1. The Dark Side of the Universe What is dark matter? Who cares?

2. What holds Galaxies together? What stops galaxy from flying apart?

3. Gravity holds stars, galaxies together Object 2 Object 1 Gravity pulls objects together Strength of gravitational pull depends on masses of object 1 and object 2

4. Fritz Zwicky 1932 computed the mass of the Coma cluster

5. Many pieces of evidence Mass of the Milky Way determined from the orbital velocities of globular clusters Distance Mass (billion solar masses) 20 kpc 200 50 kpc 440 50-100 kpc 500 100 kpc 1000 1 kpc = 1 kilo par sec = 3 x 10 19 m

6. Observe: Compute: Analysis required 400 times more mass than the mass of the stars in the galaxies

7. Much of this mass is hot gas Hot gas radiates X-rays

8. Kepler’s 3 rd Law: Measured velocities: Mercury 46 km/sec Earth 30 km/sec Jupiter 13 km/sec Neptune 5 km/sec

9. What Actually Happens To understand this, there must be mass that we don’t see

10. The Evidence Look at galaxies and measure their speed Estimate how heavy they are from what we see Look at nearby objects and see if there is enough mass to hold galaxy together –ESTIMATE IS WAY OFF…… –ASSUME THERE IS HEAVY MATTER THAT HOLDS UNIVERSE TOGETHER THAT WE DON’T SEE –We call this dark matter

11. The Picture Galaxies are surrounded by heavy dark matter which we don’t see: Galactic Halo

12. What is Dark Matter? Dark Matter Not Dark Matter Dark matter is matter which cannot be observed through the light or radio waves it emits

13. How do we know dark matter is really there? A large concentration of mass such as a cluster of galaxies will bend light –Prediction of general relativity It acts as a gravitational lens If there is a bright galaxy or a quasar behind a cluster, the bending measures the mass of the cluster

14. Gravitational Lens Orange lines show apparent source of light White line is actual light path

15. We are interested in the things we don’t see As we look into the universe, we see stars, galaxies, clusters…

16. Space Telescope Image Where the mass is

17. We live in special times We have a census of the universe Colors show distribution of temperature

18. Differences in WMAP picture result from gravitational clumping of matter

19. The Universe has an Energy Budget Crisis Stars and galaxies are only ~0.5% Neutrinos are ~0.3–10% Rest of ordinary matter (electrons and protons) are ~5% Dark Matter ~30% Dark Energy ~65%

20. The Cosmic Questions What is Dark Matter? What is Dark Energy? How much is the neutrino component? What is the evidence for dark matter and dark energy? How do we know there is missing mass? These questions connect particle physics and cosmology in an inescapable way

21. Searching for Dark Matter If dark matter particles are concentrated at the center of the galaxy, they might annihilate Maybe we can see the gamma rays We can also look for dark matter particles hitting the earth

22. Imagine that in the early universe, there were additional particles, including one that is NEUTRAL, HEAVY, AND ABSOLUTELY STABLE Created and annihilated in pairs We call this a WIMP (weakly interacting massive particle)

23. The Early Universe

24. The Universe Expands and Cools

25. Today The left-over density is density  m N 2 We get the observed density for m N 2  200 GeV

26. The density of WIMPS depends on how quickly they annihilate To get the right density today, the effective size is 1/1000 of the atomic nucleus WIMPs must be particles with weak interactions and masses 100-1000 times heavier than a proton.

27. Particle Dark Matter We’d like to detect dark matter in the lab –To show they’re in the galactic halo … And to produce them at an accelerator –To measure their properties … Why put detector underground?

28. If we saw something, how would we know what it was? We need to know the probability that a dark matter particle will hit another particle and interact

29. What is the Dark Matter? WIMPs are likely candidates: stable, electrically neutral, weakly interacting particles There are no WIMPs in the Standard Model of particle physics But most theories of unification have candidates –Lightest supersymmetric particle … –Lightest Kaluza-Klein particle in theories with extra dimensions Dark matter is evidence for new physics in the Standard Model of particle physics!

30. Supersymmetry Supersymmetry assumes that every particle has a partner particle We haven’t seen any of them The partner of the photon could be the dark matter

31. What kind of theory naturally gives the WIMPS needed to be dark matter? SUPERSYMMETRY Supersymmetry predicts many particles, not just WIMPS

32. Supersymmetric Theories Predict many new undiscovered particles (>29!) Very predictive models –Can calculate particle masses, interactions, everything you want in terms of a few parameters Any Supersymmetric particle eventually decays to the lightest supersymmetric particle (LSP) which is stable and neutral!!! Dark Matter Candidate

33. Supersymmetric explanation of Dark Matter can be tested! Supersymmetric models have natural dark matter candidate Lightest supersymmetric particle (LSP) is neutral and weakly interacting On general grounds, LSP contributes correct amount of dark matter if its mass is 300 GeV-1 TeV Supersymmetric particles within reach of LHC and a Linear Collider

34. Dark matter inescapably links particle physics and astrophysics A major goal of LHC and Linear Collider is to produce the LSP (supersymmetric dark matter candidate) and measure everything about it

35. Two kinds of particles seen in nature –Fermions: Matter is made of fermions (quarks and leptons) –Bosons: Particles which mediate forces –Is there a theory which treats these types of particles in an identical fashion? (we call this unification)

36. We say that fermions have spin ½ and bosons have spin 1 or 0. So a symmetry relating fermions and bosons would change spin by ½ This symmetry is supersymmetry

37. New particles in Supersymmetric Theory Spin ½ quarks  spin 0 squarks Spin ½ leptons  spin 0 sleptons Spin 1 gauge bosons  spin ½ gauginos Spin 0 Higgs  spin ½ Higgsino Experimentalists dream….many particles to search for! What mass scale? We haven’t seen any of these particles!

38. At the LHC Gluon –gluon collisions create pairs of new particles. These decay to lighter particles plus WIMPS An event might look like:

39. We can look for WIMPS at the LHC WIMPS produced at the LHC wouldn’t make a signal in the detector Particles produced with the WIMPS would interact however The rates are expected to be large

40. Supersymmetry Can we find it? Can we tell what it is? Masses of new particles depend on mechanism for breaking Supersymmetry Couplings of new particles predicted in terms of few parameters Simplest version has 105 new parameters

41. Do the forces unify? Coupling constants change with energy Coupling constants unify in supersymmetric models Hint for new physics?