CDMS Run 21
Ray Bunker
CDMS Collaboration Meeting – Feb. 11th , 2005
CDMS Collaboration Meeting-Feb. 11th, 2005
Outline
• Run 21 Overview
• Review of R21 results to date
• Motivation for a low threshold analysis
• Analysis Strategy
• Backgrounds & Noise
• Plans and Expectations
CDMS Collaboration Meeting-Feb. 11th, 2005
Run 21 Overview
FET cards
SQUID cards
Run Description:
• Final background exposure at the shallow Stanford Underground Facility
• 17 mwe overburden (muon flux reduced by 5x)
• Relatively low gamma background
• 1st Tower of CDMS II ZIP technology detectors
• Four 250 gram Germanium (Z1, 2, 3 & 5)
• Two 100 gram Silicon (Z4 & Z6)
• 118 live days taken during the first half of 2002
• 66 live days with 3V charge bias – published, hep-ex/0306001
• 52 live days with 6V charge bias – unpublished
Goals Achieved:
4K
0.6 K
0.06 K
0.02 K
ZIP 1 (Ge)
ZIP 2 (Ge)
ZIP 3 (Ge)
ZIP 4 (Si)
ZIP 5 (Ge)
ZIP 6 (Si)
• Confirmed CDMS I results – candidate events consistent with neutron background
• Measured the muon anti-coincident flux at SUF simultaneously with Si & Ge ZIPs
• Tested and confirmed Monte Carlo predictions of neutron suppression rates due to addition
of internal polyethylene
• Quantified gamma & beta rejection
• Established the contamination levels of the detectors before deep site run
CDMS Collaboration Meeting-Feb. 11th, 2005
Review of Results to Date
3V Background
– 28 kg-d
after cuts
Run 21 Combined
3V &Exposure
6V Background
Exposure
– 49 kg-d after cuts
Surface electrons
 Z1 () or Z5 (+)
Nuclear Recoils
10-42 cm2 →
Edelweiss 32 kg-d Limit
Published
Don
Driscoll’s
in PRL
CDMS
- hep-ex/0306001
Thesis Result
10 keV
Analysis
Threshold
5 keV
Analysis
Threshold
CDMS Collaboration Meeting-Feb. 11th, 2005
Motivation for Low Mass WIMPs
Bottino et al. – hep-ph/0307303
New (2003) SUSY calculations make
DAMA appear more plausible
10-42 cm2 →
Relax GUT scale gaugino
mass unification assumption
LEP2 chargino mass bound
does not give a neutralino
mass lower limit
10-42 cm2 →
CDMS Collaboration Meeting-Feb. 11th, 2005
WIMP Detection Rates
• Consider a 100 GeV WIMP
with a WIMP-nucleon cross
section of 10-42 cm2
• Now consider a 10 GeV WIMP
with a WIMP-nucleon cross
section of 10-42 cm2
• Detection threshold is everything!!!
Silicon → 0
Germanium → 0
Xenon → 0
CDMS Collaboration Meeting-Feb. 11th, 2005
mc = 10 GeV
1.2
0.46
0.28
2.5
3.7
4.9
0.93
1.4
1.9
0.56
0.85
1.1
Visible Energy [keV]
6.1
2.3
1.4
Run 21 Recoil Thresholds
Germanium Detectors
Z2, Z3 & Z5
Very Low, Stable
Thresholds < 1 keV !
Silicon Detectors
Z4 & Z6
Low, Stable Thresholds
< 2 keV !
CDMS Collaboration Meeting-Feb. 11th, 2005
The Analysis
• Extend analysis threshold to include lowest possible recoil energies
• Understanding of backgrounds & noise environment critical!
• Search both Germanium & Silicon data for low mass WIMPs
• Use Germanium detector Z1 as veto only – threshold & backgrounds are too high to be sensitive to recoils
from low mass WIMPs
• Select events that pass:
• Data quality cuts – selects good pulse chi-squares, remove multiple pulses & data known to be of poor quality
• Muon veto anticoincidence cut – selects events with no veto activity 50 ms before trigger (larger window for
recoils less than ~ 3.5 keV)
• Qinner cut – selects events for which the Qouter charge signal is consistent with noise (no Qinner Threshold cut)
• Single scatters cut – selects events for which only 1 detector has phonon energy over its 6-s threshold
• Nuclear recoil cut – selects events that lie within the 2-s nuclear recoil band … discriminates against
electromagnetic backgrounds
• Timing Parameters Cut – reasonable phonon start, rise & fall times (not a surface rejection cut)
• We expect a large background event rate for recoils < 5 keV
• Near perfect discrimination against electromagnetic backgrounds breaks down … gamma leakage
• Event rate from measured neutron background (above 5 keV) should increase exponentially toward low recoil
energies
• Unexpected backgrounds? … additional data quality cuts?
• With all background sources identified we assume the candidate events are all signal and make
a conservative upper limit on the spin-independent WIMP-nucleon cross section
CDMS Collaboration Meeting-Feb. 11th, 2005
R21 Germanium – Recoil Spectra – 1st Pass
Z2: 141 3V singles
3142 6V singles !?
Z3: 114 3V singles
265 6V singles ?
Z5: 148 3V singles
209 6V singles ?
CDMS Collaboration Meeting-Feb. 11th, 2005
R21 Germanium – 6V y Plots - Backgrounds
Large Z2 Candidate Excess
Near Threshold – Noise Events?
~ 1 keV Charge Feature
New Background Source
CDMS Collaboration Meeting-Feb.
11th,
Shallow Site
Neutron Background
2005
10.4 keV x-rays from 71Ge Decays
The 1.3 keV Background
• Detector activation due to neutron calibration
Thermal Neutron Activation
n +
70Ge
→
21% Natural Abundance
CDMS Collaboration Meeting-Feb. 11th, 2005
71Ge
→
70Ga (stable)
11.4 day half life
• 71Ge typically decays via Electron Capture
from the K shell – resulting in a 70Ga nucleus
with a hole in it’s K shell
Neutron Calibration (Activation!)
Beginning
of Run 21
Decays via EC
• The hole radiates outward releasing 10.4 keV
in x-rays
• The decay may also proceed via EC from the
L shell – EC(L shell)/EC(K shell) ~ 0.12
Ga L shell binding energy ~ 1.3 keV
End of
Run 21
Expected
Measured
Half Life
11.4 days
11.2 ± 0.4 days
Ratio of
EC(L)/EC(K)
0.12
0.122 ± 0.009
Charge Energy
1.298 keV
1.25 ± 0.12 keV
The Z2 Near-Threshold Excess
• Large noise-like excess of
candidates near threshold
• Event rate vs. time sporadic
probably not a true physics
process
• Clear bursts of events throughout
these runs … many occur directly
after LED flashing times
CDMS Collaboration Meeting-Feb. 11th, 2005
Cutting the Near-Threshold Excess
Phonon Pulses
Elevated phonon pre-pulse
baseline during event bursts
Percentage of events with average
baseline > mean + 2-s provides an excellent
discriminant against these event bursts
CDMS Collaboration Meeting-Feb. 11th, 2005
CDMS Collaboration Meeting-Feb. 11th, 2005
Event Burst Cut Optimization
Background Sample I
Maximize S2/B:
[S LiveTime(~cEB)]
→
2
# of Events(~cEB & cBkgnd)
[S
LiveTime(0)
]
2
# of Events(cBkgnd)
Background Sample II
CDMS Collaboration Meeting-Feb. 11th, 2005
Event Burst Cut Performance
Raw Exposure (days):
Before Cut
51.7
After Cut
→
22.3
Candidate Event Count:
Before Cut
3142
After Cut
→
121 !!!
Taking correlations into account should result
in a greater exposure & similar discrimination
CDMS Collaboration Meeting-Feb. 11th, 2005
Event Bursts In Other Z?
Worst Z2 SeriesNumber in 6V data
EventTime histograms show no
indications of event bursts for
Z3, Z4, Z5 or Z6 in 6V data
No event bursts for
any Z in the 3V data
CDMS Collaboration Meeting-Feb. 11th, 2005
R21 Silicon – 6V y Plots
Low yield band … likely leaking events into signal region.
Fixing should significantly clean up the Silicon data.
CDMS Collaboration Meeting-Feb. 11th, 2005
Plans and Expectations
• Remaining analysis for low mass WIMP search
• Finalize event bursts cut for Z2 6V & cut Qouter-like events in Silicon data
• Recalculate exposure effienciences & recoil spectra
• Subtract 1.3 keV background & estimate gamma leakage into nuclear recoil band
• Compare remaining candidate events with predicted neutron background (MC)
• Calculate Limts
• Expectations
• Set NEW world best limits on the spin-independent WIMP-nucleon cross section for
WIMP mass less than ~10 GeV ???
• Publish Si & Ge results in PRL
• Present low threshold analysis at April APS meeting
• Finish thesis & graduate !!!
CDMS Collaboration Meeting-Feb. 11th, 2005