Experimental dark matter searches

Weakly Interacting Massive Particles A WIMP c is like a massive neutrino: produced when T >> mc via annihilation through Z (+ other channels); annihilation/pair creation maintain thermal equilibrium If interaction rates high enough, the density drops as exp(- mc/T) as T drops below mc: annihilation continues, production becomes suppressed But, weakly interacting  may freeze out before total annihilation if > Gann~ nc ann v i.e., if annihilation too slow to keep up with Hubble expansion Leaves a relic abundance: c h2 10-27 cm3 s-1 ann v freeze out  if mc and ann determined by electroweak physics, then c~ 1

0 0 Detecting WIMPs  v/c  10-3 m nm Direct detection: WIMPs elastically scatter off nuclei nuclear recoils Measure recoil energy spectrum in target Indirect detection: WIMPs annihilate in halo: e+, p, g in Sun, Earth core: high energy n’s  v/c  10-3 m nm 0

Direct detection c c ~ H,h,Z q q WIMP flux s-wave scattering Ge Si WIMPs elastically scatter off nuclei in targets, producing nuclear recoils, with nc related to ann (same diagrams - via Z, h, H, and squarks) Energy spectrum of recoils is exponential with E ~ 50 keV, dependent on WIMP and target nucleus masses: Boltzmann distribution (isothermal halo) + s-wave scattering (NR) c H,h,Z q c q ~ WIMP flux Amplitude of recoil energy spectrum, i.e. event rate, normalized by nc, local WIMP number density, and nucleus-dependent A2F2 (Q) s-wave scattering I/Xe Ge Si Elastic Scattering Form Factors At low Q, scattering is coherent and ~A2. Coherence lost as Q increases; parameterized by form factor.

WIMP nucleus cross section In MSSM/CMSSM (neutralino): in general: s: 10-5 between and 10-11 pb sensitivity of current experiments: ~ 10-6 pb testing some models, will test more models in future as sensitivity improves Accelerator constraints shrink SUSY bounds: mainly constrained upper bound g-2 can provide constraint on lower bound if tentative disagreement due to SUSY 1 event kg-1 d-1 current experiments 1 event 100 kg-1 yr-1 detectors: low threshold low background large masses good event discrimination

WIMP signatures v0 Annual modulation: galactic center Sun 230 km/s WIMP Isothermal Halo (assume no co-rotation) v0~ 230 km/s WIMP wind June v0 galactic center Sun 230 km/s Dec. log dN/dErecoil Erecoil June Dec ~3% effect Earth 30 km/s (15 km/s in galactic plane)

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

WIMP signatures Diurnal modulation: vo a v0: solar motion Nuclear recoil The mean recoil direction rotates over one sidereal day The distribution of the angle a between the solar motion and recoil directions: peaks at a=180o

WIMP signatures WIMPS 40 GeV Background neutrons Material dependence: WIMPs: Ge has ~6x higher interaction rate per kg than Si neutrons: Si has ~2x higher interaction rate per kg than Ge WIMPS 40 GeV Background neutrons

Direct detection techniques CRESST ROSEBUD CUORICINO ER Phonons Ionization Scintillation CDMS EDELWEISS CRESST II ROSEBUD HDMS GENIUS IGEX MAJORANA DRIFT (TPC) DAMA ZEPLIN I UKDM NaI LIBRA XENON ZEPLIN II,III,IV Large spread of technologies: varies the systematic errors, important if positive signal! All techniques have equally aggressive projections for future performance But different methods for improving sensitivity

Where do we stand? ~ 1 event/kg/day DAMA 3s CDMS Most advanced experiments start to test the predicted SUSY parameter space One evidence for a positive WIMP signal Not confirmed by other experiments CDMS EDELWEISS ZEPLIN I Predictions: Ellis, Baltz & Gondolo, Mandic & all

The DAMA/LIBRA experiment At LNGS (3800 mwe) 9 x 9.7 kg low activity NaI crystals, each viewed by 2 PMs 2 methods of backgrd discr: PS; annual modulation -> positive signal (4 s) What next? update to LIBRA (250 kg) improved backround (~few) improved light yield Installation completed; analyze additional 3 yr of DAMA data (finished Jan 02) Day 1 = Jan 1, 1995

The CDMS II experiment gamma source neutron source neutrons gammas At SUF (16 mwe) /Soudan (2030 mwe) uses advanced athermal phonon (TES) measuring charge and phonons discrimination position resolution surface event rejection gamma source neutron source neutrons gammas electrons

The CDMS II experiment 1 tower of 4 Ge and 2 Si ZIPs operated at SUF 2001-2002; > 120 livedays > 99.98 % rejection of bulk electron recoils: 5-100 keV > 99 % rejection of surface events: 10-100 keV n background x 2.3 lower due to inner poly (as expctd); 20 Ge recoil single scatters, 2 Si single scatters, 2 triple scatter, 1 nnn double scatter; consistent with all single scatters caused by neutrons first results submitted to PRL, hep-ex/0306001 SQUID cards FET cards Si Ge Ge Si Muon anticoincident background

CDMS and DAMA assumptions of standard halo, standard WIMP interactions CDMS results incompatible with DAMA model-independent annual-modulation data (left) at > 99.8% CL even if all low-energy events were WIMPs predicted WIMP modulation predicted WIMP spectrum alone Best simultaneous fit to CDMS and DAMA predicts too little annual modulation in DAMA, too many events in CDMS (even for no neutron background) CDMS data neutron spectrum fit

The CDMS II experiment first 2 towers at the Soudan mine (2030mwe) m-flux reduced by 104, n-flux by ~ 300 first dark April 03! goal: 5 towers, 4 kg Ge, 1.5 kg Si 0.1 events/kg/keV/yr No SUSY gm-2 Baltz&Gondolo, PRL 86 (2001) 5004 SUSY gm-2 CMSSM Ellis et al. (2001) PRD 63, 065016 EDELWEISS CDMS 03 CDMS Soudan entrance to the mine

The EDELWEISS experiment In Frejus UL (4800 mwe); 320 g Ge crystals measure thermal phonons + charge EDELWEISS I: 1 kg stage fall 2000, first semester 2002, October 2002 - March 2003 total exposure: 13.8 kg  day @ Erec > 20 keV, 30.5 kg  day @ Erec > 30 keV Incompatibility with DAMA candidate (99.8% C.L.) confirmed with three different detectors and extended exposure G. Chardin 2003

The EDELWEISS experiment New run started: improved energy threshold ≈100% detection efficiency at 10 keV ER September 2003: end EDELWEISS-I run install EDELWEISS-II 21  320 g Ge-NTD detectors 7 thin film (NbSi) 200 g Ge detectors Achieve factor 100 improvement in sensitivity 100 l dilution cryostat for up to 120 detectors (36 kg Ge)

The ZEPLIN I experiment Operating at the Boulby mine (~3000 mwe) Single phase, scintillation in LiXe, PSD 3.7 kg liquid Xe (3.1 kg fid vol) 1 ton liquid scintillator veto 75 d livetime, 230 kg d of data Neutron source Gamma source 10-20keV Background: 40 dru @ 100keV implies 85Kr < 10-17 atoms/atom (standard Xe used) Fiducial Volume cut

The ZEPLIN experiment ER Xe+ Xe2+ Xe**+ Xe Xe2* Xe* 2Xe ZEPLIN II at RAL, UK ZEPLIN I ER Ionisation Excitation Xe+ +Xe Xe2+ +e- Xe**+ Xe Xe2* Xe* 2Xe 175nm Triplet 27ns Singlet 3ns Future ZEPLIN I: more data, low Kr Xenon ZEPLIN II/III: Ionization + scintillation, 2 phase Xe; 30 kg, 6kg high field II: tested at RAL, UK, PMs being produced to be installed at Boulby in 2003

The DRIFT experiment In the Boulby mine (3000 mwe) Cathode Electric Field Scattered WIMP Recoil Atom Drift direction MWPC Readout Plane CS2 Recoil Electron In the Boulby mine (3000 mwe) Resolve ionization tracks in a gas TPC filled with low-pressure EN gas (CS2) Endcap sense-planes: determination of range, orientation & energy (via ionization) e--capture by CS2 suppresses diffusion during charge-drift operates at ~40 torr , 140 g target mass discrimination through dE/dx measrmnt Future: DRIFT-II scaled-up DRIFT-I with full 3D readout & x50 sensitivity R&D: higher-resolution readout, higher-pressure operation cathode-readout of positive ions allowing event localization away from wire planes Gamma Region Overlap Region Neutron Region gamma region neutron region overlap

The ‘far’ future 1 event/100 kg yr: future projects! 1 event/kg d: EDELWEISS, CDMS, ZEPLIN 1 event/kg yr: CDMSII, CRESSTII, EDELWEISSII, ZEPLINII 1 event/100 kg yr: future projects! 1 ton is needed in order to detect 10 events per year at s = 10-46 cm2 Predictions: Bottino, Ellis, Gondolo

The ‘far’ future Project Discrimin Type Mass Location CryoArray Yes Ge/Si phonon/ioniz 1 ton NUSEL CRESST/ EDELWEISS Ge, CaWO4? phonon/ion/scint 100 kg - 1t Gran Sasso? Zeplin IV LiXe ioniz/scint 2 phase Boulby XENON (10 x 100 kg) DRIFT3 Yes + direction TPC (CS2) negative ion 100 kg GENIUS No Ge ionization in LiN 100 kg -1 ton Gran Sasso Majorana Ge ionization 500 kg