Ray Bunker (UCSB) – APS – April 17 th, 2005 CDMS SUF Run 21 Low-Mass WIMP Search Ray Bunker Jan 17 th -DOE UCSB Review

Ray Bunker (UCSB) – APS – April 17 th, 2005 Outline Motivation for a low-mass WIMP search S tanford U nderground F acility Run 21 Overview Analysis Strategy Backgrounds Summary & Expectations

Ray Bunker (UCSB) – APS – April 17 th, 2005 Bottino et al. – hep-ph/ Motivation for Low-Mass WIMPs Bottino et al. SUSY calculations make DAMA appear more plausible Relax GUT scale gaugino mass unification assumption LEP2 chargino mass bound does not give a neutralino mass lower limit cm 2 →

Ray Bunker (UCSB) – APS – April 17 th, 2005 WIMP Detection Rates ~ 100 Events/kg/year with 10 keV Ge Threshold ~ 10 Events/kg/year with 20 keV Si Threshold Consider a 100 GeV WIMP with a WIMP-nucleon cross section of cm 2 Now consider a 10 GeV WIMP with the same cross section Detection threshold is everything! Low-Mass = Low-Threshold ~ 85 Events/kg/year with 1 keV Ge Threshold ~ 22 Events/kg/year with 2 keV Si Threshold

Ray Bunker (UCSB) – APS – April 17 th, 2005 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 !

Ray Bunker (UCSB) – APS – April 17 th, keV Ga x-rays from 71 Ge Decays ~ 1 keV Charge Line New Background Source Shallow Site Neutron Background SUF Run 21 Germanium

Ray Bunker (UCSB) – APS – April 17 th, 2005 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 1 st 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days taken during the first half of live days with 3V charge bias – published, hep-ex/ live days with 6V charge bias – unpublished Goals Achieved: Confirmed CDMS I results – candidate events consistent with neutron background Quantified gamma & beta rejection Established the contamination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 ZIP 1 (Ge) ZIP 2 (Ge) ZIP 3 (Ge) ZIP 4 (Si) ZIP 5 (Ge) ZIP 6 (Si) SQUID cards FET cards 4 K 0.6 K 0.06 K 0.02 K … but can we learn more from these data?

Ray Bunker (UCSB) – APS – April 17 th, 2005 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 – Q inner cut Single detector interactions Consistent with being nuclear recoils Reasonable phonon pulse timing parameters Low-Threshold Analysis

Ray Bunker (UCSB) – APS – April 17 th, 2005 Beginning of Run 21 End of Run 21 Neutron Calibration (Activation!) The 10.4 & 1.3 keV Backgrounds Detector activation due to neutron calibration Thermal Neutron Activation Decays via EC n + 70 Ge → 71 Ge → 70 Ga (stable) 21% Natural Abundance 11.4 day half life 71 Ge typically decays via Electron Capture from the K shell – resulting in a 70 Ga nucleus with a hole in it’s 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 1.25 ± 0.12 keV1.298 keVCharge Energy ± Ratio of EC(L)/EC(K) 11.2 ± 0.4 days11.4 daysHalf Life MeasuredExpected

Ray Bunker (UCSB) – APS – April 17 th, 2005 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 The Z2 Near-Threshold Background

Ray Bunker (UCSB) – APS – April 17 th, 2005 Elevated phonon pre-pulse baseline during event bursts Phonon Pulses Cutting the Z2 Near-Threshold Excess Each phonon channel optimizes nicely in an S 2 /B fashion! Percentage of events with average baseline > mean + 2  provides an excellent discriminant against these event bursts

Ray Bunker (UCSB) – APS – April 17 th, 2005 Event Burst Cut Performance Raw Exposure (days): Before Cut After Cut 51.7 Candidate Event Count: Before Cut After Cut 3142 → 20.2 → 73 !!!

Ray Bunker (UCSB) – APS – April 17 th, 2005 R21 Silicon – The Zero-Charge Event Background Low yield band … likely leaking events into signal region. Removing should significantly clean up the Silicon data. Near the detector edges, some electric field lines terminate at the side of the detector rather than the bottom ground electrode

Ray Bunker (UCSB) – APS – April 17 th, 2005 Summary and Expectations Summary Low-Mass = Low-Threshold! Analysis of CDMS data can be pushed well below 10 keV ~ 1 keV for Ge ZIPs ~ 2 keV for Si ZIPs We see 1.3 keV Ga x-rays from 71 Ge 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 mass less than ~10 GeV ? Results expected this summer Run 119 Ge PRELIMINARY Run 21

Ray Bunker (UCSB) – APS – April 17 th, 2005

Nuclear Recoils Surface electrons  Z1 (  ) or Z5 (+) Run 21 3V Background Exposure – 28 kg-d after cuts Run 21 Combined 3V & 6V Background Exposure – 49 kg-d after cuts Edelweiss 32 kg-d Limit Published in PRL - hep-ex/ keV Analysis Threshold Review of Results to Date 10 keV Analysis Threshold Don Driscoll’s CDMS Thesis Result Edelweiss 32 kg-d Limit cm 2 →

Ray Bunker (UCSB) – APS – April 17 th, 2005 R21 Germanium – Recoil Spectra – 1 st Pass Z2: 141 3V singles V singles !? Z3: 114 3V singles 265 6V singles ? Z5: 148 3V singles 209 6V singles ?

Ray Bunker (UCSB) – APS – April 17 th, 2005 Event Bursts In Other Z? No event bursts for any Z in the 3V data EventTime histograms show no indications of event bursts for Z3, Z4, Z5 or Z6 in 6V data Worst Z2 SeriesNumber in 6V data