Results from the Full Analysis of 1998-1999 CDMS Data Runs Richard Schnee Case Western Reserve...
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Transcript of Results from the Full Analysis of 1998-1999 CDMS Data Runs Richard Schnee Case Western Reserve...
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 BarbaraD.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 ShieldActive 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
n nn
..
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
neutronsIoni
zatio
n Yi
eld
[keV
/keV
]
Ionization Yield [keV/keV]
surfaceelectronsphotons
nuclear-recoil candidate in both detectors nuclear-recoil candidate in one detector B4 / B5 B5 / B6 B4 / B6
Shared-electrode
B4B3
B5B6
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
x Total
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 S2
5
2.14.6
1.7
0.4
17
7.7
Predictions based on most likely4
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
WEISS l
imit
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
EDELW
EISS
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
EDELW
EISS
EDELWEISS
Old CDMS
17 DM2002 February 22, 2001 Richard Schnee - CDMS
Additional Comments on Upper Limits
QI QISInner & shared electrodesInner electrode
EDELW
EISS
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
EDELW
EISS
EDELWEISS
CDMS no n
eutron
s
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.