Searching for Dark Matter

1. Searching for Dark Matter Ron-Chou Hsieh CYCU

2. Outline • It seems to have something there! • What are they? • Link with physics beyond the SM. • DM searches. • Constructing a DM model • Summary

3. It seems to have something there! • Galactic rotation curves: • In 1932, Dutch astronomer Jan Oort • In 1933, Swiss astrophysicist Fritz Zwicky • In 1936, Sinclair Smith • In 1970, American astronomer Vera Rubin The stars move faster than expected!!! For

4. Gravitational lensing Spacetime around a massive object (such as a galaxy cluster or a black hole) is curved, and as a result light rays from a background source (such as a galaxy) propagating through spacetime are bent. Bending light around a massive object from a distant source. The orange arrows show the apparent position of the background source. The white arrows show the path of the light from the true position of the source Strong gravitational lensing as observed by the Hubble Space Telescope in Abell 1689 indicates the presence of dark matter

5. Collision of two clusters of galaxies (Bullet Cluster, 1E 0657-56)http://chandra.harvard.edu/photo/2006/1e0657/

6. Collision of two clusters of galaxies (Bullet Cluster, 1E 0657-56)http://chandra.harvard.edu/photo/2006/1e0657/

7. What are they? • Massive compact halo object (MACHOs) •  A MACHO is a body composed of normal baryonic matter, which emits little or no radiation and drifts through interstellar space unassociated with any planetary system.  • MACHOs may sometimes be black holes or neutron stars as well as brown dwarfs or unassociated planets. White dwarfs and very faint red dwarfs have also been proposed as candidate MACHOs.

8. The Wilkinson Microwave Anisotropy Probe (WMAP) is a NASA Explorer mission that launched June 2001 to make fundamental measurements of cosmology -- the study of the properties of our universe as a whole. • Wilkinson Microwave Anisotropy Probe (WMAP) Content of the Universe WMAP Three year data reveals that its contents include an estimate of 4% atoms, the building blocks of stars and planets. Dark matter comprises 22% of the universe. This matter, different from atoms, does not emit or absorb light. It has only been detected indirectly by its gravity. 74% of the universe, is composed of "dark energy", that acts as a sort of an anti-gravity. This energy, distinct from dark matter, is responsible for the present-day acceleration of the universal expansion.

10. Abundances of the Helium

12. Links with physics beyond the SM • Basic requirements of DM • Stability • Charge neutrality • Non negligible mass at least on timescales comparable to the age of the universe to avoid EM interactions and prevent DM to shine a lower limit on the mass of weakly interacting DM candidates to explain the formation of the smallest objects observed in the Universe, which is of a few keV

13. Free out-----DM particles which have been in thermal equilibrium with radiation at some stage of the cosmic evolution should subsequently decouple in two ways: decay and pair annihilation processes. • Free in-----DM particles are created through annihilation into DM particles, A A→DM DM, or through a decay process A→DM B. • Mechanisms to produce the DM relic density WIMP! • The observed DM relic density implies that: • The annihilation cross-section of DM particles should be comparable to the weak interaction cross section. • Fermionic DM particles should be heavier than a few GeV(Hut-Lee-Weinberg limit).

14. Weakly interacting massive particles (WIMPs) • Axions • SUSY particles • Extra Dimensions • The non-baryonic candidate zoo

15. Interaction only through the weak nuclear force and gravity, or at least with interaction cross-sections no higher than the weak scale; • Large mass compared to standard particles (WIMPs with sub-GeV masses may be considered to be light dark matter). • WIMPs

16. DM searches

18. Direct detection : elastic WIMP-atom scattering Three physical consequences of nuclear recoils are used to search for evidence of WIMP scattering. Ionization of target atoms Fluorescent radiation given off by electrons of target atoms Phonon excitations generated in crystals by the nuclear recoils

22. Indirect detection experiments search for the products of WIMP annihilation. If WIMPs are Majorana particles (the particle and antiparticle are the same) then two WIMPs colliding could annihilate to produce gamma rays or particle-antiparticle pairs. This could produce a significant number of gamma rays, antiprotons or positrons in the galactic halo. • Indirect detection

23. The EGRET gamma ray telescope observed more gamma rays than expected from the Milky Way, but scientists concluded that this was most likely due to an error in estimates of the telescope's sensitivity. The Fermi Gamma-ray Space Telescope, launched June 11, 2008, is searching for gamma ray events from dark matter annihilation. At higher energies, ground-based gamma-ray telescopes have set limits on the annihilation of dark matter in dwarf spherical galaxies and in clusters of galaxies. • The PAMELA experiment (launched 2006) has detected a larger number of positrons than expected. These extra positrons could be produced by dark matter annihilation, but may also come from pulsars. No excess of anti-protons has been observed. • A few of the WIMPs passing through the Sun or Earth may scatter off atoms and lose energy. This way a large population of WIMPs may accumulate at the center of these bodies, increasing the chance that two will collide and annihilate. This could produce a distinctive signal in the form of high energy neutrinos originating from the center of the Sun or Earth. It is generally considered that the detection of such a signal would be the strongest indirect proof of WIMP dark matter. High energy neutrino telescopes such as AMANDA, IceCube and ANTARES are searching for this signal. • WIMP annihilation from the Milky Way Galaxy as a whole may also be detected in the form of various annihilation products. The Galactic center is a particularly good place to look because the density of dark matter may be very high there.

24. Energy spectrum as detected by EGRET • Main sources of background are: • Decay of po mesons • Inelastic pp or p-He collisions • Inverse Compton scattering • Bremsstrahlung from electrons • Extra galactic background Diffuse gamma-ray spectrum as calculated with the GALPROP model.

25. Construction of the Dark Matter model • Basic requirements of DM • Stability • Charge neutrality • Non negligible mass at least on time scales comparable to the age of the universe to avoid EM interactions and prevent DM to shine a lower limit on the mass of weakly interacting DM candidates to explain the formation of the smallest objects observed in the Universe, which is of a few keV • The observed DM relic density implies that (in WIMP scenario): • Interaction only through the weak force and gravity, or at least with interaction cross-sections no higher than the weak scale. • Fermionic DM particles should be heavier than a few GeV(Hut-Lee-Weinberg limit).

26. Weakly interactive • Charge neutrality (Q=0) • Minimal DM model with

27. Elastic scattering of DM with nucleus We obtain Here c = 1 for fermionic DM and c = 4 for scalar DM Exceed observed data limit!

28. Must have However, for case , following contributions

29. We obtain Also exceed observed data limit! Must consider : Weak isospin singlet DM

30. Ansatz: • Annihilating into SM fermion pair • Renormalizable interaction • Fermionic DM SM Singlet DM SM Singlet Interacting with SM fermion through exotic vector boson

31. Minimally coupled exotic vector boson The Lagrangian describing the model is With For simplification, we only consider the vector interaction such that with

32. Thermal relic abundance The thermal relic abundance in WIMP scenario can be calculated by solving Boltzmann equation through following annihilation process Boltzmann equation: with Interaction rate per particle Numbers of particle

33. We assume

34. Elastic scattering with nucleus

35. Annihilation to muon pair

36. Summary • Astronomical observations and measurements indicate the existence of Dark Matter • Models in particle physics offer candidates for Dark Matter • We are searching for Dark Matter by • producing new particles at colliders • indirect detection of the products of WIMP annihilation • direct detection through elastic WIMP-nucleus scattering