Direct Detection of Dark Matter

1. Direct Detection of Dark Matter Ankur Deep Bordoloi, CristianGaidau

2. Outline • Cold Dark Matter (WIMP hypothesis) • Why? What? • Direct detection techniques • What to detect • Detector types • Cryogenic Dark Matter Detection • Experimental method • Results Indirect means: Indirect means

3. Motivation 1.) Galactic rotation curves Indirect means: Indirect means 2.)X-ray emission from inter-cluster medium

4. Why Cold Dark Matter? • Non-baryonic dark matter is classified as: • Hot Dark Matter (relativistic matter; e.g. neutrino) • Cold Dark Matter (non-relativistic matter; axion, neutralino) COLD DARK MATTER

5. Candidates • Astrophysical observations require the cold dark matter particle to be massive, ~ GeV. • As a result, neutrinos and axions are ruled out. • Potential cold dark matter candidates are provided by SUSY. The lightest SUSY particle is the superpartner of neutrino – the neutralino – with predicted invariant mass in the range :

6. WIMPs Relic density depends on annihilation cross section of the particle: For non-baryonic DM, At typical order of weak cross-section : 0.1 pb Any stable non-baryonic massive particle with weak interaction is a DM candidate

7. Experimental Techniques • Inelastic scattering • The WIMP will produce an excited nuclear or electronic state or ionize the atom • Background cosmic ray μ, υ, high energy e, p and n from radioactivity • Difficult to isolate WIMP from background • Elastic scattering • The WIMP exchanges energy with the nucleus as a whole, observable recoil. • Background consists of radioactive neutrons • Easier to isolate from background.

8. Elastic Scattering • Expected scattering rate Target cross-section Particle velocity WIMP particle density • Expected energy deposit Decays exponentially as E

10. What to look for? • WIMP signatures • A characteristic recoil spectrum • Uniformity in detector • Site independent WIMP parameters • Annular modulation in recoil rate & event spectrum

11. Background • The low energy regime is dominated by background event (gamma ray, radio-activity) • 3 classes of sensitivity: • Background free • Background noise is estimated as zero • Background subtraction • Pre-estimated background, later subtracted • Background limited • No idea about background, reduced sensitivity

12. Direct Detection Techniques • Direct detection techniques are based on 3 secondary effects from nuclear recoil: • Ionization (electrons) • Scintillation (photons) • Heat energy (phonons) • The more of these effects being detected, the more is the SNR.

13. Direct Detection Techniques • Solid State detectors (CDMS) • CRESST (employs CaWO4 crystals, only phonon detection) • DRIFT (directional information) • DAMA (NaIscintillator; no event by event discrimination) • KIMS (similar to DAMA, used CsI) • Xenon (noble liquid target)

14. DAMA/LIBRA • Basic technique: scintillation of thallium doped NaI. • Signal extracted from background by using the annular modulation of the dark matter flux. • In 2008 reported a positive dark matter signal, corresponding to a 60 GeV WIMP. • The only collaboration to report a positive signal.

15. DAMA/LIBRA • Annular modulation of counting rate on DM particle and earthbound target was monitored • Results found were claimed to be consistent with expected signal from standard halo model. • Not consistent with other experimental results

16. CDMS Electron recoil Ge, Si crystal : 7.6 cm φ, 1 cm thick 240 (100) g; 2 ionization channel 4 phonon sensors Nuclear recoil • Currently, the most sensitive DM detection experiment.

17. Operation Principle • A particle interacts with ZIP through e-recoil (Compton scattering) or n-recoil. • Interaction deposits energy in the crystal. • A portion of energy (6-33% depending on recoil/material) is converted into ionization then into phonons. • An electric field drifts away the charge carriers; collected at the bottom surface. • Phonons are collected at the top surface

18. Operation Principle (cont.) • The WIMP particle will interact with the nuclei. • Ordinary matter will mostly interact with the electron gas. • Thus, identifying the characteristics of an electron vs nucleon recoil provides a very powerful method of discriminating the background. • Exception: neutrons. This is the main source of backgrounds in CDMS. Energetic neutrons will produce nuclear recoils indistinguishable from WIMP recoils.

19. Sources of background noise • Radioactive contamination • U, Th, 40K decay in the cavern • Contamination from the detector & shield • Radon contamination • Cosmogenicbackgrond • High energy muons • Neutrino background

20. Reducing background • Background shielding • The ~700 m layer of dirt above the cavern reduce the muon flux by a factor of 10^4. • The outer layer veto shield for charged particles. This shield covers 99% of the detector’s total surface. • Followed by a polyethylene layer and lead to moderate neutrons. • Radioactive backgrounds are also suppressed by known gamma ray spectra of the radioactive sources • Radon reduction • All detectors and electronics are run under a purge

21. Background estimate • CDMS can discriminate nuclear recoil from electron recoil. • Measures the ratio, ionization to phonon energy (ionization yield) • A timing cut (based on phonon pulse time) is applied as a filter • A blind procedure (calibration based masking) was performed to avoid bias.

22. Search Results • WIMP search efficiency • Nuclear recoil efficiency vs. phonon recoil efficiency • Application of band cuts reduced the efficiency to 30% for Ge and 40% for Si detectors above 20 KeV phonon recoil energy.

23. Search Results • After unblinding the data No favorable event was found Before ublinding After ublinding

24. Search Results • Mass / cross section sensitivity Better sensitivity at higher mass Same minimum cross section as XENON10 • Soudan detectors have best sensitivity over wide mass range

25. Search Results • Two favorable events were found in 2009! Could not be interpreted as significant WIMP interaction given 23 % probability of it being from background fluctuation

26. Conclusion • CDMS is the most powerful detector in terms of sensitivity. • On 12.17.2009, the CDMS collaboration reported the detection of two events which met the WIMP criteria. However, because of such a small number of events, these could not be declared as true WIMP events. • Except for these two possible WIMP candidates detected by CDMS and the controversial results of DAMA, all of the DM experiments have reported null results on WIMP detection.

27. Future Experiments • SuperCDMS, a proposed successor of CDMS II. - more detector towers- improved design of the thermal sensor • This will increase the sensitivity by an order of magnitude. • Super-Kamiokande is proposed as an independent check of the DAMA results.

28. References • Bruch, Tobias (2010) Dissertation, A Search for Weakly Interacting Particles with the Cryogenic Dark Matter Search Experiment, University of Zurich. • Qiu, Xinjie (2009) Dissertation, Advanced Analysis and Background Techniques for Cryogenic Dark Matter Search, University of Minnesota. • Sumner, Timothy. J. (2002) Experimental Searches for Dark Matter, Living Reviews in Relativity, Vol. 5 • Spooner, Neil, Direct Search for Dark Matter • Cerdeno D. G, Green A M (2010), Direct Detection of WIMPs, arXiv 1002. 1912v1 • Longair, Malcolm, Galax Formation (1998) Springer