Direct and Indirect Searches For Particle Dark Matter[直接和間接搜索粒子暗物質(zhì)](-49)

1、Did Dark Matter Annihilations Reionize The Universe?Dan HooperFermilabTheoretical Astrophysics GroupFNAL Particle Astrophysics SeminarJune 1, 2009(Based on recent work with Alexander Belikov, arXiv:0904.1210)The Ionization History Of Our UniverseThe atoms in our universe have undergone two major pha

2、se transitionsDan Hooper - Did Dark Matter Annihilations Reionize The Universe?The Ionization History Of Our Universe370,000 years after the big bang, electrons and protons combine to form neutral atoms, and release the cosmic microwave backgroundDan Hooper - Did Dark Matter Annihilations Reionize T

3、he Universe?M a c in to s h P IC Tim a g e fo r m a tis n o t s u p p o r te dM a c in to s h P IC Tim a g e fo rm a tis n o t s u p p o rte dThe Ionization History Of Our Universe 370,000 years after the big bang, electrons and protons combine to form neutral atoms, and release the cosmic microwave

4、 background Between z6-20, our universes gas once again became ionizedDan Hooper - Did Dark Matter Annihilations Reionize The Universe?M a c in to s h P IC Tim a g e fo r m a tis n o t s u p p o r te dM a c in to s h P IC Tim a g e fo rm a tis n o t s u p p o rte dM a c in to s h P IC Tim a g e fo r

5、m a tis n o t s u p p o rte dEmpirical Handles On Reionization1) The Gunn-Peterson TroughNeutral hydrogen absorbs light very efficiently via the Lyman-alpha transition The lack of strong Lyman-alpha absorption in the spectra of very distant quasars demonstrates that the vast majority of hydrogen sin

6、ce z6 was ionized2) CMB Anisotropies Thompson scattering of the CMB photons with free electrons can produce observable anisotropiesWMAP has reported a Thompson optical depth of the universe of =0.017 (about 0.04 of which corresponds to full ionization at z 6, UV radiation from star forming galaxies

7、dominated reionization; at z 4, non-thermal emission from quasars became significant, enabling the double ionization of helium (Madau, Haardt, Rees, 1999)* Not to be confused with “Consensus”What Caused Reionization?Dan Hooper - Did Dark Matter Annihilations Reionize The Universe?The convention scen

8、ario, however, does not automatically lead to the full reionization of the universe by z6M a c in to s h P IC Tim a g e fo rm a tis n o t s u p p o rte dM a c in to s h P IC Tim a g e fo rm a tis n o t s u p p o rte dVolonteri and GnedinWhat Caused Reionization?Dan Hooper - Did Dark Matter Annihilat

9、ions Reionize The Universe?The convention scenario, however, does not automatically lead to the full reionization of the universe by z6This can be accommodated by reasonable (1) shifts in cosmological parameters (8, s)Volonteri and GnedinM a c in to s h P IC Tim a g e fo rm a tis n o t s u p p o rte

10、 dM a c in to s h P IC Tim a g e fo rm a tis n o t s u p p o rte dM a c in to s h P IC Tim a g e fo rm a tis n o t s u p p o rte dM a c in to s h P IC Tim a g e fo rm a tis n o t s u p p o rte dDan Hooper - Did Dark MatterAnnihilations Reionize The Universe?Current StatusMacintosh PICTimage formatis

11、 not supportedDark Matter As An Alternative Source Of Ionizing Radiation?Over the first billion years, dark matter had begun to form clumps and annihilate efficientlyDark matter annihilation products include gamma rays, which can scatter with electrons, causing gas to become ionizedIf one in 109 dar

12、k matter particles annihilate during this era, the energy released would be sufficient to completely reionize the universeDan Hooper - Did Dark Matter Annihilations Reionize The Universe?W+W-e+qq0e+Dark Matter As An Alternative Source Of Ionizing Radiation?N-Body simulations indicate that the first

13、(and smallest) clumps of dark matter formed by z60Mergers of smaller halos gradually lead to the structures observed todayThe halo mass function depends somewhat on the cosmological parameters, but otherwise can be reliably calculatedDan Hooper - Did Dark Matter Annihilations Reionize The Universe?D

14、ark Matter Halos At z6-60Macintosh PICTimage formatis not supportedReed et al., MNRAS, astro-ph/0607150Ionizing Radiation From WIMP AnnihilationsDan Hooper - Did Dark Matter Annihilations Reionize The Universe?WIMP Annihilation Typical final states include heavy fermions, gauge or Higgs bosonsW+W-Io

15、nizing Radiation From WIMP AnnihilationsDan Hooper - Did Dark Matter Annihilations Reionize The Universe?WIMP Annihilation Typical final states include heavy fermions, gauge or Higgs bosonsFragmentation/Decay Annihilation products decay and/or fragment into combinations of electrons, protons, deuter

16、ium, neutrinos and gamma-raysW+W-e+qqp0Ionizing Radiation From WIMP AnnihilationsDan Hooper - Did Dark Matter Annihilations Reionize The Universe?WIMP Annihilation Typical final states include heavy fermions, gauge or Higgs bosonsFragmentation/Decay Annihilation products decay and/or fragment into c

17、ombinations of electrons, protons, deuterium, neutrinos and gamma-raysInverse Compton Scattering Relativistic (GeV) electrons up-scatter CMB photons to MeV energiesW+W-e+qqp0e+Ionizing Radiation From WIMP AnnihilationsDan Hooper - Did Dark Matter Annihilations Reionize The Universe?WIMP Annihilation

18、 Typical final states include heavy fermions, gauge or Higgs bosonsFragmentation/Decay Annihilation products decay and/or fragment into combinations of electrons, protons, deuterium, neutrinos and gamma-raysInverse Compton Scattering Relativistic (GeV) electrons up-scatter CMB photons to MeV energie

19、sIonization, Heating and Excitation of Gas Some of these photons go on to scatter with electrons (1/3 of energy to each)W+W-e+qqp0e+The Relative Importance of Inverse Compton PhotonsDan Hooper - Did Dark Matter Annihilations Reionize The Universe?Most dark matter annihilation channels lead to a simi

20、lar quantity of energy being deposited in photons and electronsThe electrons eventually transfer their energy into a large number of lower energy photons via inverse Compton scattering with the CMBAs the photon-electron cross section is much larger at lower energies, a much larger fraction of the IC

21、 photons energy goes into ionizing atomsMa c i n t o s h P I C Ti ma g e f o r ma ti s n o t s u p p o r t e dMa c i n t o s h P I C Ti ma g e f o r ma ti s n o t s u p p o r t e dPrompt PhotonsElectronsIC PhotonsCase Example: A Typical SUSY NeutralinoConsider a typical 100 GeV neutralino which anni

22、hilates to W+W- with a cross section of v 3 x 10-26 cm3/sFor such a WIMP, annihilations in the first billion years of our universes history lead to only 1% of the atoms being reionized, and only mild heatingDan Hooper - Did Dark Matter Annihilations Reionize The Universe?Macintosh PICTimage formatis

23、 not supportedEfficiently Ionizing Dark Matter CandidatesTo provide the majority of the radiation that reionized the universe, we need another kind of WIMPFor example, we could consider WIMPs with:A considerably larger annihilation cross sectionDominant annihilation channels to electrons (more inver

24、se Compton)Dan Hooper - Did Dark Matter Annihilations Reionize The Universe?Macintosh PICTimage formatis not supportedBut what possible motivation could we have for such a dark matter candidate? Pamelas Cosmic Ray Positron MeasurementPamela Collaboration, arXivDan Hooper - Did Dark Matter Annihilati

25、ons Reionize The Universe?Pamelas Cosmic Ray Positron MeasurementPamela Collaboration, arXivFirst glance: -Is this all screwed up? Charge-dependent solar modulation important below 5-10 GeV!(Pamelas sub-10 GeV positrons appear as they should!)Dan Hooper - Did Dark Matter Annihilations Reionize The U

26、niverse?Pamelas Cosmic Ray Positron MeasurementPamela Collaboration, arXivFirst glance: -Is this all screwed up? Charge-dependent solar modulation important below 5-10 GeV!(Pamelas sub-10 GeV positrons appear as they should!)Astrophysical expectation (secondary production)Dan Hooper - Did Dark Matte

27、r Annihilations Reionize The Universe?Pamelas Cosmic Ray Positron MeasurementPamela Collaboration, arXivFirst glance: -Is this all screwed up? Charge-dependent solar modulation important below 5-10 GeV!(Pamelas sub-10 GeV positrons appear as they should!)Astrophysical expectation (secondary producti

28、on)Rapid climb above 10 GeV indicates the presence of a primary source of cosmic ray positrons!Dan Hooper - Did Dark Matter Annihilations Reionize The Universe?The Cosmic Ray Electron SpectrumIn a series of balloon flights, ATIC measured an excess of cosmic ray electrons between 300 and 800 GeV (Nat

29、ure, Nov. 21, 2008)New results from the Fermi Gamma Ray Space Telescope (and HESS) measure a less pronounced feature, but still an excess Macintosh PICTimage formatis not supportedDan Hooper - Did Dark Matter Annihilations Reionize The Universe?Dark Matter as the Source of the Pamela and/or Fermi Si

30、gnals The positrons/electrons observed by Pamela and Fermi could be generated by dark matter annihilations, but to do so would require the dark matter to have some rather special propertiesCholis, Goodenough, Hooper, Simet, Weiner arXivMacintosh PICTimage formatis not supportedBergstrom, Edsjo, Zaha

31、rijas, arXivDan Hooper - Did Dark Matter Annihilations Reionize The Universe?Dark Matter as the Source of the Pamela and Fermi Signals To produce the observed positron excess, dark Matter annihilations must proceed mostly to charged leptonsCholis, Goodenough, Hooper, Simet and Weiner, arXivDan Hoope

32、r - Did Dark Matter Annihilations Reionize The Universe?Dark Matter as the Source of the Pamela and/or Fermi Signals Bergstrom, Edsjo, Zaharijas, 2009Macintosh PICTimage formatis not supportedMacintosh PICTimage formatis not supportedTo produce the observed positron excess, dark Matter annihilations

33、 must proceed mostly to charged leptonsThe Fermi spectrum (if explained by dark matter), requires TeV-scale WIMPs that annihilate to +-Dan Hooper - Did Dark Matter Annihilations Reionize The Universe?Dark Matter as the Source of the Pamela and/or Fermi Signals To produce the observed positron excess

34、, dark Matter annihilations must proceed mostly to charged leptonsThe Fermi spectrum (if explained by dark matter), requires TeV-scale WIMPs that annihilate to +-Annihilations to leptons also enable the stringent constraints from antiproton, gamma ray and synchrotron measurements to be evadedPamela

35、Collaboration, arXivDan Hooper - Did Dark Matter Annihilations Reionize The Universe?Dark Matter as the Source of the Pamela and/or Fermi Signals The PAMELA/Fermi signals also require very large annihilations rates compared to that expected from a typical thermal relicCholis, Goodenough, Hooper, Sim

36、et and Weiner, 2008; Bergstrom, Edsjo, Zaharijas, 2009Macintosh PICTimage formatis not supportedDan Hooper - Did Dark Matter Annihilations Reionize The Universe?Dark Matter as the Source of the Pamela and Fermi SignalsOne possible solution:Annihilation rate dramatically increased by non-perturbative

37、 effects known as the “Sommerfeld Enhancement” -Very important for mmX and vXc (such as in the halo, where vx/c10-3)XXArkani-Hamed, Finkbeiner, Slatyer, Weiner, arXiv:0810.0713; Cirelli and Strumia, arXivXXDan Hooper - Did Dark Matter Annihilations Reionize The Universe?Dark Matter as the Source of

38、the Pamela and Fermi SignalsOne possible solution:Annihilation rate dramatically increased by non-perturbative effects known as the “Sommerfeld Enhancement” -Very important for mmX and vXc (such as in the halo, where vx/c10-3)If m 6)(WMAP optical depth)Did Dark Matter Reionize Our Universe? Dan Hoop

39、er - Did Dark Matter Annihilations Reionize The Universe?M a c in to s h P IC Tim a g e fo r m a tis n o t s u p p o r te dCross section range required for PAMELAObserved degree of reionization (z6)(WMAP optical depth)Belikov Did Dark Matter Reionize Our Universe? Dan Hooper - Did Dark Matter Annihi

40、lations Reionize The Universe?M a c in to s h P IC Tim a g e fo r m a tis n o t s u p p o r te dM a c in to s h P IC Tim a g e fo r m a tis n o t s u p p o r te dPAMELA OnlyPAMELA+ATICBelikov Did Dark Matter Reionize Our Universe? Dan Hooper - Did Dark Matter Annihilations Reionize The Universe?M a

41、c in to s h P IC Tim a g e fo r m a tis n o t s u p p o r te dM a c in to s h P IC Tim a g e fo r m a tis n o t s u p p o r te dPAMELA OnlyIf annihilating dark matter is responsible for the PAMELA (or ATIC) signals, then dark matter is also predicted to have played a dominant role in reionizing the

42、universe! PAMELA+ATICBelikov Cosmological parameters (8, s) impacting the halo mass functionClumping of gas (impact on recombination rate)Halo profile/concentration(Modest) UncertaintiesDan Hooper - Did Dark Matter Annihilations Reionize The Universe?How does the fraction of doubly ionized helium ev

43、olve with redshift?A closer look at gas heating - both modeling and constraintsFrom the WMAP optical depth measurement, what constraints can be placed on the dark matter annihilation cross section/channels?Open Questions/Areas For Future InquiryDan Hooper - Did Dark Matter Annihilations Reionize The

44、 Universe?Future Experiments!Planck will considerably refine the optical depth measurements, perhaps even providing information in redshift binsOpen Questions/Areas For Future InquiryDan Hooper - Did Dark Matter Annihilations Reionize The Universe?Future Experiments!The Fermi Gamma Ray Space Telesco

45、pe will be studying the extragalactic diffuse gamma ray background - if dark matter reionized the universe, it will also have generated a very bright backgroundOpen Questions/Areas For Future InquiryDan Hooper - Did Dark Matter Annihilations Reionize The Universe?Future Experiments!The Fermi Gamma R

46、ay Space Telescope will be studying the extragalactic diffuse gamma ray background - if dark matter reionized the universe, it will also have generated a very bright backgroundAlthough a typical thermal WIMP would provide only 0.5% of the 1-10 GeV background observed by EGRET, a WIMP capable of reio

47、nizing the universe would generate a background comparable to that observed (FGST would resolve very little of that flux) Open Questions/Areas For Future InquiryDan Hooper - Did Dark Matter Annihilations Reionize The Universe?Macintosh PICTimage formatis not supportedFuture Experiments!21 cm emission from neutral gas corresponds to radio frequencies (1.4 GHz)/(1+z)Very large radio observatories such as LOFAR may be able to map out the detailed history of reionization with redshiftOpen Questions/Ar