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

Loer/WIN '133

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

Loer/WIN '134

The WIMP landscape (late 2012)

2013 Sept 19

Growing interest in low-mass region

Loer/WIN '135

Have we been seeing signals for a long time already?

2013 Sept 19

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

Loer/WIN '136

CDMS ZIPs: Z-sensitive Ionization and Phonon Detectors

2013 Sept 19

~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

Loer/WIN '137

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

2013 Sept 19

Loer/WIN '138

Surface Event Rejection

Combination of yield and “timing” cuts rejects vast majority of background

10 m “dead layer” results in reduced ionization collection

2013 Sept 19

Loer/WIN '139

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

2013 Sept 19

Loer/WIN '1310

Silicon is more effective at low WIMP mass

100 GeV WIMP 10 GeV WIMP

2013 Sept 19

For lighter WIMPs, lighter nuclei win due to more efficient energy transfer

Loer/WIN '1311

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

2013 Sept 19

Loer/WIN '1312

CDMS-II Si: Three Events!

Surface Event DistributionNeutron Distribution

Surface Event DistributionNeutron Distribution

Candidate 1Candidate 2Candidate 3

2013 Sept 19

Loer/WIN '1313

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).

2013 Sept 19

Loer/WIN '1314

CDMS-II Si Confidence intervals

2013 Sept 19

Best fit is for 9.6 GeV WIMP with 1.9x10-41 cm2 WIMP-nucleon cross-section

Loer/WIN '1315

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



Loer/WIN '1316

MALBEK: 2 surface event rejection cuts

2013 Sept 19





Loer/WIN '1317

MALBEK

2013 Sept 19




Loer/WIN '1318

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+

2013 Sept 19

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

Loer/WIN '1319

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

Loer/WIN '1320

Surface events a thing of the past?

2013 Sept 19

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!

Loer/WIN '1321

CDMSlite low ionization threshold experiment

2013 Sept 19

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

Loer/WIN '1322

CDMSlite run 1 at Soudan

2013 Sept 19

• 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

Loer/WIN '1323

CDMSlite run 1 limits

2013 Sept 19

CDMS II SiCoGeNTDAMACRESST IIXENON100XENON10 (S2)CDMS II GeCDMS II Ge LTEDELWEISS IITEXONOCDEXPICASSOCRESST II LT

arXiv:1309.3259

Loer/WIN '1324

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

Loer/WIN '1325

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

Loer/WIN '1326

SuperCDMS collaboration

2013 Sept 19

Loer/WIN '1327

SNOMASS projection: next decade

2013 Sept 19

Loer/WIN '1328

Backup slides

2013 Sept 19

Loer/WIN '1329

CDMS-II Ge low threshold analysis

2013 Sept 19

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

Loer/WIN '1330

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

Loer/WIN '1331

CDMS-II Germanium results

2013 Sept 19

Observed 2 events with 0.8 ± 0.1(stat) ± 0.2(sys) expected background

PICO-2L

32 Loer/WIN '13 2013 Sept 19

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

33 Loer/WIN '13

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

2013 Sept 19

T5Z3

34 Loer/WIN '13

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

Loer/WIN '1335

WIMP Dark Matter Basics

2013 Sept 19

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

Loer/WIN '1336

Expected spin-independent detector response to WIMPs

2013 Sept 19

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

37 Loer/WIN '13 2013 Sept 19

Loer/WIN '1338

CDMS-II SiExposure vs. Recoil Energy

2013 Sept 19

Loer/WIN '1339

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

2013 Sept 19

Loer/WIN '1340

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%.

Loer/WIN '1341

Profile likelihood goodness of fit

2013 Sept 19

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%

Loer/WIN '1342

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

2013 Sept 19

Loer/WIN '1343

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

2013 Sept 19

Loer/WIN '1344

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

2013 Sept 19

Loer/WIN '1345

Candidate 2


T4Z3 12.29 keV 0.23 5.11 σ Sep 6, 2008


2013 Sept 19

Loer/WIN '1346

Candidate 3


T5Z3 8.20 keV 0.32 6.66 σ March 14, 2008


2013 Sept 19

CDMS Recent Results and Prospects

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