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Dark Matter Phenomenology

Dark Matter Phenomenology. Jae Hyeok Chang 11/09/2017 GRAD TALK. Dark matter search is like a treasure hunt We know treasure exists But we don’t know where to find it If you find it, you will become rich However, there is a problem. A treasure map we have is not like this.

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Dark Matter Phenomenology

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  1. Dark Matter Phenomenology Jae Hyeok Chang 11/09/2017 GRAD TALK

  2. Dark matter search is like a treasure hunt • We know treasure exists • But we don’t know where to find it • If you find it, you will become rich • However, there is a problem...

  3. A treasure map we have is not like this

  4. We know very little about DM

  5. What can we do? • Before starting to look for treasure, we need to make sure the treasure map is genuine • Who made the map? / Was there actually a lot of treasure? / Did the treasure disappear? / Has someone already found it?

  6. Evidences of dark matter

  7. Galaxy Rotation Curves • Rotation velocity of a galaxy can be calculated from the virial theorem • If we know masses, we can predict rotation velocity

  8. Galaxy Rotation Curve Corbelli and Salucci, 2000

  9. Dark Matter Prediction • Fritz Zwicky predicted invisible matter in 1933 • He referred the invisible matter as “dark matter”

  10. MOdified Newtonian Dynamics • Some people suggested MOND to explain the galaxy rotation curves • Newtonian dynamics does not apply to very small acceleration (or large distance like galaxy) Milgrom, 1983

  11. Bullet Cluster • Evidence of “particle” dark matter

  12. A lot of Other Evidences

  13. OK, now we believe there is treasure • What’s the next step? • We need to figure out what the treasure map indicates • A place which has a mountain, a tree, and a lake…

  14. Dark matter candidate

  15. Very wide possible mass range • , wavelength of DM exceeds the size of dwarf galaxies • , DM mass exceeds the Planck mass, so DM must be composite

  16. A Few Hints for DM • Stable • Weakly interacting • Cold (non-relativistic) • Relic abundance

  17. A Few Hints for DM • Stable • Weakly interacting • Cold (non-relativistic) • Relic abundance

  18. Relic Abundance

  19. Relic Abundance • Simple and natural guess to get correct relic abundance is that DM was in thermal equilibrium with ordinary matters in the early Universe : Thermal DM • Thermal DM relic abundance is set by freeze-out Lee and Weinberg, 1977

  20. Thermal DM • Thermal DM mass range is • Thermal DM is constrained by BBN observation • Thermal DM predicts over population

  21. Thermal DM • Thermal DM mass range is easily accessible • Most models are discoverable • Easy relic abundance calculation

  22. Freeze-out

  23. DM SM • In the early Universe, • DM and SM particles are in thermal equilibrium • Both processes are active DM SM

  24. DM SM • The Universe cools to • SM→DM process is suppressed • DM population decreases DM SM

  25. DM SM • As DM population decreases, it’s hard to find each other • DM→SM process is suppressed as well DM SM

  26. DM SM • Both interactions finally stop • DM population is fixed in the comoving frame • Called “Freeze-out” DM SM

  27. In the early Universe, DM is in thermal equilibrium SM DM SM DM

  28. SM DM When T drops down below ,DM density exponentially drops SM DM

  29. DM cannot find each other and freeze-out SM DM SM DM

  30. SM DM Freeze-out temperature depends on • is the thermal averaged annihilation cross section • Increasing yields less relic DM density SM DM

  31. Freeze-Out • To get correct relic abundance, we need to have • This is model independent • The only condition for relic abundance if DM is thermal

  32. Freeze-Out • From dimensional analysis, • For • is the weak scale! • This weak scale DM candidate is calledWeakly Interacting Massive Particle • Called WIMP miracle

  33. WIMP • DM candidate with • Generally, • The most plausible DM candidate

  34. Back to the treasure map, we have a plausible place for treasure now • What should we do? • Dig up all the land near the place to find the treasure!

  35. Dark matter search

  36. Dark Matter Search DM SM • There are three ways to search DM DM SM

  37. Indirect Search DM SM • Looking for DM annihilation signals from the galactic center DM SM

  38. Collider Search DM SM • Produce DM pairs in high energy colliders DM SM

  39. Direct Detection DM SM • DM-SM elastic scattering • We are going to concentrate on this DM SM

  40. Direct Detection • Gather many nuclei • Block all incoming SM particles • Wait for DM to hit a nucleus • Detect recoil of nucleus Goodman and Witten, 1977

  41. Direct Detection Constraints Cushman et al, 2013

  42. Direct Detection Constraints Already dug region

  43. Direct Detection Constraints Planned region

  44. Direct Detection Constraints Region that cannot be dug

  45. Direct Detection Constraints DM flux decreases

  46. Direct Detection Constraints Nucleus recoil energy decreases

  47. Some Challenges for WIMP • No clear signal yet • Small scale problems • Core-cusp problem • Missing satellites problem • Too big to fail problem

  48. We have dug up most regions near the place we first guessed • No treasure came out… • Remaining regions are hard to dig • Time to look for another place

  49. Another dark matter candidate

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