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

1. Hot on the Trail of Particle Dark Matter Dan Hooper Particle Astrophysics Center Fermi National Laboratory dhooper@fnal.gov University of Kansas April 17, 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 Vera Rubin Fritz Zwicky • Galaxy and cluster rotation curves have pointed to the presence of large quantities of non-luminous matter for many decades (conclusive evidence since the 1970’s)

5. The Existence of Dark Matter Vera Rubin Fritz Zwicky • Galaxy and cluster rotation curves have pointed to the presence of large quantities of non-luminous matter for many decades (conclusive evidence since the 1970’s) In the new age of precision cosmology, we now know much more!

6. The Density of our Universe The anisotropies in the cosmic microwave background (CMB) have been studied to reveal the curvature and density of our Universe: tot 1 (about 10-29 grams/cm3)

7. The Composition of Our Universe • In addition to matter, general relativity allows for a cosmological term, L(vacuum energy/dark energy) • Quantum field theory would suggest that WL~ 1060, 10120, or 0 • So, we had expected to measure WL= 0

8. The Composition of Our Universe • In addition to matter, general relativity allows for a cosmological term, L(vacuum energy/dark energy) • Quantum field theory would suggest that WL~ 1060, 10120, or 0 • So, we had expected to measure WL= 0 • Our expectations turned out to be wrong!

9. The Composition of Our Universe • Compare expansion history of our Universe to the CMB anisotropies and cluster masses Best fit to data Flat, all matter Universe

10. The Composition of Our Universe • Compare expansion history of our Universe to the CMB anisotropies and cluster masses • In addition to matter, our Universe contains a great deal of dark energy (WL~ 0.72) Best fit to data Flat, all matter Universe

11. What’s The Matter? • So ~30% of our Universe’s density is in the form of matter (mostly dark matter, as seen from galaxy rotation curves, clusters, etc.) • So what kind of matter is it? • First guess: Baryons (white dwarfs, brown dwarfs, neutron stars, jupiter-like planets, black holes, etc.)

12. BaryonAbundance • Big Bang nucleosynthesis combined with cosmic microwave background determine WBh2 0.024 • WB~ 0.05 • But, we also know WM~ 0.3, so most of the matter in the Universe is non-baryonic! Fields and Sarkar, 2004

13. Cold Dark Matter and Structure Formation • Observations of the large scale structure of our Universe can be compared to computer simulations • Simulation results depend primarily on whether the dark matter is hot (relativistic) or cold (non-relativistic) when structures were formed • Most of the Universe’s matter must be Cold Dark Matter

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

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

16. 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)

17. 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.

18. The Thermal Abundance of a WIMP • Stable particle, X, in thermal equilibrium in early Universe (freely created and annihilated, roughly as plentiful as ordinary types of matter) • As Universe cools, number density of X becomes Boltzman suppressed • But expansion of the Universe makes finding X’s to annihilate with difficult, suppressing the annihilation rate

19. The Thermal Abundance of a WIMP • Expansion leads to a thermal freeze-out of X particles • For a particle with weak scale interactions, freeze-out occurs at a temperature, T~MX/20 • With weak scale interactions, freeze out leads to a density of X particles of ~1

20. The Thermal Abundance of a WIMP • Expansion leads to a thermal freeze-out of X particles • For a particle with weak scale interactions, freeze-out occurs at a temperature, T~MX/20 • With weak scale interactions, freeze out leads to a density of X particles of ~1 Automatically generates observed relic density!!!

21. Supersymmetry • Elegant extension of the Standard Model • For each fermion in nature, a corresponding boson must also exist (and vice versa) • New spectrum of “superpartner” particles yet to be discovered

22. Why Supersymmetry? • Not introduced for dark matter

23. Why Supersymmetry? • Not introduced for dark matter • Higgs mass stability

24. Supersymmetry and the Mass of the Higgs Boson • Electroweak precision observables indicate the presence of a light Higgs boson (around ~100 GeV) • Large contributions to the Higgs mass come from particle loops: • Without SUSY,  ~ MGUT or ~ MPlanck ultra-heavy Higgs • With TeV scale SUSY, boson and fermion loops nearly cancel •  light Higgs

25. Why Supersymmetry? • Not introduced for dark matter • Higgs mass stability • Grand Unification

26. Supersymmetry and Grand Unification • If there is a Grand Unified Theory (GUT) in nature, then we expect the SM forces to become of equal strength at some high energy scale • In the Standard Model, couplings become similar, but not equal

27. Supersymmetry and Grand Unification • With Supersymmetry, the three forces can unify at a single scale

28. Supersymmetry and Dark Matter • For the proton to be sufficiently stable, R-parity must be conserved • Evenness or oddness of superpartners is conserved • Consequence: the Lightest Supersymmetric Particle (LSP) is stable, and a potentially viable dark matter candidate • The identity of the LSP depends on the mechanism of supersymmetry breaking

29. The Lightest Supersymmetric Particle • Dark matter candidates must be electrically neutral, not colored • Possibilities: photino Zino (neutral) higgsinos sneutrinos gravitino axino

30. The Lightest Supersymmetric Particle • Dark matter candidates must be electrically neutral, not colored • Possibilities: photino Zino (neutral) higgsinos sneutrinos gravitino axino Do not naturally generate the observed dark matter density

31. The Lightest Supersymmetric Particle • Dark matter candidates must be electrically neutral, not colored • Possibilities: photino Zino (neutral) higgsinos sneutrinos gravitino axino Ruled out by direct detection Do not naturally generate the observed dark matter density

32. The Lightest Supersymmetric Particle • Dark matter candidates must be electrically neutral, not colored • Possibilities: photino Zino (neutral) higgsinos sneutrinos gravitino axino Mix to form 4 neutralinos Ruled out by direct detection Do not naturally generate the observed dark matter density

33. How To Search For A WIMP • Direct Detection • Indirect Detection • Colliders

34. Direct Detection • Underground experiments hope to detect recoils of dark matter particles elastically scattering off of their detectors • Prospects depend on the WIMP’s elastic scattering cross section with nuclei • Leading experiments include CDMS (Minnesota), Edelweiss (France), and Zeplin (UK)

35. Direct Detection • Elastic scattering can occur through Higgs and squark exchange diagrams:     ~ q h,H q q q q SUSY Models • Cross section depends on numerous SUSY parameters: neutralino mass and composition, tan, squark masses and mixings, Higgs masses and mixings

36. Direct Detection • Current Status Zeplin, Edelweiss DAMA CDMS Supersymmetric Models

37. Direct Detection • Near-Future Prospects Zeplin, Edelweiss DAMA CDMS Supersymmetric Models CDMS, Edelweiss Projections

38. Direct Detection • Long-Term Prospects Zeplin, Edelweiss DAMA CDMS Supersymmetric Models Super-CDMS, Zeplin-Max

39. Indirect Detection • Attempt to observe annihilation products of dark matter annihilating in halo, or elsewhere • Prospects depend on both the characteristics of the dark matter particle and its distribution in the halo • Gamma-rays, neutrinos, positrons, anti-protons and anti-deuterons each provide a potentially viable channel for the detection of dark matter

40. 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

41. 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

42. 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

43. Indirect Detection: Positrons • Reduce systematics by studying the “positron fraction” • When plotted this way, HEAT experiment observes a significant excess

44. Indirect Detection: Positrons Supersymmetric (neutralino) origin of positron excess? -Spectrum generated by annihilating neutralinos can fit the HEAT data

45. Indirect Detection: Positrons Supersymmetric (neutralino) origin of positron excess? -Spectrum generated by annihilating neutralinos can fit the HEAT data -Normalization is another issue

46. 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)

47. 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 (???)

48. 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)

49. 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)

50. 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)