Detecting Dark Matter Annihilation with CMB Polarization : Signatures and Experimental Prospects

TL;DR

The paper proposes a model-agnostic framework in which DM annihilation during recombination injects energy into the IGM, quantified by $\epsilon_{dm}$ and captured by an on-the-spot approximation. This raises the residual ionization, broadens the surface of last scattering, and imprints characteristic signatures on the CMB TT, TE, and EE power spectra. By performing a likelihood-based detectability analysis that leverages polarization data, the authors show that Planck and future high-S/N polarization missions can probe DM masses in the few-to-tens of GeV range (for reasonable $f$ and $\langle\sigma_A v\rangle$), with WMAP largely insensitive above ~1 GeV. The work demonstrates that CMB polarization provides a powerful, complementary avenue to laboratory searches for constraining the cosmological abundance of DM and testing simple annihilation scenarios.

Abstract

Dark matter (DM) annihilation during hydrogen recombination (z ~ 1000) will alter the recombination history of the Universe, and affect the observed CMB temperature and polarization fluctuations. Unlike other astrophysical probes of DM, this is free of the significant uncertainties in modelling galactic physics, and provides a method to detect and constrain the cosmological abundances of these particles. We parametrize the effect of DM annihilation as an injection of ionizing energy at a rate e_{dm}, and argue that this simple "on the spot'' modification is a good approximation to the complicated interaction of the annihilation products with the photon-electron plasma. Generic models of DM do not change the redshift of recombination, but change the residual ionization after recombination. This broadens the surface of last scattering, suppressing the temperature fluctuations and enhancing the polarization fluctuations. We use the temperature and polarization angular power spectra to measure these deviations from the standard recombination history, and therefore, indirectly probe DM annihilation. (abridged)

Figures (8)

• Figure 1: A comparison of the photon cooling time to the Hubble time at $z=1000$, for different photon energies. The dominant processes (in order of increasing energy) are ionization, Compton scattering, pair production, and photon-photon scattering. All the curves (except the dotted curve) assume a neutral IGM, with a density of $2 \times 10^{-7}$ cm$^{-3}$ atoms today. The dotted curve shows the pair production rate for a completely ionized IGM. Regions where $t_{H}/t_{cool} < 1$ are transparent; photons injected at these energies lose their energy by redshifting. Note that photon-photon scattering does not transfer the energy to electrons, but simply redistributes it to lower energies. This figure ignores pair production off CMB photons since this process is subdominant for the energy range considered here; it however dominates at higher energies.
• Figure 2: The injection of energy from dark matter annihilation into the IGM, via the creation of electromagnetic cascades. Energy transfer to the IGM takes place principally through the ionization and collisional processes.
• Figure 3: The ionization fraction $x_{e}$ (top), matter temperature (center), and visibility function (bottom) as a function of $\epsilon_{dm}$. The heavy solid lines show the fiducial model with $\epsilon_{dm}=0$; from bottom to top, $\epsilon_{dm,0} = 5, 10, 100, 500 \times 10^{-25}$ eV/s. The thin dashed line in the center plot shows the evolution of CMB temperature, $T(z) = T_{0}(1+z)$. Note that the injection of additional energy does not slow recombination, but increases the residual ionization; this leaves the peak of the visibility function unchanged but broadens the surface of last scattering.
• Figure 4: The TT, TE, and EE angular power spectra for our fiducial cosmological model, with no DM annihilation (solid and dotted lines), and with $\epsilon_{dm,0}=10^{-22} {\rm ~eV/s}$. Also shown are the polarization noise spectra, for the WMAP V band, the Planck 143 Ghz channel, and a hypothetical high resolution polarization experiment (see Table \ref{['tab:expspec']} for details).
• Figure 5: (Top) The heavy lines show the ratio of damping functions $D(k)/D_{0}(k)$ for $\epsilon_{dm,0}$ of $100$, (solid), $500$ (dotted), and $1000 \times 10^{-25} {\rm eV/s}$ (dashed) and our fiducial cosmology. The light lines show this same ratio divided by $(k/k_{fid})^{-\alpha}$ where $k_{fid} = 0.05$ Mpc$^{-1}$. The horizontal dotted lines are a visual guide. (Bottom) The solid line shows the ratio of a model with no DM annihilation, but with $n_{s}$ altered using the analytic calculation above, to our fiducial model with $\epsilon_{dm,0}=500 \times 10^{-25} {\rm eV/s}$. The dashed line has the same ratio, except that all the cosmological parameters are adjusted to best fit the model with DM annihilation.