Direct Detection Lecture 2: Current Results

Transcription

1 Direct Detection Lecture 2: Current Results Outline I. Leading spin-independent measurements II. Leading spin-depending measurements III. Low Mass WIMP searches IV. Discussion on Calibration 43

2 Current and Future Searches WIPP DM-TPC Homestake LUX LZ Soudan *SuperCDMS CoGeNT SNOLAB DEAP MiniCLEAN PICASSO COUPP/PICO DAMIC SuperCDMS Boulby DRIFT Canfranc ANAIS ArDM Frejus/Modane EDELWEISS EURECA Gran Sasso DAMA/LIBRA XENON100 XENON1T/NT *DarkSide CRESST Sabre CJPL CDEX PANDAX Kamioka XMASS KamLAND-Pico Yangyang KIMS Amundsen-Scott DM-Ice 44

3 Why do we need so many experiments? (the most often asked question by funding agencies) Different targets and different detection technologies have different advantages in background rejection, WIMP scattering sensitivities and ease of construction or operation (as we will discuss). The nature of dark matter and its interactions with standard model particles is unknown. The variety of experiments allows us to maximize our exploration of possible candidates over a broad range of theoretical frameworks. Direct detection is hard! Requires exquisite background control and detailed understanding of instrumentation. Overlapping sensitivities allow experiments to cross-check each other. With a possible discovery around the corner and many reported hints, this is critical to definitively confirm reported signals. 45

4 Why do we need so many experiments? however moving forward with bigger and more sensitive detectors, in a restricted funding environment, its understood that we will need to pair down the current field for the next generation of dark matter experiments (often referred to as G2 ). In the U.S., the DOE and NSF are moving forward with this G2 downselect process as we speak (expecting to hear the final outcome any day now). As many direct detection experiments have strong U.S. involvement, this will have implications for the world-wide direct detection program. 46

5 Leading Spin-Independent Measurements 47

6 Spin-Independent Landscape SI 48

7 Dual Phase Xenon Detectors Leading the field in SI sensitivity and SD coupling to neutrons WIMP targets consist of large volumes of liquid Xe. Particle interactions produce primary scintillation light and ionization. Nuclear recoils produce less ionization compared to electron recoils Dual phase, TPC measures: S1: Primary scintillation via bottom array of PMTs S2: Electroluminescence from drifted electrons (ionization) via top array of PMTs Ratio of S1/S2 gives O(100):1 separation between ER and NR 49

8 What makes Xe TPC s such good WIMP detectors? BUT you might notice that O(100):1 discrimination is not that great. So why Xenon? Key Advantages: Large A 2 gives big coherent scattering boost and also has good SD sensitivity Easy to construct TPC s with large Xe mass (low cost per kg) Xe has no naturally radioactive isotopes and is high-z; allows for self-shielding Rejected electron recoils Approximate signal region neutron calibration mm position resolution allows for precise fiducialization Rejected background originates outside the target volume Example using 225 days of XENON100 data 50

9 Primary Challenges for Xe TPC s Xenon PURITY: Poor NR/ER discrimination means sensitivity depends critically on radiopurity of Xe and careful screening of detector materials. Purification also necessary to remove 85 Kr, Rn and other contaminants that can affect electron drifting. Rate events/kg/day/kev LUX/XENON100 achieve ER levels of ~1 events/kg/kevee/year in the interior of the target before applying discrimination. Thresholds: Large nucleus means coherence is lost at relatively low recoil energies. Low thresholds are needed to maintain good sensitivity. This is challenging given that typical Xe experiments can only detect O(few) photons per kev in the S1 channel! Si (A 28) Ge (A 72) Xe (A 131) V esc = 544 km/s σ = cm 2 51

10 XENON100 (225 live days) Operating in Gran Sasso Underground Lab since 2010 (in old Xenon10 spot) PRL (34) kg Xe active(fiducial) target, plus 99 kg Xe as active veto, optically separated ~4keV threshold, and rejection of ~99.75% of ER 2 events observed in blind analysis, consistent w/ background estimate, but not so consistent with background distributions; limit curve is result of a profile likelihood analysis favoring background only hypothesis XENON100 will continue to run but collaboration now focusing on building 1T detector 52

11 Large Underground Xenon (LUX) A very similar detector, but bigger and with a lower threshold and better light collection; running at Sanford Underground Research Facility (SURF) since late 2013 Rejected bulk electron recoils Approximate signal region 118 kg fiducial, 85.3 live days, 0 events observed (0.6 expected), limit is result of profile likelihood analysis Set s world s most stringent limit on SI WIMP-nucleon scattering Continuing to run with ~5X better sensitivity 53

12 Spin-Independent Landscape SI 54

13 Cryogenic Experiments Detectors consist of highly pure crystals (usually Ge or Si) arranged in towers CDMS II Tower 7.6 cm diameter 1.0 cm thick Simultaneous measurement of ionization or scintillation with phonons provides ER/NR separation Echarge nuclear recoils Ephonon Signature of a Nuclear Recoil: reduced ionization signal relative to phonon signal (or scintillation relative to phonon for CRESST) Operation at ~50 mk required for phonon detection, hence cryogenic nickname z r (phonon side) 4 phonon channels (each is 1036 TES sensors in parallel) (Example w/ CDMS) (charge side) inner sensor + outer guard 55

14 Why cryogenic detectors? Key Advantages: Like Xenon, target materials are intrinsically very pure and free of radioactive contaminants Exquisite energy resolution and very low energy thresholds are possible Ge is in the sweet spot of A 2 ; not so big as to suffer from lack of coherence Phased deployment is natural with reduced risk of unexpected background contamination Challenges: Cost per kg of raw material tends to be higher than other technologies (particularly liquid nobles) Detectors are fabricated in-house, almost as an artisan craft, production needs to be standardized for large-scaleup Fiducialization against surface events is key. Best technologies in this family can define a precise fiducial region. 56

15 Cryogenic Dark Matter Search Soudan Mine Z-sensitive Ionization and Phonon detectors CDMS II Five Towers (30 ZIPS) Operated kg Ge(A=73), 1.1 kg Si(A=28) 1 µ tungsten 380µ x 60µ aluminum fins Longtime leader in SI sensitivity due to superior background rejection (ER:NR is >10 6 :1 ) 57

16 The CDMS Strategy Ionization Yield Shown for Ge Most backgrounds (e, γ) produce electron recoils WIMPs and neutrons produce nuclear recoils Ionization yield (ionization energy per unit recoil energy) strongly depends on recoil type Bulk Gammas Surface Events Shown for Ge Nuclear Recoils Particles that interact in the surface dead layer result in reduced ionization yield These surface events can be rejected through a phonon pulse shape cut Ionization yield + timing result Signal Region in < 1 in 10 6 electron recoils leaking into the signal region 58

17 Final exposure for CDMS II All WIMP search data passing all cuts (except yield cut) 2 events near NR band Event 1: Tower 1, ZIP 5 (T1Z5) Sat. Oct. 27, :41 pm CDT Event 2: Tower 3, ZIP 4 (T3Z4) Sun. Aug. 5, :28 pm CDT 2-σ NR band 2 events in the signal region consistent with background expectations Moriond EW

18 EDELWEISS II The sister experiment to CDMS, operating in Modane Underground Lab. Main difference is use of NTD s to measure thermal (rather than athermal) phonons. EDELWEISS II array No fiducialization possible with phonon signal. Instead, novel interdigitated allows for rejection of surface events using the ionization signal. Closeup of one Ge detector 60

19 EDELWEISS II Final Results EDELWEISS II data from

20 Leading Spin-Dependent Measurements 62

21 Spin-Dependent Landscape Limit below is for proton coupling 63

22 Spin-Dependent Landscape Limit below is for neutron coupling 64

23 Superheated Liquid Detectors At low degrees of superheat, bubbles nucleated only by nuclear recoils (threshold detection) Chicagoland Observatory for Underground Particle Physics (COUPP) Target fluids contain F and I, which have good SD sensitivity. PICASSO Cost per kg of target is low, potentially making it cheap to scale. Better control over backgrounds may make this a competitor for SI detection Superheated droplet detector 4kg CF 3 I Bubble Chamber Latest results: Phys.Rev. D86 (2012)

24 COUPP as an example 66

25 COUPP as an example 67

26 Low mass WIMPs 68

27 Spin-Independent Landscape Next, we will focus on the low mass sector SI 69

28 Zoom on Low Mass Landscape DAMA/LIBRA CoGeNT 70

29 Unexplained Events 1998: DAMA/NaI reports annual modulation in event rate consistent w/ dark matter signal Bernabei et al., Eur Phys J C56 (2008) DAMA/LIBRA 2008: DAMA/LIBRA confirms annual modulation with high statistical significance (8.9σ) 2010/11: CoGeNT reports an overall excess of low-energy events, and an annual modulation albeit with only ~2σ significance 2012: CRESST-II reports a 4.2σ excess of low-energy events CRESST II arxiv: Phys. Rev. Lett. 107 (2011)

30 CRESST Looks for WIMP scatters off of Ca, W and O nuclei CaWO 4 crystals detect scintillation light and phonons, providing capability to identify scatters off specific nuclei Eur. Phys. J. C (2012) 72:1971 Analysis of data taken in 2011 reveals excess nuclear recoils at >4σ significance, which are consistent with WIMPs Favored masses and cross sections are in tension with other experiments; now trying to reduce Rn-daughter contaminants in detector housing components (clamps) 67 events observed, ~20 are excess over background Fermilab W&C, March

31 Recent updates to earlier hint: CoGeNT Phys. Rev. Le (2011) In 2010, CoGeNT using PPC Ge to push ionization thresholds down to <0.5 kev; reported an excess of low-energy events with spectrum consistent with a ~10 GeV/c 2 WIMP In 2011, reports a modulation of events in the kevee region with ~2σ significance, corresponding to a large fractional modulation counts/30 days In 2014, Analysis of 3.4 years of data shows persistent ~2σ modulation in low-energy region, arxiv: ; Alternative maximum likelihood analysis qualitatively supports earlier analysis, but with less significant excess seen at low energies, arxiv:1401:6234. Primary analysis Max- Likelihood analysis Fermilab W&C, March

32 Hint or background? CDMS II Silicon Search Recoil threshold PRL 111 (2013) Recoil threshold 3 candidate events observed in signal region (0.7 expected) Likelihood analysis incorpora1ng energy of events yields ~3σ significance 74

33 Why should we care about these hints? Masses < 10 GeV/c 2 are not naturally preferred by many theoretical frameworks motivated by the WIMP miracle. However. Many models predict dark matter outside of the vanilla WIMP paradigm. Fine tuning of parameters is often necessary, even if it s undesirable Expanding beyond CMSSM (even SUSY) opens up a lot of parameter space: pmssm, NMSSM, Asymmetric, Isospin Violating, Inelastic, (insert your favorite model here), We should not ignore the data. Several experiments are reporting excess events. Could these be the first indications of a major discovery? Several other experiments, done with different targets, are in tension with a dark matter interpretation Even if the experiments are only seeing backgrounds, its worth gathering enough data to definitively rule out these anomalous observations! Fermilab W&C, March

34 SuperCDMS Soudan izip Looking for SI elaskc sca+ering of relic WIMPs off Ge interleaved Z- sensi1ve Ioniza1on & Phonon detector 3- D fiducializa1on from measurement of z- symmetric ioniza1on or phonon response and outer guard channels 15 Germanium detectors 0.6 kg each Opera1onal since March of 2012 Data taken from Mar 2012 present; The two analyses presented here use a subset of the full dataset APL 103, (2013) 76

35 Search for Low Mass WIMPs with izips Using background rejection (3-D fiducialization) capability of new SuperCDMS detectors DAMA/LIBRA ~577 kg-days using 7 izips with lowest threshold; E th ~1.6 kev r SCDMS izip CoGeNT Gray bands: propagated systematic unc. from fiducial volume + nuclear recoil energy scale + trigger efficiency Expected sensitivity (prior to unblinding) Fermilab W&C, March 2014 Blind analysis using a BDT for final event selection 11 event seen; in line with expectations (6 events) except on one detector with malfunctioning channel Strongly disfavors dark matter interpretation of CoGeNT and also disfavors vanilla WIMP interpretation of CDMS II Si 77

36 (Ultra) Low Ionization Threshold Experiment: CDMSlite Neganov-Luke amplification of phonon response allows operation at very low energy thresholds PRL 112 (2014) CDMSlite (2014) Electrons and holes radiate phonons proportional to V bias as they drift to the electrodes. è Apply large V bias to amplify ionization signal World- leading limit w/ ~10 day exposure! First CDMSlite run: 170 evee (<1 kev nr ) threshold with 0.6 kg Ge, 10 live days and no background subtraction! 78

37 Dark Matter in CCD s (DAMIC) published results w/ 40 ev electron equivalent threshold Aside from CDMS, the only other direct detection experiment with a Si target, but with significantly different technology! DAMIC100 will have 100g of target and could see O(100) events per year for 8.6 GeV/c 2 WIMP and σ = 2x10-41 cm 2 DAMIC (0.5 g mass) Phys. Lett. B 711 (2012) CoGeNT CDMS II (Si central value) XENON100 UChicago, May 23,

38 Brief Discussion on Calibration and Energy Scales 80

39 Nuclear Recoil Calibration Proper calibration of the NR energy scale is always important, but it is especially true for low-mass WIMP searches Miscalibration of the NR energy scale tends to cause a systematic shift of exclusion limits to lower or higher WIMP masses This can cause large changes in sensitivity at low masses, where limit curves tend to be steep due to cutoff from detector thresholds * Xenon experiments have suffered an especially large amount of criticism for this particular issue 81

40 Measuring the NR scale Calibration of the ER scale is done with radioactive gamma sources applied directly to the detector or internal gamma lines. Is easy to do and yields subpercent accuracy. ionization energy [kev] Nuclear recoil scale calibration is more difficult. Can be done via: Example neutron scattering setup Neutron detectors Neutron scattering experiment Combination of AmBe or Cf calibration plus simulations Incident neutron Detector θ Photo-neutron sources All are difficult measurements and have significant associated systematic uncertainty. Ideally do all three if you can! 82

41 Reading the fine print Direct detection experiments will switch between the following measures of energy. Be careful it may not always be clearly labeled in the plot or table! Recoil Energy (kev or kev r ): the recoil energy Electron-Recoil Equivalent Energy (kev ee ): the recoil energy assuming its an electron-recoil. If it s really a nuclear recoil, then divide by quenching factor to determine true recoil energy Nuclear-Recoil Equivalent Energy (kev nr ): the recoil energy assuming it s a nuclear-recoil. If its really an electron recoil, then multiple by the quenching factor to determine true recoil energy Quenching Factor generically refers to the amount by which one channel (say ionization) is suppressed for nuclear recoils compared to electron recoils. Its sometimes denoted as Q. This is most often used with experiments that only measure energy in one channel such as DAMA/LIBRA and CoGeNT. 83

42 Example with LUX data Scale in electronequivalent recoil energy bulk electron recoils Approximate signal region Scale in nuclearequivalent recoil energy 84

43 How is E nr calculated for xenon TPC s? In other words, L eff accounts for the quenching of the scintillation signal for a nuclear recoil and must be measured in order to obtain the energy for a NR By convention, its defined with respect to a 122 kev gamma line (from 57 Co) 85

44 How is L eff determined? L eff is typically measured with a neutron scattering experiment using a small xenon detector at 0 field. It may also be measured in situ with neutron sources. A number of measurements exist which are roughly compatible but differ enough to be a point of contention for xenon results at low masses 86

45 New In-situ measurement from LUX Utilizing the large volume and precise position information, can reconstruct double neutron scatters inside of LUX for an in-situ neutron scattering experiment. This may enable LUX to extend sensitivity for WIMPs below 10 GeV. Conservative cut for first LUX result 87

46 End of Lecture 2 88

47 Homework 2 3a) The best-fit cross-section for SI WIMP-nucleon scattering in the CDMS II Si data was 2x10-41 cm 2 at 8 GeV/c 2 with 3 events seen. Had these three events been vanilla WIMPs, roughly estimate how many WIMPs would have been reported by LUX in their first WIMP search (115 kg fiducial and 85 live days). The CDMS II Si search had an exposure of 130 kg-days. For this part of the exercise, it is ok to neglect the form factors and phase-space corrections such as from integrating over the velocity profile, which requires taking the detector thresholds into account. 89