CDMS SUF Run 21 LowMass WIMP Search

1
CDMS SUF Run 21Low-Mass WIMP Search
Ray Bunker Sep 14th-DOE UCSB Site Visit
2
Outline

• Motivation for a low-mass WIMP search
• Stanford Underground Facility Run 21 Overview
• Analysis Strategy Backgrounds
• 1.3 keV x-rays
• Event Bursts
• Zero-Charge Events
• Results WIMP Candidates Exclusion Limits

3
Motivation for Low-Mass WIMPs
Bottino et al. hep-ph/0307303
Bottino et al. 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 ?
4
WIMP Detection Rates

• Consider a 100 GeV WIMP
• with a WIMP-nucleon cross
• section of 10-42 cm2

100 Events/kg/year with 10 keV Ge Threshold
85 Events/kg/year with 1 keV Ge Threshold

• Now consider a 10 GeV WIMP
• with the same cross section

22 Events/kg/year with 2 keV Si Threshold
10 Events/kg/year with 20 keV Si Threshold

• Detection threshold is everything!
• Low-Mass Low-Threshold

5
CDMS Run 21 Recoil Thresholds
Germanium Detectors Z2, Z3 Z5 Very Low, Stable
Thresholds 1 keV !
Silicon Detectors Z4 Z6 Low, Stable
Thresholds 2 keV !
6
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• 52 live days with 6V charge bias unpublished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
• Quantified gamma beta rejection
• Established the contamination levels of the
  detectors before deep site run

but can we learn more from these data?
7
Low-Threshold 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 top detector Z1 (Ge) as veto only
• Select candidates based on the following
  criteria
• Good data quality
• Anticoincident in time with activity in the muon
  veto
• Within the fiducial volume Qinner cut
• Single detector interactions
• Consistent with being nuclear recoils

8
The 10.4 1.3 keV Backgrounds

• 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

9
The Z2 Near-Threshold Background

• Large noise-like excess of
• candidates near threshold
• Event rate vs. time is sporadic
• probably not a true physics
• process
• Clear bursts of events throughout
• the run many occur directly
• after LED flashing times

10
Cutting the Z2 Near-Threshold Excess
Phonon Pulses
Elevated phonon pre-pulse baseline during event
bursts
Percentage of events with average baseline gt mean
2s provides an excellent discriminant against
these event bursts
Each phonon channel optimizes nicely in an S2/B
fashion!
11
Event Burst Cut Performance
Raw Exposure (days) Before Cut
After Cut 51.7
? 20.2
Candidate Event Count Before Cut
After Cut 3142
? 73 !!!
12
R21 Silicon The Zero-Charge Event Background
Near the detector edges, some electric field
lines terminate at the side of the detector
rather than the bottom ground electrode
Ionization Yield Threshold
Low yield band likely leaking events into
signal region. Removing should significantly
clean up the Silicon data.
13
WIMP Candidates

• Example Si y-plot Candidate Events in Red
• Z4 3V bias
• 66.12 live days
• 1 keV recoil energy threshold
• 22 candidates
• g Leakage
• Shallow site neutron background
• Misidentified surface scatters
• Residual zero-charge event background

• Example Ge y-plot Candidate Events in Red
• Z5 6V bias
• 51.66 live days
• ½ keV recoil energy threshold
• 146 candidates
• g Leakage (mostly 1.3 keV)
• Shallow site neutron background
• Misidentified surface scatters
• Residual zero-charge event background

• Combined Ge Signal Events
• Z2, Z3 Z5
• 322 live days
• ½ keV recoil energy threshold
• 594 Candidate events

• Combined Si Signal Events
• Z4 Z6
• 263 live days
• 1 keV recoil energy threshold
• 381 Candidate events

14
Summary and Expectations

• Summary
• Low-Mass Low-Threshold!
• Analysis of CDMS data can be pushed well below
  10 keV
• ½ keV for Ge ZIPs
• 1 keV for Si ZIPs
• We see 1.3 keV Ga x-rays from 71Ge decays !
• We have a good understanding of the
• near-threshold backgrounds.
• Expectations
• Set NEW world best limits on the
• spin-independent WIMP-nucleon
• cross section for WIMP masses less

Ge
Si
15
SUF Run 21 Germanium
1 keV Charge Line New Background Source
Shallow Site Neutron Background
10.4 keV Ga x-rays from 71Ge Decays
16
Ionization Yield Threshold Cut

• Set Minimum Ionization
• Yield Threshold (magenta curves)
• Choose threshold in S2/B
• fashion
• Use 60Co data zero-charge
• events for B estimate
• Use 252Cf data efficiency for
• S estimate
• This cut successfully optimized for
• all detectors and both charge biases!

17
Ge Exclusion Limit
Detectors Z2, Z3 Z5 Raw Exposure
322 Live Days Number of Candidates
594 Recoil Threshold ½ keV
18
Si WIMP Candidates
Detector Z4 3V Bias Z4 6V Bias Raw
Exposure 66.12 Live Days 51.66 Live
Days Number of Candidates 22 18 Recoil
Threshold 1 keV
19
Si WIMP Candidates
Detector Z6 3V Bias Z6 6V Bias Raw
Exposure 66.12 Live Days 51.66 Live
Days Number of Candidates 172 169 Recoil
Threshold 1 keV
20
Si Exclusion Limit
Detectors Z4 Z6 Raw Exposure
236 Live Days Number of Candidates
381 Recoil Threshold 1 keV
21
Review of Results to Date
Run 21 Combined 3V 6V Background Exposure 49
kg-d after cuts
10-42 cm2 ?