Results from the Full Analysis of 1998-1999 CDMS Data Runs

Richard SchneeCase Western Reserve University

2 DM2002 February 22, 2001 Richard Schnee - CDMS

CDMS Collaboration

Santa Clara UniversityB.A. Young

Stanford UniversityL. Baudis, P.L. Brink, B. Cabrera, C. Chang, T. Saab

University of California, Berkeley

M.S. Armel, S.R. Golwala, V. Mandic, P. Meunier, M. Perillo Isaac, W. Rau, B. Sadoulet, A.L. Spadafora

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 UniversityR.J. Gaitskell, J.-P. Thompson

Case Western Reserve UniversityD.S. Akerib, A. Bolozdynya, D. Driscoll,S. Kamat, T.A. Perera, R.W. Schnee, G.Wang

Fermi National Accelerator LaboratoryM.B. Crisler, R. Dixon, D. Holmgren

Lawrence Berkeley National Laboratory

R.J. McDonald, R.R. Ross, A. Smith

National Institute of Standards and Technology

J. Martinis

Princeton UniversityT. Shutt

4 DM2002 February 22, 2001 Richard Schnee - CDMS

•Detectors provide near-perfect event-by-event discrimination against otherwise dominant , very good (>95%) against surface electron-recoil backgrounds

electron-recoil backgroundsbulk electron-recoil backgrounds

616 Neutrons (external source)

1334 Photons (external source)

CDMS Background Discrimination

•Ionization Yield (ionization energy per unit recoil energy) depends strongly on type of recoil•Most background sources (photons, electrons, alphas) produce electron recoils

•WIMPs (and neutrons) produce nuclear recoils

Ionization Threshold

233 Electrons (tagged contamination)•Particles (electrons) that interact

in surface “dead layer” of detector result in reduced ionization yield

5 DM2002 February 22, 2001 Richard Schnee - CDMS

10.6 m earth

DetectorsInner Pb shieldPolyethylene

Pb Shield

Active Muon Veto

Fridge

Current CDMS Site: Stanford

•Expect neutron background ~2 / kg / day produced outside shield; measure using

Two materials (Si more sensitive to neutrons, Ge more sensitive to WIMPs)

Multiple-detector neutron scatters

Copper

•Shielded, low-background environment •Shallow (17 mwe rock)

Hadronic cosmic-ray flux reduced by >1000x

Muons reduced by ~5x

•Active muon veto >99.9% efficient Reject ~100 neutrons

per kg-day produced by muons within shield

nnn

..

6 DM2002 February 22, 2001 Richard Schnee - CDMS

CDMS Results• Results of 1998-1999 runs announced at UCLA DM2000

Published in Phys. Rev. Letters v.84, #25, pp.5702-6 (19 June, 2000)

• Thesis at http://cosmology.berkeley.edu/preprints/cdms/Dissertations/Sunil 1999 : 4x165 g Germanium BLIP 10.6 kg-days after cuts

• 13 single-scatter nuclear recoil events observed (WIMPs or neutrons)• 4 multiple-scatter nuclear recoil events observed (neutrons)

1998 : 100 g Silicon ZIP 1.6 kg-days after cuts • 4 nuclear recoil events observed (mostly neutrons)

Most sensitive upper limits on WIMP-nucleon cross-section

• Improved analysis of these runs recently completed Relaxation of fiducial volume cut 15.8 kg-days after cuts Better quantitative estimates of systematic errors

• More conservative treatment of data from 1998 Silicon ZIP Long paper on results will be submitted soon (PRD)

• Description of analysis, cuts, and calculation of efficiencies

7 DM2002 February 22, 2001 Richard Schnee - CDMS

Inner Ionization Electrode

Outer IonizationElectrode

Increasing the 1999 Run BLIP Fiducial Volume

•The less restrictive cut yields our “ultimate results” for the data set.

•Inner ionization electrode shielded from background events.

Top View

Region ofSharedEvents

•Events near inner-outer gap have ionization energy shared between the two electrodes

•Internal multiple scatters also appear as shared events

•Including “shared” events increases exposure

by ~40% for WIMPs by ~60% for neutrons.

8 DM2002 February 22, 2001 Richard Schnee - CDMS

1999 Run Ge BLIP Muon-Anticoincident Data Set

all single-scattersnuclear recoil candidates

NR Band (-3,+1.28) 90% efficient NR Band (-3,+1.28) 90% eff.

Inner-Electrode11.9 kg-days for WIMPs13 nuclear-recoil candidates >

10 keV

Shared-Electrode4.4 kg-days for WIMPs 10 nuclear-recoil candidates >

10 keV

9 DM2002 February 22, 2001 Richard Schnee - CDMS

Neutron Multiple Scatters• Require that at least one

hit be in fiducial volume

• Observe 4 neutron multiple scatters in 10-100 keV multiple events

• Calibration indicates negligible contamination by electron multiples ≈1 with one misidentified <0.05 with both

misidentified

neutronsIon

izati

on

Yie

ld [

keV

/keV

]

Ionization Yield [keV/keV]

surfaceelectrons

photons

nuclear-recoil candidate in both detectors nuclear-recoil candidate in one detector B4 / B5 B5 / B6 B4 / B6

Shared-electrode

B4

B3

B5

B6

Inner-electrode

low-yield hit in outer electrode

10 DM2002 February 22, 2001 Richard Schnee - CDMS

mostly neutrons

Not WIMPs: Si cross-section too low (~6x lower rate per kg than Ge)

Misidentified electrons?•Calibration predicts < 0.26 events in 20-100 keV range at 90% CL, but we cannot rule out systematic error due to fact that conditions of calibration and low-background data-taking were different

Using conservative assumptions about a calibration taken under same conditions as low-background data predicts contamination of 2.2 events in NR band (<7.3 events at 90% CL)

Use this very conservative estimate (7.3 events) in calculating limits

1998 Run Si ZIP Data Set

bulk events NR candidates

Early-design Si ZIP measured external neutron background

11 DM2002 February 22, 2001 Richard Schnee - CDMS

Consistency of Neutron Hypothesis

•4 Ge multiples and 4 Si singles imply expected background of 29 neutron singles in Ge, with large statistical uncertainty

xTotal

Shared-electrode

Inner-electrode

•Most likely neutron background predicts fewer inner-electrode Ge multiples than seen (1.7 vs. 4). Overall, data in good agreement.

•Likelihood ratio test: expect worse agreement 30% of the time

+ Data w/ 68% confidence intervalX Prediction based on Ge M, Si S

25

2.1

4.6

1.7

0.4

17

7.7

Predictions based on most likely

4

23

13

10

4

12 DM2002 February 22, 2001 Richard Schnee - CDMS

CDMS Upper Limits

•Most constraining upper limit of any experiment for WIMPs with 10-70 GeV mass

EDELWEISS better above 70 GeV

•Rules out DAMA NaI/1-4 most likely point (x) at >99.9% CL (for standard WIMPs, halo)•Rules out DAMA NaI/0-4 most likely point (circle) at >99% CL (for standard…)•Compatible with less likely points in DAMA 3 allowed regions

X marks DAMA NaI/1-4 most likely point

90% CL upper limits assuming standard halo, A2 scaling

DAMA NaI/1-4 3region

DAMA limit

EDEL

WEI

SS li

mit

Expected CDMS sensitivity

13 DM2002 February 22, 2001 Richard Schnee - CDMS

Compatibility of CDMS and DAMA

•Likelihood ratio test

asymptotic approximations

“standard” halo

standard WIMP interactions

•CDMS results incompatible with DAMA model-independent annual-modulation data (left) at > 99.99% CL

Best simultaneous fit to CDMS and DAMA predicts too little annual modulation in DAMA, too many events in CDMS (even for small neutron background)

predicted WIMP spectrum with n backgroundCDMS data

n background (1.1 multiples)

predicted WIMP

modulation

DAMA annual modulation data

14 DM2002 February 22, 2001 Richard Schnee - CDMS

Compatibility of CDMS and DAMA

DAMA annual modulation data

predicted WIMP

modulation

•Likelihood ratio test

asymptotic approximations

“standard” halo standard WIMP

interactions

•CDMS results incompatible with DAMA model-independent annual-modulation data (left) at > 99.8% CL, even under assumption that none of the CDMS events are neutrons

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

predicted WIMP spectrum

15 DM2002 February 22, 2001 Richard Schnee - CDMS

Additional Comments on Upper Limits

Inner & shared electrodesInner electrode

EDELWEIS

S

EDELWEISS

•Limits from inner-electrode data are near expected sensitivity (dots)

16 DM2002 February 22, 2001 Richard Schnee - CDMS

Additional Comments on Upper Limits•Limits from inner-electrode data are near expected sensitivity (dots)•New limits from inner-electrode data are worse than old CDMS limits (light blue) due to more conservative treatment of Si data

Inner & shared electrodesInner electrode

EDELWEIS

S

EDELWEISS

Old CDMS

17 DM2002 February 22, 2001 Richard Schnee - CDMS

Additional Comments on Upper Limits

QI QIS

Inner & shared electrodesInner electrode

EDELWEIS

S

EDELWEISS

Old CDMSIgnoring Si Ignoring Si

•Results slightly better if Si data is ignored (dashed red curves)

18 DM2002 February 22, 2001 Richard Schnee - CDMS

Additional Comments on Upper Limits•Results slightly better if Si data is ignored (dashed red curves)•Even without neutron subtraction (blue dash-dot curves), better limits than any other experiment for low-mass WIMPs (10-45 GeV)

Inner & shared electrodesInner electrode

EDELWEIS

S

EDELWEISS

CDMS no

neutro

ns

CDMS no

neutro

ns

Old CDMSIgnoring Si Ignoring Si

(See Yellin, soon-to-be astro-ph for method)

20 DM2002 February 22, 2001 Richard Schnee - CDMS

Conclusions

• No significant change from original conclusions New data from relaxing the fiducial volume cut are consistent with

our earlier results Ultra-conservative treatment of Si data has small impact on results Two years of scrutiny of the data resulted in only small changes See long paper to be submitted very soon for (extensive) details

• Background is dominated by neutrons at shallow site at Stanford

• Best upper limits of any experiment for WIMPs with 10-70 GeV mass

• Results incompatible with signal claimed by DAMA at high confidence level If signal is from scalar-coupled WIMP in a standard dark matter

halo

• We are focusing on CDMS II First complete tower of 6 ZIPs now running at Stanford

Underground Facility with internal neutron shield (factor >2) To be transported in Soudan in Summer 2002.