Detecting dark matter annihilation with CMB polarization: Signatures and experimental prospects

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 ǫ 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. The modifications to the temperature power spectrum are nearly degenerate with the primordial scalar spectral index and amplitude; current CMB data are therefore unable to put any constraints on the annihilation power. This degeneracy is broken by polarization; Planck will have the sensitivity to measure annihilation power ǫ dm (z = 1000) > 10 −15 eV/s/proton, while high sensitivity experiments (eg. NASA's CMBPOL) could improve that constraint to ǫ dm (z = 1000) > 4 × 10 −16 eV/s/proton, assuming a fractional detector sensitivity of ∆T /T ∼ 1µK and a beam of 3 ′ . These limits translate into a lower bound on the mass of the DM particle, M dm > 10 − 100 GeV, assuming a single species with a cross section of σAv ∼ 2 × 10 −26 cm 3 /s, and a fraction f ∼ 0.1 − 1 of the rest mass energy used for ionization. The bounds for the WMAP 4y data are significantly lower, because of its lack of high S/N polarization measurements, but it can strongly constrain O( MeV) particles such as those proposed by .

Cosmological constraints in the presence of ionizing and resonance radiation at recombination

“…These particles may then decay further, leading to a shower /cascade that could, amongst other products, generate a bath of lower energy photons that could interact with the primordial gas and cosmic microwave background. Interestingly these models, as well as injecting energy at recombination, z ∼ 1000, boost the ionization fraction after recombination and can distort the ionization history of the universe at even later times, during galaxy formation and reionization z ∼ 5 − 10 [9,39,40,41,42]. Other mechanisms include evaporation of black holes [27,44] or inhomogenities in baryonic matter [27].…”

Early-Universe constraints on dark matter-baryon scattering and their implications for a global 21 cm signal

“…For example, DM annihilation (e.g., Refs. [23][24][25][26][27]) or decay [28][29][30][31] to SM particles can inject energy into the photon-baryon fluid between recombination and reionization, heating and ionizing the hydrogen gas and changing the CMB anisotropy spectrum accordingly.…”

A tale of two portals: testing light, hidden new physics at future e + e − colliders

“…Measurements of the CMB generally give constraints on DM annihilation or decay processes, which inject extra energy into the CMB and thus delay recombination [52][53][54][55][56][57]. The constraint is calculated using the energy deposition yield, f i eff , where i denotes a particular annihilation or decay channel and f eff describes the efficiency of energy absorption by the CMB from the energy released by DM in particular channel.…”