Searching For Dark Matter in the Universe:

1. Searching For Dark Matter in the Universe: Direct (indirect) methods for the detection of Weakly Interacting Massive Particles (WIMPs) PIC -2004 Nader Mirabolfathi University of California, Berkeley

2. Evidence of Dark Matter: At Galactic scales… m M halo bulge sun disk • Rotation curve of spiral galaxies imply the presence of dark matter • Expect v2 1/r • Velocity is measured using atomic lines from stars or the 21cm H line for the hydrogen clouds around the galaxy • If WIMPs are the halo, detect them on earth via scattering on nuclei in targets Bergstrom, Rept.Prog.Phys. 63 (2000) 793 E. Corbelli & P. Salucci astro-ph/9909252

3. Evidence of Dark Matter: At Cosmological scales… 2003 Many different approaches: • Cosmic Microwave Background • Clusters of Galaxies • Supernovae SN1a • Large-Scale structure formation ΩL • All agree that mattermakes up approx. 27 % of the Universe and… ... Big Bang Nucleosynthesis, CMB, and Structure Formation require that approx. 85% of the matter is Non Baryonic Cold Dark Matter Ωmatter

4. Standard Model of Cosmology • Many CDM candidates: • SUSY neutralinos • Axions • Gravitinos • Kaluza-Klein states • ...

5. Candidate: Weakly Interacting Massive Particles • WIMP  : produced when T >> m via annihilation through Z (+other channels). • If interaction rates high enough, comoving density drops as exp(- m/T) as T drops below m: • Annihilation continues • Production suppressed. Production = Annihilation (T≥mc) Production suppressed (T<mc) Freeze out ~exp(-m/T) • Freeze out when annihilation too slow to keep up with Hubble expansion • Leaves a relic abundance: • ch2 10-27cm3 s-1 annvfr • ForW c~0.3: • M ~ 10-1000 GeV • A ~ electroweak 100 1000 10 1 mc / T (time )

6. WIMP detector Measure recoil energy Direct Detection of WIMPs If WIMPs are the halo, detect them via elastic scattering on nuclei in targets (nuclear recoils) • Energy spectrum & ratedepend on target • nucleus masses and WIMP distribution in • Dark Matter Halo: • Standard assumptions: • Isothermal and spherical • Maxwell- Boltzmann velocity distribution • V0=230 km/s <V>= 270 km/s,  = 0.3 GeV / cm3 Energy spectrum of recoils ~falling exponential with <E> ~ 15 keV Rate(based onncand ) is of the order of a fraction of 1 event /kg/day Log(rate) Erecoil

7. Experimental Challenges WIMPs: Extremely small scattering rate, small energy of the recoiling nucleus, and subtle signatures… Requirements: • Low (keV) energy threshold • Large target mass • Suppression of backgrounds from radioactivity and cosmic rays (,,, neutrons) • Deep sites • Passive/active shielding • Discrimination of residual background • Use WIMPS signatures • WIMPs Signatures: • Nuclear recoils, not electron recoils • Absence of multiple scattering • Annual modulation • Directionality

8. WIMPS Detection Methods (strategies) Increase the mass of the absorber and keep the background as low as possible. But how to distinguish WIMPs? Cosmological signature for the WIMPs assuming standard halo model. Statistically remove the background. g,b,a • Electron recoil Sensitivity improves by 1/(MT)1/2 2)Discriminate WIMPs against dominant back ground (, , ). EVENT BY EVENT How? WIMPs are interacting with nucleons whereas , ,  interact with electrons. Increase the mass. 0 , neutrons  Nuclear recoil Sensitivity improves by 1/(MT)

9.          Current Direct Detection Experiments

10. DAMA-NaI Experiment

11. NaI NaI PMT PMT NaI NaI DAMA - 100 kg NaI Experimental Apparatus • Very elegant experimental setup - in place >1996 • Low Activity NaI scintillator9  9.7 kg NaIcrystals, each viewed by 2 PMTs • Located at Gran Sasso Underground Lab (3.8 kmwe) + Photon and Neutron shielding • Two modes of Background discrimination • Pulse shape • Annual modulation: ~2% modulation amplitude POSITIVE SIGNAL Copper Lead Polyethelene

12. Annual Modulation of Rate & Spectrum WIMP Isothermal Halo (assume no co-rotation) v0~ 230 km/s galactic center Sun 230 km/s Dec. v0 June Earth 30 km/s (15 km/s in galactic plane) Dec log dN/dErecoil June ~5% effect Erecoil

13. ±2% Dec June Dec June Annual Modulation • Not distinguish between WIMP signal and Background directly • From the amplitude of the modulation, we can calculate the underlying WIMP interaction rate WIMP Signal Background Dec June Dec June

14. Best FitDAMA NaI/1-4 mean over 2-6 keVee(22 – 66 keV recoil) DAMA 2000 paper Figure 2 DAMA 15,000 kg-day DAMA 215,000 kg-day DAMA 3 + 438,000 kg-day Best fit to Ann Mod data alone Modulation Amplitude • Best-fit WIMP model’s expected annual modulation does not appear to fit data; lowest point of 3s contour is much worse. • Why? Additional constraint applied during max likelihood analysis: DC WIMP signal implied by AC signal must not exceed observed DC count rate  best-fit cross-section is decreased • There is clearly a modulation (4s - compared to null hypothesis) MinimumDAMA NaI/1-4 (3s)

15. 3 more annual cycles acquired 58,000 + 49,800 = 107,800 kg-d 7 cycles total Improved DAQ Multiple scatters? LIBRA Large sodium Iodide Bulk for RAre processes 250 kg with improved radiopurity Taking data. Results have not been announced. Further R&D toward 1-ton NaI(Tl) radiopurification started DAMA → LIBRA

16. ZEPLIN ZonEd Proportional scintillation in LIquid Noble gasses Or Zoned Electroluminescence and Primary Light In Noble gasses Location: Boulby Mine UK: UKDMC

17. Why Xe? • Available in large quantities. • High atomic number (A=131) gives a high rate due to WIMP-NucleonA2 (if E is low). • High density (~ 3g/cm3 liquid). • High light (175 nm) and ionization yield. • Can be highly purified. • long light attenuation (m). • long free electron life time (~5ms). • Easy to scale up to large volume. • No long lived radioisotopes.

18. Principle Of Detection • Excitation • Production and Decay of excited Xe2* states: • 1)Through singlet (3ns) • 2)Through triplets (27 nS) • dE/dxdetermines the proportion of different channels=> • Nuclear more dense give more singlets or faster • Ionization • Ionized state Xe2+, recombine with e- => Xe2* =>Above relaxation • dE/dxdetermines the recombination time channels=> • Nuclear recoils (ps scale) electrons (40 nS) Nuclear recoils are faster

19. ZEPLIN I • Recombination allowed. • Only scintillation signal measured. • Discrimination is based on the pulse shape. • Discrimination is statistical.

20. ZEPLIN I Results Linear response 1.5 p.e/keV (E)=1.24E1/2 122 keV&136 keV 30 keV 90 keV

21. ZEPLINI Results (continued) • Fiducial mass =3.2 Kg • Mean event rate 2Hz. • Trigger three fold coincidences at 1pe. • 2keV threshold. • Light yield 1.5-2.5pe/keV. • Statistics 293 kg.day in Three runs. 2003 2004 2002 2003 (SUF) 2002 No in situ neutron calibration

22. LTD-9 2001 LTDs, phonon sensors and beyond! • Who? • CDMS (Cyogenic Dark Matter Search) • EDELWEISS (Expérience pour DEtecter Les WIMPS En Sites Sousterrain) • CRESST (Cryogenic Rare Event Search with Superconducting Thermometers) Low Temperature Detectors LTD-1 1987

23. Advantages: • Detecting the overall T  No position dependence. • Best resolution obtained with this kind of detectors:~100 eV at 5 MeV ? • Weak points: • CMass  Hard to increase the detector mass. • Unable to reconstruct the history of evts. • Advantages: • Could reconstruct the history of an event. • Thermometer collects constant fraction of phonons  independent of the absorber Mass. • Weak points: • Dispersion or position dependence of E. • Homogeneity of thermometers. LTDs, phonon sensors and beyond! Why? Advantages? • After an interaction (event), all the excitations transform to heat. •  Good resolution • Phonon excitation~10-6eV compare to ~1or few eV for conventional semiconductor detectors. •  Low threshold How to measure: Two methodes Temperature: Equilibrium Lattice excitations (phonons) T=E/C C(T/D)3  T could be big even with keV interaction. Using thermometers (Mott-Anderson or Superconducting thermometers) to measure T. Low T  Density of thermally excited phonons (noise) is very low. But we need to collect phonons before they reach the Equilibrium in the absorber. At low T Electron-phonon interaction is more effective than ph-ph interaction  evaporated thermometers (electron bath).

24. d4> d3> d2> d1 d4 d1 d2 d3

25. Comparison between the two types of signals B) Phonon measurement A) Temperature measurement Thermometer Thermometer To cold bath To cold bath Absorber Absorber T=E/Cfilm T=E/Ctotal T=E/Ctotal

26. Heat is not enough! Need another measurement to achieve event by event discrimination. Charge? • The amount of charge created in a Semiconductor after an event depends on the type of interaction: Quenching factor (Q). • Quenching factor for an electron recoil event (Most of the radioactive background) is bigger than for nuclear recoil events (WIMPs). • By simultaneously measuring the charge and heat, one can discriminate - event by event WIMPs from the background. This defines the principle of detectors for CDMS and EDELWEISS experiments. Scintillation? CRESST: The same principle but scintillation instead of charge.

27. Electron recoil Nuclear recoil Dead layer What is different between CDMS and EDELWEISS • Collection E field needs to be very low ~3Volts/cm. • Dead layer (~50 m) > than traditional SM detectors (~1 m). limits discrimination! • Most of the  bkgnd falls into DL region. • very important to deal with. Solutions Identify near surface events: 1) Using phonon signal. Only possible if athermal phonons measured. (CDMS current, EDELWEISS R&D) 2) Using charge signal rise time. Needs a large bandwidth electronics. (EDELWEISS R&D ) Avoid surface event by: 1) Carefully dealing with surface contamination. 2) Introducing a blocking layer against the charge back diffusion  Introducing an amorphous Si layer below charge electrodes.DecreaseDL to < 10 m

28. Use of Ge NTD thermistors : FET readout • The guard electrode ~50% volume

29. FET cards SQUID cards 4 K 0.6 K 0.06 K 0.02 K ZIP 1 (Si) ZIP 2 (Si) ZIP 3 (Ge) ZIP 4 (Si) ZIP 5 (Ge) ZIP 6 (Si) • CDMS Soudan first result with towerI • Tower I: 4 Ge (250 g) and 2 Si (100 g) • CDMS now running two towers 6 Ge and 6 Si • Si and Ge combination helps to better understand the neutron bkgnd. • Edelweiss 2002 1 Ge (320g) detectors No Si • Edelweiss 2003 3 Ge (320g)

30. Active Veto (reject events associated with cosmics) Hermetic, 2” thick plastic scintillator veto wrapped around shield Reject residual cosmic-ray induced events Information stored as time history before detector triggers Expect > 99.99% efficiency for all m, > 99% for interacting m MC indicates > 40% efficiency for m-induced showers from rock Shielding 30 cm parafin, 20cm Pb ,1 cm Cu No active veto Dilution fridge : 17mK base. Layered shielding (reduce g, b, neutrons) ~1 cm Cu walls of cold volume (cleanest material) Thin “mu-metal” magnetic shield (for SQUIDs) 10 cm polyethylene (further neutron moderation) 22.5 cm Pb, inner 5 cm is “ancient” (low in 210Pb) 40 cm polyethylene (main neutron moderator)

31. Electron recoil Charge spectrum Nuclear recoil Phonon Spectrum CDMS 2004 Results (Calibration  cuts) 4 Ge (850g) 2 Si (170 g) * 52 live days during 92 calendar days • Neutron calibration after the run and Systematically check for gamma (e-recoil) calibration. • Phonon position dependence removed. • Nuclear and electron recoil bands defined (+/- 2) • Phonon timing cuts defined with calibration data. • Guard charge electrode defined. • Veto coincident events defined (window 50 s).       • Selection criteria and nuclear recoil efficiency • Veto coincidence (50 us window) - 97% • Baseline stable (pileup, noise,…) - 95% • Nuclear Recoil band (2 sigma) - 95% • Phonon Timing cuts - 80% • Charge outer electrode cut - 75% • TOTAL - 53%

32. WIMP search data with Ge detectors • Exposure • 92 days (October 11, 2003 to January 11, 2004) • 52.6 live days • 20 kg-d net (after cuts) • Data: Yield vs Energy • Timing cut off • Timing cut on • Yellow points from neutron calibration Charge yield Recoil energy (keV)

33. WIMP search data with Ge detectors • Exposure • 92 days (October 11, 2003 to January 11, 2004) • 52.6 live days • 20 kg-d net (after cuts) • Data: Yield vs Energy • Timing cut off • Timing cut on • Yellow points from neutron calibration Charge yield Recoil energy (keV)

34. WIMP search data with Ge detectors • Exposure • 92 days (October 11, 2003 to January 11, 2004) • 52.6 live days • 20 kg-d net (after cuts) • Data: Yield vs Energy • Timing cut off • Timing cut on • Yellow points from neutron calibration No nuclear-recoil candidates Charge yield Recoil energy (keV)

35. Comparing Cross section-WIMP Mass plots

36. Future • The presented results are from one tower • CDMS II is now running two towers (6 *Ge (250 g) 6 *Si (100 g) • Background of the second tower is very similar to tower I. • Run stops mid July of this year • New three towers of detectors will be installed October this year • CDMS II ends by the end of 2005. • March 2004 end EDELWEISS I • Install EDEWEISS II with 21*320 g Ge NTD+ • Install 7*400 g NbxSi1-x athermal phonon detectors (Dead layer rejection) • The 100 liter dilution fridge has been successfully tested. Capacity for 120 detectors or 35 Kg Ge

37. CRESST : Scintillation/Heat instead of Charge/Heat Gran Sasso • Background discrimination by simultaneously measuring light/heat. • Uses a cryogenic detector (the same as phonon detector) for light measurement. • Works with different absorber materials: CaWO4 (mainly), PbWO4, BaF,..Advantage to change the absorber • Phonon channel:320 g CaWO4 (=40mm,h=40mm) , W-SPT (4*6 mm2). • Light channel:30*30*.4 mm3 W-SPT. • Reflector: Polymer foil, Teflon. Need 33 Modules to complete CRESST II goal

38. CRESST Sensitivity and rejection • High rejection: • 99.7% E > 15 keV • 99.9% E>20 keV • 9.7 kg.day data • Only half of the data analyzed. • Data without neutron shield. • Sensitivity limited by n.

39. Future direct detection experiments

40. DRIFT experiment Directional Recoil Identification From Tracks • Standard halo model for WIMPs in our galaxy suggests that the axis of recoils changes in the 24 hours (earth). • Axis of recoil is a cosmological signature for WIMPs. • Ionization track in a low pressure gas (CS2) depends on the type of interaction (Discrimination). • Multi wire proportional chamber ?

41. WIMPS e- C+,S+  Principle of DRIFT E Si Time of flightz

42. DRIFT setup • Low Prsure CS2 (40 Torr) 1 m3, 0.167 kg, 20 micron diameter wires 2 mm pitch. • 1 mm track for nuclear recoils • Many calibration runs with 55Fe (5.9 keV X-rays) • Neutron Calibration with 252Cf. • Polypropylene shielding (~ 50 cm). • Dark matter run started. • Energy threshold 15 keV. gammas C recoils S recoils

43. Discrimination in ZeplinII and III,IV,… Double phase Xe : Ionization Calibration of the prototype with gamma and neutron sources showed very good gamma/neutron discrimination (Cline et al. Astroparticle Phys. 12(2000) 373)

44. ZEPLIN I ~3Kg ZEPLIN II ~30kg ZEPLIN IV ~1000 kg ZEPLIN projected

45. Xenon: Perspective • Dual phase Xe experimnent • Light/Ionization • Very-low BG PMT • Prototype 1 cm drift • 10 kg prototype underway • 100 kg phase : 1 TPC • Modular: each module 100 kg • Self protected by outer Xe • 1 Ton scale • 99.5 % discrimination eff • 16 keV threshold Reach: ~10-46 cm2

46. WIMPS indirect detection experiments • AMANDA , ICECUBE (Southe po;e) • ANTARES • NESTOR • Superkamiokande, Hyperkamiokande •  -ray telescopes: CANGAROO, MAGIC, HESS • Satellite experiments: AMS-02, GLAST

47. WIMP indirect detection • WIMP elastic scattering. But in average it will lose energy: • V<Vescape accumulates in the center of large massive objects like the sun earth or galaxy. • Neutralino : Majorana particle  its own anti particle. • If massive annihilates. • Annihilation ;b,c,t quarks;gauge and Higgs Bosons • ,,,e+, p-. • Signature: • search for excess of up-going muons • Form direction from center of sun galaxy or Earth. • Search for annihilation lines (galactic center, cosmological…)

48. Neutrinos from the center of the earth, sun, galaxy. • Assumptions: • Dark matter in the galaxy due to  • Density~ 0.3 GeV/cm3 AMANDA, Super K…

49. AMANDA South Pole Dome road to work AMANDA Summer camp 1500 m Amundsen-Scott South Pole station 2000 m [not to scale]

50. AMANDA Optical Module PMT noise: ~1 kHz AMANDA-II 19 strings 677 OMs Trigger rate: 80 Hz Data years: 2000-