Search for Cosmic Dark Matter at CDMS Laura Baudis Stanford University SLAC Topical Conference, August 16, 02

What is our Universe made of? We live in a very interesting Universe! matter 1/3 energy 2/3 ordinary matter : 5% cold dark matter : 30% cosmological constant : 65% apparently flat universe :  T =  M +   = 1

data from 26 experiments (including VSA and CBI) Evidence for a flat Universe large angular scales:  ~ 1/l Tegmark & Zaldarriaga 2002

CMB + IRAS PCSz power spectrum + Hubble param. prior h=0.7  dm = 0.24+/-0.06  b = 0.04+/-0.02 wang,tegmark,zaldarriaga,astro-ph/  m = /  b = / Galaxy clustering: the 2dF galaxy redshift survey > galaxies percival et al.,,astro-ph/ : likelihood surfaces for the best fit linear power spectrum The matter budget

Cold dark matter candidates axions strong CP, m~10 -5 eV WIMPs SUSY, m~ GeV were NOT invented to solve the dark matter problem! exotic: primordial black holes superheavy dark matter (WIMP/SIMPzillas)...

Weakly interacting massive particles long lived or stable particles left over from the BB coincidence or big hint??? actual abundance:  annihilation <  equilibrium abundance:  l l ~~

Searching for WIMPs WIMP searches fall into 3 main categories WIMP production at accelerators Indirect detection via :  ’s & cosmic rays due to WIMP annihilation in the halo ‘s from WIMP annihilation in the Solar/Earth cores Direct detection experiments Recoils due to WIMPs elastically scattering from nuclei 00 measure the energy deposited by the recoiling nucleus in a terrestrial, low background detector v/c  ERER 00

What do we need to know? Particle physics: a good candidate M W,  el Astrophysics: density of WIMPs in the halo    velocity distribution f(v)  Nuclear physics: nuclear form factor F(q) Detector physics: energy threshold E T, resolution ionization efficiency, discrimination,... Input from several fields required to estimate the event rates in a direct detection experiment :

Favourite WIMP candidate: neutralino if SUSY exists and R-parity (-1) 3(B-L)+2S is conserved => LSP is stable: potential DM candidate! in general: mixture of photino, zino and higgsinos  prediction of masses, scattering cross sections  elastic  nucleus cross section dominated by SI part

Study the low energy SUSY theories which arise from GUT, supergravity or string theories, reduce > 100 MSSM parameters to When masses and couplings fixed: calculate the WIMP-nucleus cross section (from  -quark cross section, QCD, nuclear physics...) Theorists survey a large set of models with masses and couplings within a plausible range; impose laboratory and relic density constraints plots of elastic scattering cross sections versus neutralino mass in general:  : and pb sensitivity of current experiments: ~ pb WIMP nucleus cross section

Local dark matter density measured rotation curve of Milky Way: flat out to 50 kpc v circ ~ 220 km/s; assumption: halo is spherical  0 ~ GeV/cm 3, v ~ 220 km/s, MB-distrib.  R 220 km/s   R

Differential rate for WIMP elastic scattering spin-independent interaction form factor velocity distribution

An example.... m  ~ 100 GeV,  0 = cm 2  0 = 0.4 GeV/cm 3, v  = 220 km/s rate of scattering from Ge (m N ~73 GeV) R < 1 event/kg keV d E R max = 2m r 2 v 2 /m N < 250 keV Detectors: low threshold low background large masses 100 GeV WIMP Ge

Comparing target nuclei Selected Target Nuclei NucleusA  Ionization/ scintillation Phonons Si Ge I Xe

Elastic scattering rates   ~ m N 4 favors heavier nuclei but - form factor suppression favors lighter nuclei integrated visible rate: threshold as low as possible!

The CDMS experiment

CDMS collaboration Case Western Reserve University D.S. Akerib,D. Driscoll, S. Kamat, T.A. Perera, R.W. Schnee, G.Wang Fermi National Accelerator Laboratory M.B. Crisler, R. Dixon, D. Holmgren Lawrence Berkeley National Lab R.J. McDonald, R.R. Ross A. Smith Nat’l Institute of Standards & Tech. J. Martinis Princeton University T. Shutt Santa Clara University B.A. Young Stanford University L. Baudis, P.L. Brink, B. Cabrera, C. Chang, T. Saab University of California, Berkeley S. Armel, V. Mandic, P. Meunier, W. Rau, B. Sadoulet University of California, Santa Barbara D.A. Bauer, R. Bunker, D.O. Caldwell, C. Maloney, H. Nelson, J. Sander, S. Yellin University of Colorado at Denver M. E. Huber Brown University R.J. Gaitskell, J.P. Thomson

Ultra-pure Si and Ge crystals: 1cm thick; 7.5cm diameter. measure phonons and ionization signals after an interaction discrimination between nuclear and electron recoils nuclear recoils: WIMPs, n electron recoils:  e CDMS detectors gamma source neutron source Ionization threshold

ZIP: advanced athermal phonon detectors 1 Z - dependent ionization and phonon detectors superconducting thin films of W/Al

Phonon signal Interaction creates THz (~ 4meV) phonons Phonons propagate to SC Al-fins on the surface, break Cooper pairs and create quasiparticles Quasiparticles diffuse in 10  s through the Al-fins and are trapped in the W transition-edge sensors (TES) where they release their binding energy to the W electrons The electron system T is raised increased R The TES is voltage biased and operated in the Electro-Thermal Feedback (ETF) mode,P J = V B 2 /R: when R increases, I decreases Current change is measured by SQUIDs Al Si or Ge W qp-trap Al Collector W TES qp diffusion R T Electro- Thermal Feedback

Ionization signal An interaction breaks up the electron-hole pairs in the crystal An electric field through the crystal separates the electrons and holes The charge is collected by electrodes on the surface of the crystal Two charge channels: Main electrode: a disk in the center of the crystal surface. Second electrode: a ring at the edge of the crystal surface. TES side Shared Event Fully contained Q in event V bias Readout electrons holes

Position Measurement Ionization signal is ~ instantaneous: measure of time Speed of sound in Si (Ge) crystal of ~ 1 (0.5) cm/  s results in measurable delays between the pulses of the 4 phonon channels => (x,y) position of the interaction (x= delay(A) - delay(D), y = delay(B) - delay(A)) A BC D many-hole collimated, large surface Cd-109 source

Particle identification Different types of interactions in crystal => different signatures in the phonon and charge signals gammas neutrons betas Risetime of the phonon pulses => information about the depth of the event position => discriminate against betas (surface events) surface bulk neutrons gammas electrons

How do we identify neutrons? 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 neutrons 1. use Si and Ge detectors: 2. look at multiple scattered events: measure the neutron background

Active Muon Veto Inner Pb shield Polyethylene Pb Shield Stanford Underground Facility 17 mwe of rock hadronic component: >1000 muon flux: ~5 Active Scintillator Muon Veto muon veto >99.9% efficient rejects “internal” neutrons produced by muons within shield Low-Background Environment 15 cm Pb shield:  flux  > cm PE:  -induced n-flux >100 radiopure cold volume (10 kg) 43 kg additional internal (ancient) lead shielding 11 kg internal PE shielding Current location: SUF detectors muon veto

Stanford Underground Facility CDMS operations since fall of 1996, Runs Data acquisition systemIcebox and shielding

SUF icebox and tower SQUET card Tower The Tower provides support for the ZIP detectors and their low T readout electronics spans various thermal stages in the fridge (4K --> 10mK) Phonon readout 600 mK Ionization readout 4K 6 ZIP detectors stacked vertically, 2 mm separation between each pair.

Current CDMS SUF: since July 01 First physics run with 6 ZIP tower and last SUF: > 100 livedays 6 new ZIP detectors (4x 250 g Ge, 2x 100 g Si) lower n background due to internal PE and better discrimination against  Goal: Improved physics results: 3x better WIMP sensitivity Performance test of Soudan Tower 1 (-> Soudan end 2002) Ge Si SQUID cards FET cards 4K 0.6 K 0.06 K 0.02 K

Baseline performance Observed noise spectra for all 4 phonon channels and both charge channels of one Si detector As expected, nV/  Hz As expected, 10 pA/  Hz Ionization signal  rise ~ 1  s,  fall = 40  s Resolution < 1 keV FWHM Phonon signal  rise ~ 10  s,  fall ~ 400  s Resolution ~ 400 eV FWHM Ge ZIP

Recoil Energy Threshold Sensitivities of direct detection searches depend strongly on E th Threshold energy depends, in turn, on the detector energy resolution ZIP detectors able to achieve 5 keV (recoil energy) energy threshold Ge ZIPs Si ZIPs

137 Cs calibration source provides a gamma line at 662 keV 71 Ga decay provides an x-ray line at keV in the Ge detectors Lines/features to calibrate the charge and phonon responses 137 Cs calibration source provides a gamma line at 662 keV 73m Ge provides a gamma line at 66.7 keV in the Ge detectors Calibrating the Energy Scale 73m Ge provides a gamma line at 66.7 keV in the Ge detectors 71 Ga decay provides an x-ray line at keV in the Ge detectors  = 10.4 keV  = 0.34 keV

2D Plots with gammas and neutrons Response of the detectors to calibration gamma and neutron sources Si ZIPGe ZIP

Y plots with gammas & neutrons Discrimination parameter Y (=E ionization /E recoil ) black lines: +/- 2  bounds of the electron/nuclear recoil band Si ZIPGe ZIP

Effectiveness of the discrimination parameter Histogram of yield parameter as a function of energy for gammas >99.99% of  ‘s rejected between keV 90% CL) > 99.8% rejection in 5-10 keV bin No gamma events leaked into the nuclear recoil band : CL limited by size of calibration data set Vertical black lines: +/- 2  bounds of the nuclear recoil band Ge ZIP

Background data Yield plots for background data from the current run (muon coincident!): gamma background band clearly visible muon coincident neutrons populate the nuclear recoil band Si ZIP Ge ZIP

Gamma background rates Gamma background rate ~ 1 evt/keV/kg/day in Ge and ~ 3 evt/keV/kg/day in Si With a discrimination ability of > 99.8% the gamma background is reduced to < 2 x evt/keV/kg/day Si ZIPGe ZIP

Current and projected limits Current CDMS SUF Limit (PRL 84, 2000) Projected CDMS SUF Limit Projected CDMS Soudan Limit Edelweiss 2002 CDMS Edelweiss 2002 already rule out DAMA!

Location: 2000 mwe muon flux suppressed by factor 10 4 neutron flux suppressed by ~ 300 expected sensitivity 1 year ~ 0.07 evt/kg/day final expected sensitivity ~ 0.01 evt/kg/day Identical Icebox: 7 towers each with 3 Ge & 3 Si ZIPs (M tot > 7kg) Soudan: end 2002

CDMS experimental enclosure

CDMS II icebox and shield

Goal of Soudan CDMS II: 0.01 events/kg/d, (MT) eff =230kg.d CRESST II CDMS II MSSM models DAMA 3  claim ‘g-2’ constraint New results with improved stats (hep-ex/ ) ~ 2.6  SM deviation: M  < GeV  < pb (Baltz&Gondolo, astro-ph/ )

WIMPs are excellent candidates for dark matter their discovery would have deep implications for cosmology and particle physics ! CDMS well suited to search for WIMPs 1999 results EDELWEISS results: incompatible with DAMA (independent of halo model, Copi & Krauss 2002) last started July01-July02, > 100 livedays (48 kg d) 6 ZIP tower, excellent background discrimination: final results soon! SUF limited by external n-background fridge is being commissioned at Soudan mine first tower in Soudan: late 2002 reach 100 times better sensitivity ~1 event / kg / year Conclusions

‘The constitution of the universe may be set in first place among all natural things that can be known. For coming before all others in grandeur by reason of its universal content, it must also stand above them all in nobility as their rule and standard.’ Galileo Galilei, Dialogue

more slides.....

CDMS Spectrum and DAMA Predictions Observed CDMS Nuclear Recoils versus expected WIMP Spectra WIMP spectra corrected for CDMS eff. function DAMA NaI/1-4 (3  ) DAMA NaI/1-4 (3  CL) contour lowest cross-section (40 GeV, cm 2 ) DAMA NaI/1-4 best-fit WIMP (52 GeV, cm 2 ) ~2 event/kgGe/day Expected WIMPs Detected Recoil Energy (keV)

Corrections Large Tc non-uniformity can distort the phonon signals Use position and risetime information to correct for this non-uniformity Z1 in Run2: 2-step correction: 1) First correct the phonon energy based on risetime - for good detectors, the yield (ratio of charge and phonon signals) is not correlated with the risetime (i.e. the blob is vertical) 2) Then correct the phonon energy based on event position Not corrected Corrected (s) Charge Energy (keV) Phonon energy (keV)Phonon Energy (keV)

Annual Modulation of Rate & Spectrum galactic center v0v0 Sun 230 km/s Earth 30 km/s (15 km/s in galactic plane) log dN/dErecoil Erecoil June Dec ~5% effect June Dec. WIMP Isothermal Halo (assume no co-rotation) v 0 ~ 230 km/s

ZIP Detector 380 mm Al fins 60  m wide Phonon Sensor Design Crystals: 7.6cm diam, 1cm thick. The films deposited are: Amorphous Si (40 nm) Al (300 nm on phonon side) W for traps (35 nm on phonon side) W, again, for TES There are 4 phonon sensors, each covering a quadrant of the crystal. Each sensor is split into 37 cell. Each TES is 1  m wide and 250  m long and it is connected to Al fins.

Ionization Readout The expected noise level after this amplification is nV/  Hz, implying that the lowest observable signal is ~1.5 keV. Ionization signal read-out: based on FETs An electric field is maintained through the crystal. Charge created in an interaction is effectively collected on the feedback capacitor and then dissipated in the feedback resistor. Fall-time: R F C F time constant (~40  s). Rise-time: determined by the amplifier (~1  s). The noise level of the readout system is given by: Dominant source of noise: the FET itself FET operated at 150 K to minimizes its noise (to ~0.5 nV/  Hz at ~50 kHz).

Phonon Readout Phonon read-out based on SQUIDs. Phonon sensor is voltage biased. Interaction changes sensor resistance, which changes current through input coil of the SQUID. Current signal in feedback coil: larger by the turns ratio (10) Pulse shape determined by the detector physics The noise level is given by: SQUID: operated at 600 mK, with responsivity r = 100  (RTFb). SQUID and amplifier noise contributions suppressed The dominant voltage noise source is the shunt resistor For typical R s = 100 m  at T = 35 mK, we expect the noise level to be at 10 pA/  Hz. R sh = 20 m  RsRs RpRp

Relevant Processes The phonon behavior is governed by two processes, both strongly dependent on frequency: Anharmonic decay with rate  D ~ f -5 Isotopic scattering with  I ~ f -4 These two processes define the “quasidiffusion” of phonons. An interaction creates phonons at Debye energy (13.4 THz for Si) - they are almost still and they decay VERY quickly (within a few us) to phonons of energy ~1THz, at which the mean free path is ~1cm (Si) - i.e. they are ballistic

Adjustment of W Tc for Optimization Target Tc W substrate 50 keV Fe implantation Tc measurements at Stanford in KelvinOx 15 AG limit 1st order RKKY B. A. Young, et al, JAP 86, 6975 (1999)

ZIP detector fabrication: CIS, Stanford Al/W Grid 60% Area Coverage mm Squares 888 X 1 µm tungsten TES in parallel Aluminum Collector Fins 8 Traps

Superconducting Transition Edge Sensors Steep Resistive Superconducting Transition Voltage bias is intrinsically stable T W T c ~ 70 mK 10-90% <1 mK unitless measure of transition width R