cosmic-variance limit near $5.0\times10^{-8}$. The results place stringent constraints on Sommerfeld-enhanced cross-sections at very low relative velocities, providing an early-Universe probe that complements local Universe bounds and could test DM scenarios proposed to explain cosmic-ray anomalies. Overall, the work demonstrates the CMB as a sensitive laboratory for constraining DM microphysics and velocity-dependent annihilation, with significant implications for Planck-era cosmology and beyond.

Abstract

The injection of secondary particles produced by Dark Matter (DM) annihilation around redshift 1000 would inevitably affect the process of recombination, leaving an imprint on Cosmic Microwave Background (CMB) anisotropies and polarization. We show that the most recent CMB measurements provided by the WMAP satellite mission place interesting constraints on DM self-annihilation rates, especially for models that exhibit a large Sommerfeld enhancement of the annihilation cross-section, as recently proposed to fit the PAMELA and ATIC results. Furthermore, we argue that upcoming CMB experiments such as Planck, will improve the constraints by at least one order of magnitude, thus providing a sensitive probe of the properties of DM particles.

Figures (5)

• Figure 1: Evolution of the free electron fraction as function of redshift for different values of $p_{ann}=[0, 10^{-6},5\times10^{-6},10^{-5}]$$m^3/s/Kg$.
• Figure 2: TT, TE, EE angular power spectra (from Top to Bottom) for different values of $p_{ann}=[0, 10^{-6},5\times10^{-6},10^{-5}]$$m^3/s/Kg$ .
• Figure 3: Constraints on the $\omega_b$, $n_s$ and $\omega_c$ parameters in the case of standard recombination (solid line), or including dark matter annihilation (dashed line).
• Figure 4: Constraints on the self-annihilation cross-section at recombination $(\sigma v)_{z_r}$ times the gas--shower coupling parameter $f$. The dark blue area is already excluded by WMAP5 data, whereas the more stringent limit (dashed area) refers to the constraints which will be possible to apply with Planck. The light blue area is the zone ultimately allowed to probe by a cosmic variance limited experiment with angular resolution comparable to Planck.
• Figure 5: Constraints on the self-annihilation cross-section at recombination $(\sigma v)_{z_r}$, assuming the gas--shower coupling parameter $f$=0.5, see text for details. Regions above the solid (/long dashed/short dashed) thick lines are ruled out by WMAP5 (/Planck forecast/Cosmic Variance limited); the thin dotted and dashed-dotted lines are the predictions of the "Sommerfeld" enhanced self--annihilation cross sections with force carrying bosons of m$_\phi$=1GeV/c$^2$ and m$_\phi$=90GeV/c$^2$ respectively, see text for details. Notice that these constraints apply to $\langle\sigma v\rangle$ at very low temperatures such that it is in saturated Sommerfeld regime, and therefore directly comparable with results from galatic substructures and dwarf galaxies constraints as from SommDMsubMW.