Results from DarkSide-50 with underground argon

Transcription

1 Dark Matter 2016 Los Angeles, CA February 2016 Results from DarkSide-50 with underground argon Alden Fan UCLA for the DarkSide collaboration 1

2 DarkSide WIMP dark matter search using direct detection Dual-phase Liquid Argon Time Projection Chamber (LArTPC) Ultra low background Deep underground at LNGS Low-background materials, including Ar target Powerful background rejection Pulse Shape Discrimination (PSD) Ionization/Scintillation ratio (S2/S1) Surface rejection using 3D position reconstruction Active neutron and muon vetoes In situ background measurement 2

3 Why Argon? Relatively dense Ionization and scintillation Easy to purify (chemically) Scales to large mass Transparent to its own scintillation light Exceptional discrimination power PSD S2/S1 Main challenge: 39 Ar contamination Atmospheric argon: high concentration of 39 Ar to 40 Ar cosmogenically activated (1 Bq/kg) β decay (T1/2: 269 yr, Q: 565 kev) Underground argon: significantly reduced 39 Ar activity 3

4 Multi-stage DarkSide program Gran Sasso National Laboratory, Italy DarkSide DarkSide x DarkSide-20k ? ARGO 202?-20?? See C.J. Martoff talk 4

5 Dual-phase LArTPC 6 μs NR S1 time [μs] [arb] [arb] 6 μs ER PSD parameter: F90 = fraction of light in first 90 ns time [μs] 5

6 Dual-phase LArTPC [arb] 65 μs S2 S1 e - e - e - e - e - e - Eextr Edrift NR time [μs] [arb] 65 μs ER S2 allows for 3D position reconstruction and additional discrimination power time [μs] 6

7 DarkSide-50 TPC 46 kg active volume 36 cm diameter, 36 cm height 38 3 PMTs Cold pre-amps High reflectivity Teflon walls Fused silica anode and cathode windows Coated with transparent conductor (Indium Tin Oxide) All inner surfaces coated with wavelength shifter (Tetraphenyl Butadiene) 0.2 kv/cm drift, 2.8 kv/cm extraction 7

8 Vetoes Liquid Scintillator Veto 4 m diameter sphere Boron-loaded: PC + TMB 1 8 PMTs Active neutron veto tag neutrons in TPC in situ measurement of neutron BG Neutron and gamma shielding Water Tank 11 m diameter x m high Existing Borexino CTF tank 80 PMTs Active muon veto tag cosmogenic neutrons Neutron and gamma shielding 8

9 DarkSide-50 Assembly e-50 Assembly 9

10 Calibrations Insertion system deployed Sept 2014 Calibrations: 83mKr (injected), 57Co, 133Ba, 137Cs, AmBe, AmC Validate NR band obtained from SCENE Evaluate Light Yield f90 Validate MC P. Agnes et al. / Physics B 743 (2015) Energy [kevnrletters ] FIG left: CALIS installation inside the CRH radon-suppressed clean room atop the WCD. right: Photograph taken after200 a camera looking into the LSV from the WCD. It shows a source deployed next to the cryostat of the LAr-TPC. 7 the first be S2, and the in DarkSide pulse is assumed to be S1, the second to AmBe 83m + 39Arthe source is brough 95 Source Deployment: In order to deploy a radioactive source next to Kr the LAr-TPC, third to be S the radon-suppressed clean room atop the WCD tank, and mounted in a source holder connected t Several corrections are applied to the S1 and S2 CRH integrals to aclight Yield: 97 deployment device within CALIS. This is lowered into the LSV through a dedicated access port, to a position 0.7 count for geometrical variations of light production and collection. 98 to the cryostat. The source holder is on the end of a 60 cm long articulated arm, which can be moved and rota ± 0.4 nulla slightly field pressu Due surface,thelight col99 position source in 3-D relative to the cryostat (Fig.7.9 2, right). Throughout the deployment 0.6to total internal reflection at the liquid nitrogenthe atmosphere to avoid exposing the liquid@ scintillator to oxygen or w lection of scintillation pulses varies by 19% 0 between top andhas to be maintained in order7.0 ± 0.3 PE/keV 200 V/cm which would degrade it. All materials used for CALIS, such as stainless steel, teflon, and viton, that com bottom of the TPC. An empirical z-dependent correction, derived 2 contact with the scintillator are certified for contact with the liquid scintillator. The system includes several s 83m from Kr and Ar calibration data and normalized totothe center 3 features ensure safe deployment and retrieval of the source without affecting the stability and perform 4 of the LSV. After data taking, the deployment device is retracted and a sequence of N2 purging and evacu of the0.3 TPC, is applied to S1. 5 removes all traces of scintillator from the deployment device. Thereafter the source holder can be extracted fro Electronegative impurities in the LAr capture drifting electrons deployment device and the radioactive source returned to storage. The CALIS concept has the built-in flexibil This results in the number of drifting electrons, and hence S2,devices de- next to the cryostat, e.g. the deployment of a (d,d) neutron generator is considered 7 deploy different 0.1 creasing exponentially with the time taken 8to future. drift between the Calibration Campaigns: After the calibration device and the deployment procedures were approved by the 0 point and the liquid-gas interface.9 We interaction fit for this elec Side scientific committee, two extensive calibration campaigns were performed during October through Dece tron drift lifetime, then correct S2, normalizing to the top S1 of through the [PE] February The performance and stability of the LSV and LAr-TPC was not aff and January TPC. Our very high electron mean drift lifetime a maxi- campaigns. 112 byinduces the two calibration Fig. 3. The primary scintillation (S1) spectrum from a zero-field run of the mum 7% correction for the runs acquired through February Li(p,n) in SCENE 241

11 UAr Extracted from Doe Canyon CO2 wells Transported to Fermilab for distillation 6 yr effort to obtain 155 kg of UAr Shipped to LNGS by sea (15.6 kg by air) Map data 2015 Google, INEGI 500 mi 11

12 AAr vs. UAr - null field s] kg Events / [50 PE AAr Data (0 V/cm) UAr Data (0 V/cm) S1 Late [PE] LY unchanged from AAr to UAr 12

13 AAr vs. UAr V/cm s] kg Events / [50 PE AAr (200 V/cm) UAr (200 V/cm) S1 [PE] 13

14 AAr vs. UAr V/cm s] kg Events / [50 PE AAr (200 V/cm) UAr (200 V/cm) AAr (200 V/cm, LSV Anti-coinc.) UAr (200 V/cm, LSV Anti-coinc.) S1 [PE] Slight excess at 39 Ar endpoint 14

15 39 Ar depletion s] kg Events / [50 PE AAr (200 V/cm) UAr (200 V/cm) AAr (200 V/cm, LSV Anti-coinc.) UAr (200 V/cm, LSV Anti-coinc.) MC total (Global Fit) 85 Kr (Global Fit) 39 Ar (Global Fit) saturation S1 [PE] MC fit prefers 85 Kr component to explain excess See P. Agnes talk for MC 15

16 39 Ar depletion 39 Ar reduction factor: 1400 s] kg Events / [50 PE 1 AAr Data Bi 609 kev (C+P) 60 Co 1.17 MeV (C+F) 60 Co 1.33 MeV (C+F) 40 K 1.46 (P) MeV C: Cryostat P: PMTs F: Fused Silica 214 Bi 1.77 MeV (C+P) UAr Data UAr MC Total MC MC 85 Kr 39 Ar 208 Tl 2.62 (P) MeV S1 Late Fitted 85 Kr activity in UAr: 2.05 ± 0.13 mbq/kg Fitted 39 Ar activity in UAr: 0.73 ± 0.11 mbq/kg 39 Ar activity in AAr: 00 mbq/kg [PE] 16

17 85 Kr delayed coincidences 85 Kr: 0.4% BR to 85m Rb (T1/2 = 1 μs, 514 kev γ) Signature: two S1s (β+γ) in delayed coincidence Events / [0.1 µs] Decay time 1.64 ± 0.12 µs Δt [µs] Rates Observe: 33.1 ± 0.9 events/d From spectral fit: 35.3 ± 2.2 events/d 17

18 Dark Matter search I Dark Matter search with UAr begins immediately after turning on fields. First WIMP search with UAr AmC calibration 83m Kr calibration Accumulated livetime with UAr ~1.5 Hz trigger rate, predominantly ER events 18

19 F90 Use analytic model for F90 distributions Fit to high statistics AAr data Scale to UAr data Derive 0.01 ER leakage events / S1 bin S1: [60.00, 70.00] PE S1: [250.00, ] PE 4 3 AAr UAr 4 3 AAr UAr f f 90 19

20 NR acceptance Cuts: select single scatters (single S1 + single S2) with no signal in veto. Efficiencies evaluated using UAr data + AmBe data + MC Dominant acceptance loss: accidental coincidences in veto NR Acceptance Energy [kev nr ] Total Cut Efficiency f 90 NR Acceptance Overall NR Acceptance S1 [PE] 20

21 Veto efficiency Veto neutrons via thermalization or capture in LSV >99.1% efficiency to veto neutrons from capture alone (AmBe + simulation) arxiv: Will increase efficiency using neutron thermalization signal Analysis in progress using new AmC source data (Dec 15 - Jan 16) AmC 21

22 Dark Matter search II 70.9 live-days, 36.9 kg fiducial volume Expect <0.15 ER leakage events f Energy [kev nr ] WIMP Search Region Ar + 85 Kr + γ 50% 90% 99% 200 NR acceptance contours from AmBe in DS50 + SCENE S1 [PE] 0 No events in the WIMP search region. 22

23 Dark Matter search III Combined limit of UAr and AAr exposures in DS50: minimum at 0 GeV/c 2 : 2 x -44 cm 2 [cm 2 ] σ PandaX-I (2014) PICO (2015) WARP (2007) DarkSide-50 (AAr, 2014) DarkSide-50 (UAr, 2015) DarkSide-50 (combined) CDMS (2015) XENON0 (2012) LUX (2015) M χ 2 [GeV/c ] 4 arxiv:

24 DS50 3 yr projection [cm 2 ] σ CDMS (2015) PandaX-I (2014) PICO (2015) XENON0 (2012) WARP (2007) DarkSide-50 (AAr, 2014) DarkSide-50 (UAr, 2015) DarkSide-50 (combined) DarkSide-50 (3 yr proj.) 45 LUX (2015) M χ 2 [GeV/c ] 4 24

25 Summary DarkSide-50 performed first ever dark matter search using Underground Argon. Measured 39 Ar level in UAr to be factor 1400 smaller than in AAr. DarkSide-50 has the strongest WIMP limit using an Ar target, third best limit. Currently in stable WIMP search mode. 25

26 Backup 26

27 50d AAr DM search 1422 ± 67 kg-day exposure f Energy [kev nr ] % 90% 99% S1 [PE] 0 27

28 S2/S1 f S2/S1 cut calibrated on AmBe data in DS50 50% NR acceptance in S2/S1 Energy [kev nr ] % 90% 99% S1 [PE] Should we ever see a potential WIMP signal, S2/S1 cut is powerful additional handle. 0 28