CMB constraints on dark matter annihilation

Transcription

1 CMB constraints on dark matter annihilation Tracy Slatyer, Harvard University NEPPSR 12 August 2009 arxiv: with Nikhil Padmanabhan & Douglas Finkbeiner

2 Dark matter!standard cosmological model: ~5/6 of the matter in the universe is nonbaryonic dark matter.!evidence: galactic rotation curves, cluster mass-to-light ratios, gravitational lensing, cosmic microwave background, structure formation. The observed rotation curve remains constant, contradicting predictions ( Gravitational lensing in the Bullet Cluster (

3 The WIMP miracle! Suppose dark matter was once in thermal equilibrium with Standard Model particles.! As the universe cools below the mass of the DM particle, annihilations of DM to lighter SM particles deplete the dark matter.! However, the expansion of the universe competes with annihilation: co-moving dark matter density eventually freezes out. Higher annihilation xsec = lower relic density.! Present-day DM density thus sets annihilation xsec, if DM is a thermal relic.! WIMP miracle: annihilation xsec is around the weak scale, where we expect new physics anyway. DM is a Weakly Interacting Massive Particle?

4 Signatures of dark matter annihilation!if dark matter is a thermal relic, we know it annihilates to SM particles with a weak-scale cross section. Can we see annihilation products?!many experiments search for these annihilation products from DM annihilation in the Galaxy s dark matter halo: Cosmic rays (PAMELA, ATIC, Fermi, etc) Gamma rays (EGRET, Fermi) Neutrinos (SuperKamiokande, IceCube, etc)!large uncertainties in astrophysical backgrounds, dark matter distribution, and propagation of cosmic rays: consequently, difficult to identify signals as coming from dark matter, or make precise predictions.

5 Dark matter and the history of the universe z ~ 1000 z ~ 30 z ~ 6 z ~ 0! Dark matter annihilation injects extra high energy particles can these annihilation products modify the universe s history?! Physics free of present-day astrophysical uncertainties.! Cosmic microwave background radiation carries information from around z ~ 1000, the epoch of hydrogen recombination.

6 The cosmic microwave background radiation The cosmic microwave temperature fluctuations from the 5-year WMAP data seen over the full sky (taken from

7 The cosmic microwave background radiation! Before recombination, most electrons are free: the universe has large optical depth due to Thomson scattering. At z ~ 1000, electrons and protons combine into hydrogen atoms, universe becomes largely transparent.! CMB photons propagate freely from the last scattering surface at z ~ 1000 to the present day. Density / temperature fluctuations in the plasma at the time of last scattering are therefore imprinted on the CMB.! Various cosmological parameters can be deduced from the temperature and polarization angular power spectra of the CMB: The spectral index of the primordial density fluctuations The ratio of energy density of matter / energy density of radiation The ratio of dark matter (interacts only gravitationally, to a first approximation) to baryons And more

8 WIMP annihilation during recombination Chen and Kamionkowski 04, Padmanabhan and Finkbeiner 05! WIMP annihilation injects high energy particles, which decay to stable SM states: 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, modifying the last scattering surface and the CMB. 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. #(z) Define effective efficiency f(z): = energy deposited to the IGM by DM annihilation per baryon per second, at redshift z = f(z) (2 M DM ) < $ v> (1+z) 3 (n DM ) 02 /(n baryon ) 0 deposition efficiency energy injected per annihilation annihilations per second per baryon! To constrain specific WIMP models, we need to compute f(z).

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

10 The transparency window(s)! t cool << t H at energies > 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.

11 Evaluating f! Numerically simulate photonelectron cascades from WIMP annihilation, track energy absorption with respect to z.! Take into account energy deposited by products of annihilations at earlier times.! Calculation performed for a range of annihilation channels and WIMP masses. leptons XDM e, % quarks XDM &'()

12 Effects of WIMP annihilation on electron fraction Ionization fraction Visibility function! At z > 1000 there are many free e - : energy injection has no effect.! At z < 1000 => annihilation products ionize the gas giving rise to residual ionization, broader last scattering surface.

13 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, )

14 Constraints on DM annihilation Galli et al ! Average f(z) over z= , to get approximate constant f. Then consider a new cosmological parameter p ann = f <$ v> / M parametrizing the power deposited by DM annihilation.!re-fit the WMAP5 data taking this new parameter into account, obtaining constraints on its value.! This gives upper limits on the DM annihilation xsec. It can also shift the best-fit values of the other cosmological parameters (especially n s ).

15 The PAMELA(/Fermi/ATIC/HESS) saga Recent cosmic ray measurements show excess in e+e-. IF interpreted as DM, implies high annihilation cross section <$v> ~ cm 3 /s Strongly constrained by CMB! e + fraction e - + e + E [GeV] By courtesy of M. Cirelli and F. Iocco If we want to keep WIMP miracle, then this suggests a velocity-dependent annihilation xsec, <$v> = <$v>(v) DM freezeout: * ~1 Milky Way: * ~ Small halos: * Recombination: * ~10-8 If <$v> rises monotonically with falling v, even MORE strongly constrained by CMB!

16 Constraints on specific DM models TRS, Padmanabhan and Finkbeiner ; Cholis et al WMAP5: models that fit PAMELA / ATIC / Fermi are close to 95% confidence limit. However, there are large astrophysical uncertainties in the fits to cosmic-ray data. Planck can test these models! Note: CMB constraints on DM explanations for these excesses are nearly modelindependent to first order, both cosmic-ray measurements and CMB just measure the total power injected in e+e-. The CMB is largely insensitive to the injection spectrum.

17 Summary The CMB can be used to place robust constraints on dark matter annihilation, which do not rely on assumptions about structure formation or Galactic astrophysics. These constraints are especially stringent for models where the annihilation rate rises at low velocities. We have performed the first detailed calculation of the energy deposition efficiency for products of WIMP annihilation, allowing direct comparison of models to the CMB constraints. Cross sections + annihilation channels which fit recently measured cosmicray anomalies lie close to WMAP5 95% limits (but fits have large astrophysical uncertainties). A broad range of DM explanations for these cosmic-ray excesses can be ruled out by Planck at 95% confidence at the factor of 10 level.