CMB constraints on WIMP annihilation: energy absorption during recombination

1. CMB constraints on WIMP annihilation: energy absorption during recombination Tracy Slatyer – Harvard University TeV Particle Astrophysics SLAC, 14 July 2009 arXiv:0906.1197 with Nikhil Padmanabhan & Douglas Finkbeiner

2. DM annihilation at recombination z ~ 0 z ~ 1000 z ~ 30 z ~ 6 • Extra ionization from annihilation products modifies the CMB temperature/polarization angular power spectra. • Physics free of present-day astrophysical uncertainties (e.g. dark matter density profile, propagation parameters). CMB

3. Effects of WIMP annihilation on electron fraction • WIMP annihilation injects highly energetic particles, which ionize the hydrogen and helium gas as they cool. • At z > 1000 there are many e- : energy injection has no effect. • At z < 1000 => residual ionization, broader last scattering surface. Visibility function Ionization fraction

4. Effects of extra e- on CMB TT, EE, TE angular power spectra for different values of the energy injection from WIMP annihilation. (Galli et al, 0905.0003)

5. Energy absorption from DM annihilation Chen and Kamionkowski 04, Padmanabhan and Finkbeiner 05 • WIMP annihilation injects high energy particles: e+e-, , , p. Energy injected in neutrinos and protons largely escapes. • e+e- with E < 1 MeV and photons with E < 1 keV efficiently heat and ionize the IGM. Higher energy photons, e+e-must first lose their energy (by redshifting, downscattering, pair production, etc). Energy not absorbed by IGM is redshifted away - unabsorbed photons may appear in diffuse gamma backgrounds today. • Define effective efficiency f(z): (z) = energy deposited to the IGM by DM annihilation per baryon per second, at redshift z = f(z) 2 MDM< v> (1+z)3 (nDM)02/(nbaryon)0 • If f(z) ~ constant w.r.t z, then previous work constrains pann = f < v> / M. To constrain specific WIMP models, need to compute f(z).

6. Energy loss mechanisms PHOTONS Injected  ray ELECTRONS Pair production on the CMB. Photon-photon scattering. Pair production on the H/He gas. Compton scattering. Photoionization. CMB Inverse Compton scattering on the CMB. e- e+ e- e- Excitation, ionization, heating of gas. Positronium annihilation. e- H, He e- Need to consider redshifting. All fast relative to Hubble time.

7. The “transparency window(s)” • tcool << tH at energies > 100 GeV, <1 keV at z ~ 1000. At intermediate energies, tcool ~ tH; dominant processes are pair production on gas, Compton scattering. At later redshifts universe becomes more transparent. • Below transparency windows, most energy => heating, ionization. Transparent region => diffuse gamma background.

8. Evaluating “ f ” leptons quarks XDM e, XDM , All channels, all secondaries, redshift dependence Branching ratio of DM annihilation essential for determining absorption Leptophilic channels give best fits to cosmic ray excesses

9. Constraints on DM modelsGalli et al 0905.0003; Cholis et al 0811.3641 • Average f(z) over z=800-1000, compare specific DM annihilation channels to constraints on f < v>. • WMAP5: models that fit PAMELA / ATIC / Fermi are close to 95% confidence limit – but with large uncertainties. • Planck can test these models.

10. The Pamela(/Fermi/ATIC/HESS) saga IF interpreted as DM: High annihilation cross-section <v> ~10-24-10-21cm3/s Forget about thermal decoupling WIMP miracle e+ fraction Unless <v> = <v>(v) DM decoupling:  ~1 Recombination:  ~10-8 Small halos: 10-4 Milky Way:  ~10-3-10-4 “Sommerfeld” enhancement fulfils the requirements (higher DM masses preferred) e-+ e+ E [GeV] By courtesy of M. Cirelli and F. Iocco

11. Sommerfeld enhancement Sommerfeld 1931, Hisano et al 2005, Cirelli et al 2007, March-Russell et al 2008 • Nonperturbative boost to annihilation at low relative velocities v, due to force mediated by new particle of mass mV. Proposed to explain large annihilation xsec in Galactic halo, relative to xsec at freezeout. • Enhancement saturates when v/c ~ mV/mDM. • Non-resonant saturated value ~ /(mV/mDM), where  = coupling of the DM to the force carrier. Yukawa potential a benchmark model Saturated enhancement [Galli et al. 09]

12. Constraining the force carrier mass • Cross section required for PAMELA / ATIC / Fermi ~ maximum saturated cross section allowed by WMAP5. If Sommerfeld-enhanced models fit cosmic-ray excesses => enhancement MUST be saturated by v/c ~ 10-4-10-3. • In simplest case, this implies force carrier masses mV/mDM << 10-4 are disfavored. • For models where WIMP annihilates to the same particle that mediates the Sommerfeld enhancement,  is set by freeze-out xsec: again, in the simplest case, models with mV/mDM << 10-4 violate CMB constraints.

13. Conclusions • First detailed calculation of f(z) for WIMP annihilation allows direct comparison of models to CMB constraints. • Cross sections + annihilation channels which fit cosmic-ray anomalies lie close to WMAP5 95% limits (but fits have large astrophysical uncertainties). • Broad range of DM explanations for cosmic-ray excesses can be ruled out by Planck at 95% confidence at the factor of 10 level. • Sommerfeld-enhanced models which fit cosmic-ray data: enhancement is ~saturated at v/c ~ 10-3-10-4. For the simplest (Yukawa potential) case, force carriers with mV << 10-4 mDM are disfavored (relevant for collider expts).