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

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 ~ 1000 z ~ 30 z ~ 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 < 1000 => 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, 0905.0003)

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).

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-

e-

e-

CMBe-

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.

Evaluating “ f ”

All channels,all secondaries,redshift dependence

Branching ratio ofDM annihilation

essential fordetermining absorption

Leptophilic channels give best fits to

cosmic ray excesses

leptons quarks

XDM e, XDM

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.

The Pamela(/Fermi/ATIC/HESS) sagaIF interpreted as DM:

High annihilation cross-section<v> ~10-24-10-21cm3/s

Forget about thermal decoupling

WIMP miracle

Unless<v> = <v>(v)

DM decoupling: ~1

Recombination: ~10-8

Small halos: 10-4

Milky Way: ~10-3-10-4

By courtesy of M. Cirelli and F. Iocco

“Sommerfeld” enhancementfulfils 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 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.

Saturated enhancement

[Galli et al. 09]

Yukawa potentiala 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 ~ 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.

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).