1 / 46

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

chuong
Download Presentation

CDMS Recent Results and Prospects

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related