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

DM annihilation at recombination  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). z ~ 1000z ~ 30z ~ 6 z ~ 0 CMB

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 residual ionization, broader last scattering surface. Ionization fraction Visibility function

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, )

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 M DM (1+z) 3 (n DM ) 0 2 /(n baryon ) 0  If f(z) ~ constant w.r.t z, then previous work constrains p ann = f / M. To constrain specific WIMP models, need to compute f(z).

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

The “transparency window(s)”  t cool 100 GeV, <1 keV at z ~ At intermediate energies, t cool ~ t H ; 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.

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

Constraints on DM models Galli et al ; Cholis et al  Average f(z) over z= , compare specific DM annihilation channels to constraints on f.  WMAP5: models that fit PAMELA / ATIC / Fermi are close to 95% confidence limit – but with large uncertainties.  Planck can test these models.

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

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 m V. Proposed to explain large annihilation xsec in Galactic halo, relative to xsec at freezeout.  Enhancement saturates when v/c ~ m V /m DM.  Non-resonant saturated value ~  /(m V /m DM ), where  = coupling of the DM to the force carrier. Saturated enhancement [Galli et al. 09] Yukawa potential a benchmark model

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 ~  In simplest case, this implies force carrier masses m V /m DM << 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 m V /m DM << violate CMB constraints.

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 ~ For the simplest (Yukawa potential) case, force carriers with m V << m DM are disfavored (relevant for collider expts).