CDMS Recent Results and Prospects
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Transcript of CDMS Recent Results and Prospects
CDMS Recent Results and Prospects
Ben Loer, Fermilab Center for Particle AstrophysicsOn behalf of the SuperCDMS Collaboration
Loer/WIN '132
Outline Some history The CDMS approach to dark matter The CDMS-II silicon analysis (April 2013) The low mass WIMP landscape The next generation iZIP and SuperCDMS
Soudan CDMSlite (Sept 2013) What’s next?
2013 Sept 19
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Dark Matter direct detection basics
2013 Sept 19
GeV – TeV WIMP mass Inefficient transfer to e-
Active discrimination against beta/gamma events common feature
Isotropic halo with Maxwellian velocity profile and escape velocity cutoff
Coupling to nuclei goes as A2 with nuclear form factor correction
Roughly exponential low energy nuclear recoil spectrum
Predict annual modulation from revolution about sun
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The WIMP landscape (late 2012)
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Growing interest in low-mass region
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Have we been seeing signals for a long time already?
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Starting in 1997, DAMA/LIBRA observes annual modulation with correct phase, now at >9
2010: CoGeNT observes low energy rate excess
2011: CoGeNT sees 2.8 annual modulation (updated last week!)
2011: CRESST sees excess in nuclear recoil band, disfavored as background at 4
CoGeNT
CRESST
Only CRESST has particle discrimination, and all have backgrounds
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CDMS ZIPs: Z-sensitive Ionization and Phonon Detectors
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~15%
• Phonons read out via Transition Edge Sensor
• Gives unique energy measurement
• Charge readout allows particle discrimination: NRs (WIMPS, neutrons) produce less ionization per unit edep than ERs (beta, gamma)
• Separate outer charge ring allows fiducialization to reject edge events in non-uniform field region
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Main Problem for CDMS-II ZIPs: Surface Events~10 μm“dead layer”
-3V
carrier back diffusion
Reduced ionization collection in surface events can mimic low yield of NR signals!
Can distinguish by phonon pulse shape
(timing)
surface event
nuclear recoil
rising edge slope
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Surface Event Rejection
Combination of yield and “timing” cuts rejects vast majority of background
10 m “dead layer” results in reduced ionization collection
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CDMS-II 19 Ge and 11 Si detectors in 5 “towers” Multiple runs since 2003 Early focus on Ge for high-mass
sensitivity; 3 published results
7.6 cm diameter 1.0 cm thick
Six detectorsstacked in each tower Five towers arranged in cold volume
4.75 kg of Ge, 1.1 kg of Si
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Silicon is more effective at low WIMP mass
100 GeV WIMP 10 GeV WIMP
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For lighter WIMPs, lighter nuclei win due to more efficient energy transfer
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CDMS-II SiBackground Estimate Neutrons
Indistinguishable from WIMPs!
Cosmogenic: active veto Radiogenic: passive
shielding & materials screening
<0.13 expected events Surface events
Discriminate using phonon timing
Optimize in 3 energy bins 0.47 expected events
estimated before unblinding.
Neutrons
Surface Events
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CDMS-II Si: Three Events!
Surface Event DistributionNeutron Distribution
Surface Event DistributionNeutron Distribution
Candidate 1Candidate 2Candidate 3
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0.7 expected eventsSurface + n + Pb
WIMP model
Surface LeakageNeutrons Pb recoils
Note: these are the Normalized
Distributions!
Tower 4, Detector 3
CDMS-II SiProfile Likelihood Analysis
Monte Carlo simulations of the background-only model give a p-value of a statistical fluctuation producing three or more events anywhere in our signal region of 5.4%.
A likelihood ratio test favors a WIMP+background hypothesis over the known background estimate as the source of our signal at the 99.81% confidence level (~3σ).
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CDMS-II Si Confidence intervals
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Best fit is for 9.6 GeV WIMP with 1.9x10-41 cm2 WIMP-nucleon cross-section
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CoGeNT updated annual modulation
2013 Sept 19
• “Persistent annual modulation exclusively at low energy and for bulk events. Best-fit phase consistent with DAMA/LIBRA (small offset may be meaningful). Similar best-fit parameters to 15 mo dataset, but with much better bulk/surface separation (~90% SA for~90% BR)”
• Unoptimized frequentist analysis yields ~2.2σ reference over null hypothesis.
• Modulation amplitude is 4-7 times larger than that predicted by the SHM.
https://conferences.lbl.gov/contributionDisplay.py?contribId=175&confId=36
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MALBEK: 2 surface event rejection cuts
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https://conferences.lbl.gov/contributionDisplay.py?contribId=64&confId=36
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MALBEK
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https://conferences.lbl.gov/contributionDisplay.py?contribId=64&confId=36
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SuperCDMS:Surface events a thing of the past?
iZIPs have > 30X better surface event rejection w/ 50% better efficiency to WIMPs!
Cross-section of electric field in iZIPe-
h+
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Bulk events collect charge on both iZIP faces
Surface events only detect charge on one face
iZIP: Interleaved phonon and charge sensors on both sides
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SuperCDMS Soudan9 kg of Ge arranged in 5 towers (15 iZIPs) Running now! (data collection since ~May
2012)iZIP
profileCDMSII ZIP
profile
1cm 2.5 cm (thickness of Ge crystals)
iZIP0.25 kg 0.6 kg per detector
2 iZIPs have 210Pb source to study surface event rejection WIMP Search analysis underway, first results due out this
Fall2013 Sept 19
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Surface events a thing of the past?
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arXiv:1305.2405
• 0 events leak into signal region out of 71,525 (38,178) electron and 16,258 (7,007) Pb-206 surface events on T3Z1 (T3Z3)
• Leakage into NR band < 1.7x10-5 at 90% CL with 50% nuclear recoil acceptance (8-115 keVr)
<0.6 expected leaked events in 4 years with 200 kg Ge at SNOLAB!
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CDMSlite low ionization threshold experiment
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Electrons/holes propagating in crystal reach “terminal velocity”
Excess energy from bias field transferred to lattice as Luke phonons
High field high “gain” charge measurement Can reach very low thresholds BUT: lose phonon
readout, more ERbackgrounds
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CDMSlite run 1 at Soudan
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• 6.3 kg-day exposure• No background
subtraction, only basic quality cuts
• ~14 eVee baseline resolution
• 840 eVnr (170 eVee) threshold
1.3 keVee neutron activation lineBest fit CoGeNT and CDMS-II Si
results
arXiv:1309.3259
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CDMSlite run 1 limits
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CDMS II SiCoGeNTDAMACRESST IIXENON100XENON10 (S2)CDMS II GeCDMS II Ge LTEDELWEISS IITEXONOCDEXPICASSOCRESST II LT
arXiv:1309.3259
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Coming up next: SuperCDMS SNOLABMove to North America’s deepest underground lab for >100X reduction in cosmogenic neutron backgrounds; deploy 200 kg of advanced Ge iZIPsNow seriously considering including silicon iZIPs
SNOLAB: Ladder Lab(and future home)
2013 Sept 19
10cm X 3.8cm SNOLAB prototype
iZIP
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Conclusions
2013 Sept 19
CDMS-II Si analysis observed 3 WIMP-candidate events with ~0.5 event expected background, best fit is to 8.6 GeV WIMP with 1.9E-41 cm2
Silicon best fit result in tension with XENON; many other experiments will probe some of the silicon ROI soon
SuperCDMS iZIPs have demonstrated significantly improved surface event rejection, sufficient for 4 background-free years at SNOLAB
CDMSlite sets best low mass WIMP limits < 6 GeV with only 6.3 kg-day exposure and no background subtraction; rules out significant portions of CDMS Si and CoGeNT regions of interest
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SuperCDMS collaboration
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SNOMASS projection: next decade
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Backup slides
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CDMS-II Ge low threshold analysis
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2011: Analysis of 8 best germanium ZIPs down to 2 keV shows no evidence of signal above expected background in 241 kg-days of exposure
DAMACDMS limit
CoGeNT
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CDMS-II Ge annual modulation analysis
2013 Sept 19
No annual modulation of low energy events observed over 2 year period in 8 best Ge ZIPs
CoGeNTCDMS
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CDMS-II Germanium results
2013 Sept 19
Observed 2 events with 0.8 ± 0.1(stat) ± 0.2(sys) expected background
PICO-2L
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Change the target in a COUPP-style bubble chamber to C3F8 to gain low mass sensitivity
Projected sensitivity shown is for 4 month run of 2.8 kg target
Deployment underway, expect to start taking data by October
T4Z3
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Below charge threshAbove charge thresh
Candidate event
Events shown pass all cuts except those indicated by the marker label
Peeking below the charge threshold, does not reveal a large background below. Keep in mind, the timing cut degrades below threshold so this alone doesn’t rule
out a missed background
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T5Z3
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Events shown pass all cuts except those indicated by the marker label
Below charge threshAbove charge thresh
Candidate event
Peeking below the charge threshold, does not reveal a large background below. Keep in mind, the timing cut degrades below threshold so this alone doesn’t rule
out a missed background
2013 Sept 19
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WIMP Dark Matter Basics
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WIMPs act like heavy neutrino with only neutral-current interactions
Standard assumptions: WIMPs are a non-interacting gas on average at
rest w.r.t. the galaxy Energies follow Maxwellian velocity distribution
with average velocity ~250 km/s with cutoff at galactic escape velocity
Local density ~0.3 GeV/cm3; per-particle mass is a free parameter. For 100 GeV wimp, flux would be ~105 /cm2/s
With large mass and low velocity, WIMPs are very non-relativistic. Interaction details don’t matter to first order: it’s billiard ball physics
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Expected spin-independent detector response to WIMPs
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WIMP-nucleon cross section and local WIMP density Coherent scattering factor. If fp=fn (isospin
symmetry), reduces to A2
Nuclear form factor, accounts for imperfect coherence at larger momentum transfer (i.e. smaller propagator wavelength) and larger nucleus
Velocity distribution function. vE term introduces seasonal modulation. Only upper tail of velocity distribution above vmin can cause recoil of energy ER
Dark Matter in CCD’s (DAMIC)
Aside from CDMS, the only other direct detection experiment with a Si target, but with significantly different technology!
published results w/ 40 eV “electron
equivalent” threshold
DAMIC(0.5 g mass)CoGeNT
XENON10
XENON100
CDMS II (Si central value)
Phys. Lett. B 711 (2012) 264-269
DAMIC100 will have 100g of target and could see O(100) events per year for 8.6 GeV/c2 WIMP and σ = 2x10-41cm2
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CDMS-II SiExposure vs. Recoil Energy
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Candidate 1Candidate 2Candidate 3
Post-Unblinding Checks After unblinding, the data quality
was re-checked. Events occurred during high-quality
data series Events were well-reconstructed Checked energy in other detectors
to verify events were single scatters
Surface event background fully estimated from the tails of three different NR sideband distributions 0.41 (-.08 +.20 stat.) (-.24 +.28
syst.) Checked for the possibility of
206Pb recoils from 210Po decay, and limited this to be <0.08 events.
0.41 events
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Si nuclear recoil energy scale
2013 Sept 19
Possible ~10% underestimation of Si nuclear recoil energy scale
Below 20 GeV/c2 the change is well approximated by shifting the limits parallel to the mass axis by ~7%. In addition, neutron calibration multiple scattering effects improve the response to WIMPs by shifting the upper limit down parallel to the cross-section axis by ~5%.
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Profile likelihood goodness of fit
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WIMP+background is preferred over background alone to high significance, but is it a good fit?
Goodness of fit for known-background-only hypothesis is 4.2%
Goodness of fit for WIMP+background hypothesis is 68.6%
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Event Details
Detector Recoil Energy Yield
Charge Signal to
NoiseDate
Event 1 T4Z3 9.51 keV 0.27 4.87 σ July 1, 2008
Event 2 T4Z3 12.29 keV 0.23 5.11 σ Sep 6, 2008
Event 3 T5Z3 8.20 keV 0.32 6.66 σ March 14, 2008
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Several years of CDMS II Data
raw exposure
Total raw exposure is 612 kg-days
this work
2008 result
some detectors not analyzed for WIMP
scattersperiods of poor data quality
removed
recorded data
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Candidate 1
Detector Recoil Energy Yield Charge Signal to Noise Date
T4Z3 9.51 keV 0.27 4.87 σ July 1, 2008
Raw Phonon Traces Raw Ionization Traces
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Candidate 2
Detector Recoil Energy Yield Charge Signal to Noise Date
T4Z3 12.29 keV 0.23 5.11 σ Sep 6, 2008
Raw Phonon Traces Raw Ionization Traces
2013 Sept 19
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Candidate 3
Detector Recoil Energy Yield Charge Signal to Noise Date
T5Z3 8.20 keV 0.32 6.66 σ March 14, 2008
Raw Phonon Traces Raw Ionization Traces
2013 Sept 19