Transcript First Results from the Cryogenic Dark Matter Search at Soudan

First Results from the Cryogenic Dark
Matter Search at the Soudan
Underground Laboratory
Priscilla Cushman
University of Minnesota
ICHEP Beijing, China
August 16-22, 2004
Professor Priscilla Cushman
University of Minnesota
Composition of the Cosmos
WMAP best fit
WIMPs
ICHEP Beijing, China
August 16-22, 2004
Professor Priscilla Cushman
University of Minnesota
Moving from a Shallow Site at Stanford to a Deeper Site at
the Soudan Underground Mine in Northern Minnesota
Reduce neutron background from
~1 / kg / day to ~1 / kg / year
Reduce cosmic muon flux by ~ 30,000
Log10(Muon Flux) (m-2s-1)
Stanford Underground Site
Depth 713 m (2090 mwe)
500 Hz muons
in 4 m2 shield
1 per
minute
in 4 m2 shield
Depth (mwe)
ICHEP Beijing, China
August 16-22, 2004
Professor Priscilla Cushman
University of Minnesota
The CDMS
Collaboration
…in the mine
Brown University
M. Attisha, R.J. Gaitskell, J.P.
Thomson,
Case Western Reserve University
D.S. Akerib, M. Dragowski, S. Kamat,
R.W. Schnee, G.Wang
Fermi National Accelerator Laboratory
D. Bauer, M.B. Crisler, D. Holmgren, E.
Ramberg
Lawrence Berkeley National Laboratory
J.H. Emes, A. Smith
University of Florida
L. Baudis
Santa Clara University
ICHEP Beijing, China
B.A. Young
August 16-22, 2004
University of Minnesota, Minneapolis
L. Duong, P. Cushman, A. Reisetter
Stanford University
P.L. Brink, B. Cabrera,
C.Chang,R.W. Ogburn
University of California, Berkeley
V. Mandic, P. Meunier, N.
Mirobalfathi, B. Sadoulet, D. Seitz,
B. Serfass, K. Sundqujst
University of California, Santa
Barbara
R. Bunker, D. O. Caldwell, R.
Mahapatra, H. Nelson, J. Sander, S.
Yellin. Professor Priscilla Cushman
University ofUniversity
ColoradoofatMinnesota
Denver
The Soudan Underground Laboratory
Operated by the University of Minnesota,
in cooperation with Fermi National Accelerator Laboratory
and the Minnesota Department of Natural Resources
MINOS
CDMS II
Old: Soudan2 proton decay calorimeter
New: Screening and Prototyping Area
Applications welcome, see
http://www.hep.umn.edu/~prisca/soudan
ICHEP Beijing, China
August 16-22, 2004
Professor Priscilla Cushman
University of Minnesota
CDMS II Facilities
Electronics room
Clean
Room
Loft
Offices
Main floor
Detector
Prep
Clean
room
MINOS staging
DAQ /El ectroni cs
Main floor
Soudan
entrance
HVAC
Minos
Mezzanine
R F -shi el ded
Clean room
Mezzanine
Mezzanine
Shield
Mechani cal
Pumps,
Cryogenics
ICHEP Beijing, China
August 16-22, 2004
2
Mezzanine
F ridge
Icebox
Front-end
Electronics
Detector
Prep
Clean Benches
Professor Priscilla Cushman
University of Minnesota
CDMS Icebox and Shield
plastic
scintillators
Dilution
Fridge
polyethylene
lead
ancient
lead
inner
polyethylene
|------------------ 2.18 m --------------------------|
ICHEP Beijing, China
August 16-22, 2004
Professor Priscilla Cushman
University of Minnesota
ZIP Detectors and Tower Construction
250 g Ge or 100 g Si crystal
1 cm thick x 7.5 cm diameter
P honon D
SQUID array
R bias
R feedb ack
I bias
D
A
C
B
Q outer
Q inner
Vqbias
4 Phonon Channels: XY position
Z position using timing and signal shape
2 Charge Channels: radial position
ICHEP Beijing, China
August 16-22, 2004
Professor Priscilla Cushman
University of Minnesota
Detector Traces & Event Reconstruction
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Charge rise time is fast (~ 1 us) compared to the phonon rise time (~10-20 us)
Phonon pulse time of arrival allows for event position reconstruction
Event energy reconstructed using optimal filter (time => FFT => fit in freq)
Example of a typical 20 keV event in a Si & Ge ZIP
Si ZIP
Ge ZIP
(very good signal/noise for a 20 keV true recoil energy event)
ICHEP Beijing, China
August 16-22, 2004
Professor Priscilla Cushman
University of Minnesota
Discrimination of nuclear recoils to electron recoils
Calibration Data
Yield = Ionization Energy
Total Recoil Energy
Surface betas are electron recoils
with reduced charge collection
ICHEP Beijing, China
August 16-22, 2004
Professor Priscilla Cushman
University of Minnesota
Timing provides further discrimination
Use phonon risetime
n-recoil is slower than
e-recoil
And
Charge to phonon delay
smaller for surface e’s
neutrons
gammas
Ejectrons=e- products of incident radiation
ICHEP Beijing, China
August 16-22, 2004
Professor Priscilla Cushman
University of Minnesota
Source runs also provide energy calibration
Ge ZIP with 133Ba source
Ionization energy in keV
Phonon energy in keV
Excellent agreement between data and Monte Carlo
ICHEP Beijing, China
August 16-22, 2004
Professor Priscilla Cushman
University of Minnesota
Possible Sources of Beta Backgrounds
• Ejected betas from incident gammas.
Monte Carlo of detector response and risetime analyses:
50% of our “beta contamination”, but less than 3% of our beta background
1 ejectron per 25k incident gammas appears in the nuclear recoil band
• Radon contamination on the copper cans or the detectors themselves.
Alpha analyses (a:b ~ 1:1 for 210Pb)  30-60% of our background.
• K contamination introduced during fabrication & adventitious surface C
Ion beam characterization  20-30% of the beta background.
ICHEP Beijing, China
August 16-22, 2004
Professor Priscilla Cushman
University of Minnesota
Expected Backgrounds
Source
Expected # events
(WIMP search Ge data set)
Cosmogenic
punch-through neutrons
< 0.3 events (90% CL) given
no veto-coincident neutrons
Beta leakage
Cuts applied to extra Cal. sets
0.67 events (90% CL)
Beta leakage
Cal. distribution extrapolation
0.88 +/- 0.45
ICHEP Beijing, China
August 16-22, 2004
Professor Priscilla Cushman
University of Minnesota
Blind Analysis of WIMP Search Data
All detector evaluations and cut definitions were done with calibration runs.
Veto anti-coincident WIMP search data in the nuclear recoil region was blinded.
Same Tower 1 as run at Stanford: 52.6 live days raw, 19.4 kg-d of Ge after cuts
ICHEP Beijing, China
August 16-22, 2004
Professor Priscilla Cushman
University of Minnesota
Cuts applied to WIMP search data
(all detectors summed)
CUTS
Evts
Raw
Data Quality
Q and P thresh
Veto anti-coinc
Single scatters
< 100 keV recoil
Qinner electrode
Pileup cut
Timing cut
N-recoil band
968,680
807,419
199,338
194,088
87,596
13,947
8,845
8,240
1,249
1
ICHEP Beijing, China
August 16-22, 2004
Raw Wimp Search Data
Professor Priscilla Cushman
University of Minnesota
After applying Cuts:
Wimp search data for individual detectors
Saw no events in nuclear
recoil band in Blind Search
But after unblinding,
we found a software error:
Fit for saturated pulses had
been also applied to many
unsaturated events.
Ge
Ge
Ge
Si
Contaminated by
calibration source
Correction improves cut
efficiency, but Z5 event
now passes.
All plots shown are for
corrected data.
ICHEP Beijing, China
August 16-22, 2004
Ge
Si
seen in previous
shallow run
Professor Priscilla Cushman
University of Minnesota
Calculate limits
New Limit is
4 x better than Edelweiss
10 x better than CDMS I
Not consistent with WIMPs
being the DAMA annual
modulation signal
ICHEP Beijing, China
August 16-22, 2004
Professor Priscilla Cushman
University of Minnesota
CDMS II Future Plans
Current data run (Mar 25th to Aug 9th, 2004)
~65 live days with 2 towers of detectors (5 Ge ZIPs)
Charge thresholds lowered by ~20% (improved electronics grounding)
More 133Ba calibration data to improve beta systematic analyses
New analysis methods
Improved Z-position reconstruction
5-parameter phonon timing cut w/covariance matrix
Improve timing resolution of phonon leading edge by pulse fitting
Warm up Aug-Oct, 2004 to install 5 towers of detector
Cryocooler (already tested) to handle the added heat load
Run until Dec 2005 to achieve ~ x 20 increase in sensitivity
ICHEP Beijing, China
August 16-22, 2004
Professor Priscilla Cushman
University of Minnesota
Summary
• CDMS has successfully collected data at Soudan
Finished the analysis of that first data run
results submitted to PRL: astro-ph/0405033
• No WIMP signal was observed.
Current CDMS limits improve by a factor of ~4
(assuming standard DM halo and WIMP scalar interactions)
• No anticipated problems to achieving x 20 improvement
ICHEP Beijing, China
August 16-22, 2004
Professor Priscilla Cushman
University of Minnesota