CDMS Run 21

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CDMS Run 21
Ray Bunker CDMS Collaboration Meeting Feb. 11th
, 2005
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Outline

• Run 21 Overview
• Review of R21 results to date
• Motivation for a low threshold analysis
• Analysis Strategy
• Backgrounds Noise
• Plans and Expectations

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Run 21 Overview

• 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• 52 live days with 6V charge bias unpublished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
• 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

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Review of Results to Date
Run 21 Combined 3V 6V Background Exposure 49
kg-d after cuts
10-42 cm2 ?
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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 ?
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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!!!

mc 10 GeV
Silicon ? 0 1.2 2.5
3.7 4.9 6.1 Germanium ?
0 0.46 0.93 1.4
1.9 2.3 Xenon ? 0
0.28 0.56 0.85 1.1
1.4 Visible Energy keV

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Run 21 Recoil Thresholds
Germanium Detectors Z2, Z3 Z5 Very Low, Stable
Thresholds lt 1 keV !
Silicon Detectors Z4 Z6 Low, Stable
Thresholds lt 2 keV !
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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

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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 ?
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R21 Germanium 6V y Plots - Backgrounds
Large Z2 Candidate Excess Near Threshold Noise
Events?
1 keV Charge Feature New Background Source
Shallow Site Neutron Background
10.4 keV x-rays from 71Ge Decays
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The 1.3 keV Background

• Detector activation due to neutron calibration

Thermal Neutron Activation Decays via EC
n 70Ge ? 71Ge ? 70Ga
(stable) 21 Natural Abundance 11.4 day
half life

• 71Ge typically decays via Electron Capture
• from the K shell resulting in a 70Ga nucleus
• with a hole in its K shell
• 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

Neutron Calibration (Activation!)
Beginning of Run 21
End of Run 21
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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

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Cutting the Near-Threshold Excess
Phonon Pulses
Elevated phonon pre-pulse baseline during event
bursts
Percentage of events with average baseline gt mean
2-s provides an excellent discriminant against
these event bursts
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Event Burst Cut Optimization
Maximize S2/B
?
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Event Burst Cut Performance
Raw Exposure (days) Before Cut
After Cut 51.7
? 22.3
Candidate Event Count Before Cut
After Cut 3142
? 121 !!!
Taking correlations into account should result
in a greater exposure similar discrimination
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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
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R21 Silicon 6V y Plots
Low yield band likely leaking events into
signal region. Fixing should significantly clean
up the Silicon data.
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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