Dan Hooper Particle Astrophysics Center Fermi National Laboratory dhooper@fnal.gov

1. In Search of Particle Dark Matter Dan Hooper Particle Astrophysics Center Fermi National Laboratory dhooper@fnal.gov Massachusetts Institute of Technology March 13, 2006

2. What do we know about dark matter?

3. What do we know about dark matter? Ask An Astrophysicist:  A Great Deal!

4. The Existence of Dark Matter • Galaxy and cluster rotation curves have pointed to the presence of large quantities of non-luminous matter for many decades (compelling since the 1970’s) • White dwarfs, brown dwarfs, Jupiter-like planets, neutron stars, black holes, etc?

5. The Dark Matter Density • WMAP best-fit LCDM model (for a flat Universe): • 73 % Dark energy (WL= 0.73) • 27 % Matter (Mh2 = 0.27) • h2  0.0076

6. BaryonicAbundance • Big Bang nucleosynthesis combined with cosmic microwave background determine WBh2 0.024 • But, we also know WM~ 0.3, so most of the matter in the Universe is non-baryonic dark matter! Fields and Sarkar, 2004

7. A Cosmological Concordance Model LCDM D. Spergel et al., ApJ, 2003 J. Tonry et al, 2003 SDSS (and 2dF), 2005 M. Tegmark et al, 2004

8. “The world is full of obvious thing which nobody by any chance ever observes.” -Sherlock Holmes

9. What do we know about dark matter? Ask An Astrophysicist:  A Great Deal!

10. What do we know about dark matter? Ask An Astrophysicist:  A Great Deal! Ask A Particle Physicist: Next to Nothing (but we have some good guesses)

11. The Particle Nature of Dark Matter Axions, Neutralinos,Gravitinos, Axinos, Kaluza-Klein States, Heavy Fourth Generation Neutrinos, Mirror Particles, Stable States in Little Higgs Theories, WIMPzillas, Cryptons, Sterile Neutrinos, Sneutrinos, Light Scalars, Q-Balls, D-Matter, SuperWIMPS, Brane World Dark Matter,… A virtual zoo of dark matter candidates have been proposed over the years. 100’s of viable candidates. Weakly Interacting Massive Particles (WIMPs) are a particularly attractive class of dark matter candidates.

12. The Thermal Abundance of a WIMP • Freeze-out process: Stable particle X in thermal equilibrium in early Universe; later annihilation suppressed by Hubble expansion • Freeze-out occurs at a temperature: Automatically generates observed relic density!!!

13. The Thermal Abundance of a WIMP • Relic abundance of a WIMP is naturally in Ω h2 ~ 0.01-1 range • Strong motivation for WIMP dark matter • Other types of dark matter only generate observed dark matter density if fixed by hand (axions, thermal gravitinos, WIMPzillas, etc.) • Exception: SuperWIMP scenario •  Focus on WIMP candidates for dark matter

14. Supersymmetry • Introduces new bosons for fermions and vice versa • Elegant extension of the Standard Model • Natural solution to hierarchy problem (stabilizes quadradic divergences to Higgs mass) • Restores unification of couplings • Necessary in most string theories • Likely to be discoverd at Tevatron and/or LHC

15. Supersymmetry • To obtain sufficent proton stability, R-parity must be introduced • R-parity ensures that the Lightest Supersymmetric Particle (LSP) is stable • Possible supersymmetric dark matter candidate if LSP is not electrically charged or strongly interacting • The identity of the LSP depends of the mechanism supersymmetry breaking

16. The Minimal Supersymmetric Standard Model (MSSM) • Introduce minimal new particle content • New bosons: squarks, sleptons • New fermions: photino, zino, neutral higgsinos (neutralinos) charged wino and higgsino (charginos) and gluino • Of these, only the lightest neutralino and sneutrino are electrically neutral and non-strongly interacting (gravitino, axino are also possibilities in models beyond the MSSM) • Only the lightest neutralino naturally generates the observed dark matter density The lightest neutralino: the most natural SUSY dark matter candidate

17. The Lightest Neutralino Higgsinos Bino Wino • Neutralinos are Majorana particles (their own antiparticles) • Properties of the lightest neutralino can vary wildly depending on its composition • Tree-level annihilation to heavy fermions, higgs or gauge bosons, any of which can dominate • Magnitudes of the low velocity annihilation cross section and the elastic scattering cross section with nucleons can vary by many orders of magnitude • Very rich phenomenology

18. The Particle Dark Matter Candidate Zoo Axions, Neutralinos,Gravitinos, Axinos, Kaluza-Klein States, Heavy Fourth Generation Neutrinos, Mirror Particles, Stable States in Little Higgs Theories, WIMPzillas, Cryptons, Sterile Neutrinos, Sneutrinos, Light Scalars, Q-Balls, D-Matter, SuperWIMPS, Brane World Dark Matter, etc… Some old saying about eggs and baskets comes to mind…

19. Universal Extra Dimensions • All Standard Model Particles Propagate in Bulk • Kaluza-Klein (KK) Towers Appear, M ≈ R-1 ~ TeV • Extra-Dimensional Momentum Conservation  KK-Number Conservation • Realistic Models Must Be Orbifolded, KK-Number Violated, but KK-Parity Conserved  Lightest KK Particle (LKP) Stable • B(1) Most Natural Choice For LKP • Excellent Dark Matter Candidate

20. Kaluza Klein Dark Matter • t-channel KK fermion exchange diagrams dominate • Unlike neutralinos, annihilations are NOT helicity suppressed  light fermions final states frequent • Annihilations mostly to (large hypercharge) fermion pairs • 20% taus, 20% muons, 20% electrons, 35% quarks, 3% neutrinos, 2% higgs bosons • Yields observed dark matter density for m~ 900 GeV • Coannihilations could accomodate 550-3000 GeV • EWPO constrain m > 700 GeV (Flacke, Hooper, March-Russell, 2005)

21. How To Search For A WIMP: Colliders • If mDM~ mEW (along with associated particles), • discovery likely at LHC and/or Tevatron • Strong constraints from LEP data

22. How To Search For A WIMP: Astrophysics • Direct Detection • - Momentum transfer to detector • through elastic scattering • Indirect Detection • - Observation of annihilation • products (, e+, p, , etc.)

23. Indirect Detection: Anti-Matter • Matter and anti-matter generated equally in dark matter annihilations (unlike other processes) • Cosmic positron, anti-proton and anti-deuteron spectrum may contain signatures of particle dark matter • Upcoming experiments (PAMELA, AMS-02) will measure the cosmic anti-matter spectrum with much greater precision, and at much higher energies

24. Indirect Detection: Anti-Matter • Anti-protons/anti-deuterons • -Energy loss length of tens of kpc (samples entire dark matter halo) • -Depends critically on understanding of halo profile, galactic magnetic fields, and radiation (diffusion parameters and background difficult) • -For anti-deuterons, very low background and very low rate (single event discovery?) • Positrons • -Few kpc (or less) energy loss length (samples only local volume) • -Less dependent on understanding of halo profile, diffusion parameters • -Possible hints for dark matter present in existing data(?)

25. Indirect Detection: Positrons • Positrons produced through a range of dark matter annihilation channels: • (decays of heavy quarks, heavy leptons, gauge bosons, etc.) • Positrons move under influence of galactic magnetic fields • Energy losses through inverse compton and synchotron scattering with starlight, CMB

26. Indirect Detection: Positrons • Determine positron spectrum at Earth by solving diffusion equation: Energy Loss Rate Diffusion Constant • Inputs: • Diffusion constant • Energy loss rate • Annihilation cross section/modes • Halo profile (inhomogeneities?) • Boundary conditions • Dark matter mass Source Term

27. Indirect Detection: Positrons • Observed spectrum depends on dark matter particle properties:

28. Indirect Detection: Positrons Supersymmetric (neutralino) origin of positron excess? Spectrum can fit HEAT data Large annihilation rate required (non-thermal cross section and/or large degree of inhomogeneities)

29. Indirect Detection: Positrons • Kaluza-Klein Dark Matter • -Hard annihilation modes preferred (20% to each of e+e-, +-, +-) •  still good fit to data • -As with neutralinos, ultimately requires • a large quantity of local inhomogeneities to • normalize to HEAT data Hooper and G. Kribs PRD (hep-ph/0406026)

30. Indirect Detection: Positrons • The Annihilation Rate (Normalization) • -If a thermal relic is considered, a large degree of local • inhomogeneity (boost factor) is required in dark matter halo • -Might local clumps of dark matter accommodate this? • Two mass scales: • -Sum of small mass (~10-1 - 10-6 M)clumps •  Small boost (2-10, whereas ~ 50 or more is required) • -A single large mass clump (~104 - 108 M) •  Unlikely at 10-4 level Hooper, J. Taylor and J. Silk, PRD (hep-ph/0312076) H. Zhao, J. Taylor, J. Silk and Hooper (hep-ph/0508215)

31. Indirect Detection: Positrons Where does this leave us? • Future cosmic positron experiments hold great promise • PAMELA satellite, planned to be launched in 2006 • AMS-02, planned for deployment • onboard the ISS (???)

32. Indirect Detection: Positrons With a “HEAT sized” signal: • Dramatic signal for either PAMELA or AMS-02 • Clear, easily identifiable signature of dark matter Hooper and J. Silk, PRD (hep-ph/0409104)

33. Indirect Detection: Positrons With a smaller signal: • More difficult for PAMELA or AMS-02 • Still one of the most promising dark matter search techniques Hooper and J. Silk, PRD (hep-ph/0409104)

34. Indirect Detection: Positrons Prospects for Neutralino Dark Matter: • AMS-02 can detect a thermal (s-wave) relic up to ~200 GeV, for any boost factor, and all likely annihilation modes • For modest boost factor of ~ 5, AMS-02 can detect dark matter as heavy as ~1 TeV • PAMELA, with modest boost factors, can reach masses of ~250 GeV • Non-thermal scenarios (AMSB, etc), can be easily tested Value for thermal abundance Hooper and J. Silk, PRD (hep-ph/0409104)

35. Indirect Detection: Positrons Prospects For Kaluza-Klein Dark Matter: • For KKDM, AMS-02 can exclude thermal mass range for modest boost factors • Coannihilation scenarios with large masses (m > 1 TeV) may remain untested • Other DM models with annihilations to charged leptons will be highly constrained, especially for lower masses Lower limit from EWPO Value for thermal abundance Value for thermal abundance Cross section for KKDM Hooper and J. Silk, PRD (hep-ph/0409104)

36. Indirect Detection: Neutrinos • WIMPs elastically scatter with massive bodies (Sun) • Captured at a rate ~ 1018 s-1 (p/10-8 pb) (100 GeV/m)2 • Over billions of years, annihilation/capture rates equilibrate • Annihilation products absorbed, except for neutrinos

37. Indirect Detection: Neutrinos • The IceCube Neutrino Telescope • Full cubic kilometer instrumented volume • Technology proven with predecessor, AMANDA • First string of detectors deployed in 2004/2005, • 8 more strings deployed in 2005/2006 (80 in total) • Sensitive to muon neutrinos above ~ 100 GeV • Similar physics reach to KM3 in • Mediterranean Sea

38. Indirect Detection: Neutrinos • Neutrino flux depends on the capture rate, which is in turn tied to the elastic scattering cross section • Direct detection limits impact rates anticipated in neutrino telescopes

39. Indirect Detection: Neutrinos • WIMPs become captured in the Sun through spin-independent and spin-dependent scattering • Direct detection constraints on spin-dependent scattering are still very weak Spin-Dependent Spin-Independent

40. Indirect Detection: Neutrinos What Kind of Neutralino Has a Large Spin-Dependent Coupling?  Z q q q q Always Small  |fH1|2 - |fH2|2 Substantial Higgsino Component Needed

41. Indirect Detection: Neutrinos What Kind of Neutralino Has a Large Spin-Dependent Couplings? Large Rate At IceCube/KM3 Large Rate in IceCube/KM3 F. Halzen and Hooper (hep-ph/0510048)

42. Indirect Detection: Neutrinos B(1) B(1) Kaluza-Klein Dark Matter q(1) • Spin-dependent scattering naturally dominates • Annihilations to +- (20%),  (3%) lead to large number of neutrinos • Spin-independent cross section well beyond reach of direct detection q q  SD ≈ 2 x 10-6 pb (TeV/m)4 (0.1/rq)2

43. Indirect Detection: Neutrinos Kaluza-Klein Dark Matter • Low masses, quasi-degenerate KK quarks, lead to large rates • Relic density and EWPO • considerations lead to prediction of • 0.5-30 events/yr in km-scale telescopes • (IceCube, KM3) Hooper and G. Kribs, PRD (hep-ph/0208261), F. Halzen and Hooper (hep-ph/0510048)

44. Indirect Detection: Gamma-Rays Advantages of Gamma-Rays • Propagate undeflected (point sources possible) • Propagate without energy loss (spectral information) • Distinctive spectral features (lines), provide potential “smoking gun” • Wide range of experimental technology (ACTs, satellite-based) Disadvantages of Gamma-Rays • Flux depends critically on poorly known inner halo profiles •  predictions dramatically vary from model to model • Astrophysical backgrounds

45. Indirect Detection: Gamma-Rays The Galactic Center Region • Likely to be the brightest source of dark matter annihilation radiation • Detected in ~TeV gamma-rays by three • ACTs: Cangaroo-II, Whipple and HESS • Possible evidence for dark matter?

46. Indirect Detection: Gamma-Rays The Cangaroo-II Observation • Consistent with WIMP in ~1-4 TeV mass range • Roughly consistent with Whipple/Veritas Hooper, Perez, Silk, Ferrer and Sarkar, JCAP, astro-ph/0404205

47. Indirect Detection: Gamma-Rays The Cangaroo-II Observation • Consistent with WIMP in ~1-4 TeV mass range • Roughly consistent with Whipple/Veritas The HESS Obsevation • Superior telescope • Inconsistent with Cangaroo-II • Extends at least to ~10 TeV • WIMP of ~10-40 TeV mass needed D. Horns, PLB, astro-ph/0408192

48. Indirect Detection: Gamma-Rays Can A Neutralino Be As Heavy As 10-40 TeV? • Very heavy neutralinos tend to overclose the Universe • Largest annihilation cross sections (lowest relic abundance) are found for Wino-like or Higgsino-like neutralinos • h2~0.1 for ~1 TeV Higgsino, or ~3 TeV Wino • Significantly larger masses are possible only if coannihilations are carefully arranged (for example, S. Profumo, hep-ph/0508628)

49. Indirect Detection: Gamma-Rays Can A Neutralino Be As Heavy As 10-40 TeV? • Electroweak precision observables indicate the presence of a light higgs boson (near the EW scale) • Large contributions to the higgs mass come from particle loops: • In unbroken SUSY, boson and fermion loops exactly cancel • If mSUSY >> mHiggs , extreme fine tuning required • mSUSY below ~1 TeV is strongly preferred

50. Indirect Detection: Gamma-Rays Messenger Sector Dark Matter • In Gauge Mediated SUSY Breaking (GMSB) models, SUSY is broken in ~100 TeV sector • LSP is a light gravitino (1-10 eV), poor DM candidate • Lightest messenger particle is naturally stable, multi-TeV scalar neutrino is a viable dark matter candidate Dimopolous, Giudice and Pomarol, PLB (hep-ph/9607225) Han and Hemfling, PLB (hep-ph/9708264) Han, Marfatia, Zhang, PRD (hep-ph/9906508) Hooper and J. March-Russell, PLB (hep-ph/0412048)