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CDMS Recent Results and Prospects

CDMS Recent Results and Prospects. Ben Loer , Fermilab Center for Particle Astrophysics On behalf of the SuperCDMS Collaboration. Outline. Some history The CDMS approach to dark matter The CDMS-II silicon analysis (April 2013) The low mass WIMP landscape

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CDMS Recent Results and Prospects

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  1. CDMS Recent Results and Prospects Ben Loer, Fermilab Center for Particle Astrophysics On behalf of the SuperCDMS Collaboration

  2. 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? Loer/WIN '13

  3. Dark Matter direct detection basics • 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 '13

  4. The WIMP landscape (late 2012) Growing interest in low-mass region Loer/WIN '13

  5. Have we been seeing signals for a long time already? CoGeNT • 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 CRESST Only CRESST has particle discrimination, and all have backgrounds Loer/WIN '13

  6. CDMS ZIPs: Z-sensitive Ionization and Phonon Detectors • 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 ~15% Loer/WIN '13

  7. -3V ~10 μm “dead layer” carrier back diffusion rising edge slope surface event nuclear recoil Main Problem for CDMS-II ZIPs: Surface Events Reduced ionization collection in surface events can mimic low yield of NR signals! Can distinguish by phonon pulse shape (timing) Loer/WIN '13

  8. Surface Event Rejection 10 m “dead layer” results in reduced ionization collection Combination of yield and “timing” cuts rejects vast majority of background Loer/WIN '13

  9. 7.6 cm diameter 1.0 cm thick 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 Six detectors stacked in each tower Five towers arranged in cold volume • 4.75 kg of Ge, 1.1 kg of Si Loer/WIN '13

  10. Silicon is more effective at low WIMP mass 100 GeV WIMP 10 GeV WIMP For lighter WIMPs, lighter nuclei win due to more efficient energy transfer Loer/WIN '13

  11. 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 Loer/WIN '13

  12. Candidate 1 Candidate 2 Candidate 3 CDMS-II Si: Three Events! Surface Event Distribution Neutron Distribution Surface Event Distribution Neutron Distribution Loer/WIN '13

  13. Note: these are the Normalized Distributions! WIMP model Surface Leakage Neutrons Pb recoils CDMS-II SiProfile Likelihood Analysis Tower 4, Detector 3 • 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). 0.7 expected events Surface + n + Pb Loer/WIN '13

  14. CDMS-II Si Confidence intervals Best fit is for 9.6 GeV WIMP with 1.9x10-41 cm2 WIMP-nucleon cross-section Loer/WIN '13

  15. CoGeNT updated annual modulation https://conferences.lbl.gov/contributionDisplay.py?contribId=175&confId=36 • “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)” • Unoptimizedfrequentist analysis yields ~2.2σ reference over null hypothesis. • Modulation amplitude is 4-7 times larger than that predicted by the SHM. Loer/WIN '13

  16. MALBEK: 2 surface event rejection cuts https://conferences.lbl.gov/contributionDisplay.py?contribId=64&confId=36 Loer/WIN '13

  17. MALBEK https://conferences.lbl.gov/contributionDisplay.py?contribId=64&confId=36 Loer/WIN '13

  18. SuperCDMS:Surface events a thing of the past? iZIP: Interleaved phonon and charge sensors on both sides Bulk events collect charge on both iZIP faces e- Surface events only detect charge on one face Cross-section of electric field in iZIP h+ iZIPs have > 30X better surface event rejection w/ 50% better efficiency to WIMPs! Loer/WIN '13

  19. SuperCDMS Soudan 9kg of Ge arranged in 5 towers (15 iZIPs) Running now! (data collection since ~May 2012) iZIP profile CDMSII ZIP profile 1cm 2.5 cm (thickness of Ge crystals) 0.25 kg 0.6 kg per detector iZIP 2 iZIPshave 210Pb source to study surface event rejection WIMP Search analysis underway, first results due out this Fall Loer/WIN '13

  20. Surface events a thing of the past? 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 '13

  21. CDMSlitelow ionization threshold experiment • 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 phononreadout, more ERbackgrounds Loer/WIN '13

  22. CDMSlite run 1 at Soudan arXiv:1309.3259 • 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 line Best fit CoGeNT and CDMS-II Si results Loer/WIN '13

  23. CDMSlite run 1 limits arXiv:1309.3259 CDMS II Si CoGeNT DAMA CRESST II XENON100 XENON10 (S2) CDMS II Ge CDMS II Ge LT EDELWEISS II TEXONO CDEX PICASSO CRESST II LT Loer/WIN '13

  24. Coming up next: SuperCDMS SNOLAB Move to North America’s deepest underground lab for >100X reduction in cosmogenic neutron backgrounds; deploy 200 kg of advanced GeiZIPsNow seriously considering including silicon iZIPs SNOLAB: Ladder Lab (and future home) 10cm X 3.8cm SNOLAB prototype iZIP Loer/WIN '13

  25. Conclusions • 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 • SuperCDMSiZIPs 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 '13

  26. SuperCDMS collaboration Loer/WIN '13

  27. SNOMASS projection: next decade Loer/WIN '13

  28. Backup slides Loer/WIN '13

  29. CDMS-II Gelow threshold analysis • 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 CDMS limit DAMA CoGeNT Loer/WIN '13

  30. CDMS-II Ge annual modulation analysis • No annual modulation of low energy events observed over 2 year period in 8 best Ge ZIPs CoGeNT CDMS Loer/WIN '13

  31. CDMS-II Germanium results • Observed 2 events with 0.8 ± 0.1(stat)± 0.2(sys) expected background Loer/WIN '13

  32. PICO-2L 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 Loer/WIN '13

  33. T4Z3 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 Below charge thresh Above charge thresh Candidate event Events shown pass all cuts except those indicated by the marker label Loer/WIN '13

  34. T5Z3 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 Below charge thresh Above charge thresh Candidate event Events shown pass all cuts except those indicated by the marker label Loer/WIN '13

  35. WIMP Dark Matter Basics • 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 '13

  36. Expected spin-independent detector response to WIMPs • 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 Loer/WIN '13

  37. Dark Matter in CCD’s (DAMIC) published results w/ 40 eV “electron equivalent” threshold Phys. Lett. B 711 (2012) 264-269 DAMIC (0.5 g mass) CoGeNT XENON10 CDMS II (Si central value) Aside from CDMS, the only other direct detection experiment with a Si target, but with significantly different technology! XENON100 DAMIC100 will have 100g of target and could see O(100) events per year for 8.6 GeV/c2 WIMP and σ = 2x10-41cm2 Loer/WIN '13

  38. CDMS-II SiExposure vs. Recoil Energy Loer/WIN '13

  39. Candidate 1 Candidate 2 Candidate 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 Loer/WIN '13

  40. Si nuclear recoil energy scale • 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 '13

  41. Profile likelihood goodness of fit • 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 '13

  42. Event Details Loer/WIN '13

  43. Total raw exposure is 612 kg-days raw exposure this work 2008 result Several years of CDMS II Data recorded data some detectors not analyzed for WIMP scatters periods of poor data quality removed Loer/WIN '13

  44. Raw Phonon Traces Raw Ionization Traces Candidate 1 Loer/WIN '13

  45. Raw Phonon Traces Raw Ionization Traces Candidate 2 Loer/WIN '13

  46. Raw Phonon Traces Raw Ionization Traces Candidate 3 Loer/WIN '13

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