Abstract
We compute in detail the rate at which energy injected by dark matter (DM) annihilation heats and ionizes the photon-baryon plasma at z{approx}1000, and provide accurate fitting functions over the relevant redshift range for a broad array of annihilation channels and DM masses. The resulting perturbations to the ionization history can be constrained by measurements of the CMB temperature and polarization angular power spectra. We show that models which fit recently measured excesses in 10-1000 GeV electron and positron cosmic rays are already close to the 95% confidence limits from WMAP. The recently launched Planck satellite will be capable of ruling out a wide range of DM explanations for these excesses. In models of dark matter with Sommerfeld-enhanced annihilation, where rises with decreasing WIMP velocity until some saturation point, the WMAP5 constraints imply that the enhancement must be close to saturation in the neighborhood of the Earth.
Highlights
Dark matter (DM) annihilation around the redshift of last scattering (z ∼ 1000) can modify the observed temperature and polarization fluctuations of the CMB, which have been measured to high precision by experiments such as WMAP, ACBAR and BOOMERANG [1, 2, 3]
Other indirect astrophysical probes of DM annihilation must contend with the complexities of Galactic astrophysics – for example, the DM distribution and clumpiness, ISM density, magnetic field strength and degree of tangling, Galactic photon energy density, etc
Less precise fits are given for higher and lower redshifts. Note that these fitting functions do not apply to models of decaying dark matter: while our numerical approach can be applied to decaying WIMP models with only trivial changes, we defer such an analysis to future work
Summary
Dark matter (DM) annihilation around the redshift of last scattering (z ∼ 1000) can modify the observed temperature and polarization fluctuations of the CMB, which have been measured to high precision by experiments such as WMAP, ACBAR and BOOMERANG [1, 2, 3]. The electron and positron spectra produced by such annihilation channels provide excellent fits to the ATIC and PAMELA e+e− spectra, up to a boost factor attributed to the Sommerfeld enhancement [20], and fit the Fermi data well [29] Such models have been proposed to explain anomalous results from the DAMA [30] experiment with inelastic WIMPnucleon scattering [31, 32, 33], and the INTEGRAL [34] signal with inelastic WIMP-WIMP scattering [35, 36], which naturally arises in this framework [23].
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