Results from DarkSide-50 with underground argonAlden Fan

UCLA

for the DarkSide collaboration

1

Dark Matter 2016Los Angeles, CA

17-19 February 2016

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

Why Argon?• Relatively dense

• Ionization and scintillation• Transparent to its own scintillation light

• Exceptional discrimination power• PSD• S2/S1

3

Main challenge: 39Ar contaminationAtmospheric argon:high concentration of 39Ar to 40Ar• cosmogenically activated (1 Bq/kg)• β decay (T1/2: 269 yr, Q: 565 keV)

Underground argon:significantly reduced 39Ar activity

• Easy to purify (chemically)

• Scales to large mass

Multi-stage DarkSide program

4

DarkSide-102011-2013

DarkSide-502013-201x

DarkSide-20k2020-202?

ARGO202?-20??

Gran Sasso National Laboratory, Italy

See C.J. Martoff talk

Dual-phase LArTPC

5

1

S1

NR

ER

PSD parameter: F90 = fraction of light in first 90 ns

time [μs]

time [μs]

6 μs

6 μs

[arb

][a

rb]

Dual-phase LArTPC

6

1

Edrift

Eextr

e-

e-

e-e-e-

e-

S2NR

ER

S1

S2 allows for 3D position reconstruction and additional discrimination power

time [μs]

time [μs]

65 μs

65 μs

[arb

][a

rb]

DarkSide-50 TPC

7

• 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

VetoesLiquid Scintillator Veto• 4 m diameter sphere• Boron-loaded: PC + TMB• 110 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 10 m high• Existing Borexino CTF tank• 80 PMTs• Active muon veto

• tag cosmogenic neutrons• Neutron and gamma shielding

8

DarkSide-50 Assembly

9

DarkSide-50 Assembly

19 Feb 2014 Alden Fan (UCLA) -- LLWI 11

38

Calibrations• Insertion system deployed Sept 2014• Calibrations: 83mKr (injected), 57Co, 133Ba,

137Cs, AmBe, AmC• Validate NR band obtained from SCENE• Evaluate Light Yield• Validate MC

10

3

are even better than upper limits previously established by counting small samples of UAr[4]. Details of the TPC88

performance with UAr are presented below in Sec. IV.89

II. RADIOACTIVE SOURCE CALIBRATION OF DARKSIDE-5090

During 2014, calibration hardware to deploy radioactive sealed sources within the LSV was jointly developed91

by Fermilab and University of Hawaii. CALIS (CALibration Insertion System) is designed to deploy radioactive92

sources to calibrate both the LAr-TPC and the LSV. CALIS has been constructed, tested at Fermilab and LNGS,93

precision cleaned, and successfully installed in the DarkSide clean room (Fig. 2, left).94

FIG. 2. left: CALIS after installation inside the CRH radon-suppressed clean room atop the WCD. right: Photograph taken witha camera looking into the LSV from the WCD. It shows a source deployed next to the cryostat of the LAr-TPC.

Source Deployment: In order to deploy a radioactive source next to the LAr-TPC, the source is brought into95

the CRH radon-suppressed clean room atop the WCD tank, and mounted in a source holder connected to the96

deployment device within CALIS. This is lowered into the LSV through a dedicated access port, to a position next97

to the cryostat. The source holder is on the end of a 60 cm long articulated arm, which can be moved and rotated to98

position the source in 3-D relative to the cryostat (Fig. 2, right). Throughout the deployment a slightly pressurized99

nitrogen atmosphere has to be maintained in order to avoid exposing the liquid scintillator to oxygen or water,100

which would degrade it. All materials used for CALIS, such as stainless steel, teflon, and viton, that come in101

contact with the scintillator are certified for contact with the liquid scintillator. The system includes several safety102

features to ensure safe deployment and retrieval of the source without affecting the stability and performance103

of the LSV. After data taking, the deployment device is retracted and a sequence of N2 purging and evacuation104

removes all traces of scintillator from the deployment device. Thereafter the source holder can be extracted from the105

deployment device and the radioactive source returned to storage. The CALIS concept has the built-in flexibility to106

deploy different devices next to the cryostat, e.g. the deployment of a (d,d) neutron generator is considered in the107

future.108

Calibration Campaigns: After the calibration device and the deployment procedures were approved by the Dark-109

Side scientific committee, two extensive calibration campaigns were performed during October through December110

2014 and January through February 2015. The performance and stability of the LSV and LAr-TPC was not affected111

by the two calibration campaigns.112

S1 [PE]50 100 150 200 250 300 350 400 450

90 f

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1) in SCENEn,pLi(7

AmBe in DarkSide-50241

]nrEnergy [keV20 40 60 80 100 120 140 160 180 200

P. Agnes et al. / Physics Letters B 743 (2015) 456–466 461

the first pulse is assumed to be S1, the second to be S2, and the third to be S3.

Several corrections are applied to the S1 and S2 integrals to ac-count for geometrical variations of light production and collection.

Due to total internal reflection at the liquid surface, light col-lection of scintillation pulses varies by 19% between the top and bottom of the TPC. An empirical z-dependent correction, derived from 83mKr and 39Ar calibration data and normalized to the center of the TPC, is applied to S1.

Electronegative impurities in the LAr capture drifting electrons. This results in the number of drifting electrons, and hence S2, de-creasing exponentially with the time taken to drift between the interaction point and the liquid-gas interface. We fit for this elec-tron drift lifetime, then correct S2, normalizing to the top of the TPC. Our very high electron mean drift lifetime induces a maxi-mum 7% correction for the runs acquired through February 2014. During a gap in the data taking that followed, the lifetime contin-ued to improve. We do not apply any correction to data collected after this period, about 75% of the total, since the electron drift lifetime has become too long to be measured reliably.

We discovered during commissioning that the amplitude of S2 has a strong radial dependence, where events under the central PMT exhibit greater than three times more electroluminescence light than events at the maximum radius. Only basic cuts on S2 are used in the present analysis, and this does not affect our results. Preliminary x–y reconstruction algorithms indicate that the radial variations can be empirically corrected using calibration data.

6. Veto event reconstruction

Due to the use of DAQ-level zero-suppression, reconstruction of LSV and WCD signals is different from the TPC reconstruction. Pulses are naturally defined as the non-zero portion of each raw waveform for each channel. The DAQ records 20 samples (16 ns) before and after each pulse. The first 15 samples before the pulse are averaged to define a baseline, which is subtracted from the waveform. Each channel is then scaled by the corresponding SPE mean and the channels in each veto detector are summed together.

A clustering algorithm on the sum waveform identifies physical events in the LSV. To handle the high pile-up rate due to 14C, the algorithm is a “top-down” iterative process of searching for clusters from largest to smallest. These clusters are used only for building the 14C and 60Co spectra and determining the light yield of the LSV. Identification of coincident signals between LSV and TPC uses fixed regions of interest of the sum waveform and is described in Section 9. For tagging of muons in the LSV and WCD, the total integrated charge of each detector is used.

7. TPC energy calibration and light yield

In the data presented here, taken with atmospheric argon, the TPC trigger rate is dominated by 39Ar ! decays, with their 565 keV endpoint. The spectrum observed in the presence of the 83mKrsource at zero drift field, clearly dominated by 39Ar decay, is shown in Fig. 3. The measured rate of 83mKr events is 2 to 3 Hz. Because it affects the optics of the detector, the gas pocket was maintained even when operating the detector at zero drift field to collect reference data for light yield. The spectrum is fit to obtain the measurement of the light yield of the detector at the 41.5 keV reference line of 83mKr. The fit of the entire spectrum, encompass-ing the 39Ar and 83mKr contributions, shown in Fig. 3, returns a light yield of (7.9 ± 0.4) PE/keV at zero drift field, after including systematic errors. Fitting the light yield from the 83mKr peak alone gives the same result within the fitting uncertainty. The resolution is about 7% at the 83mKr peak energy. The dominant uncertainty in

Fig. 3. The primary scintillation (S1) spectrum from a zero-field run of the DarkSide-50 TPC. Blue: S1 spectrum obtained while the recirculating argon was spiked with 83mKr, which decays with near-coincident conversion electrons sum-ming to 41.5 keV. Red: fit to the 83mKr + 39Ar spectrum, giving a light yield of (7.9 ± 0.4) PE/keV at zero drift field. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 1Reference 83mKr values of the light yields from SCENE and DarkSide-50 used to correlate the S1 and S2 scales of DarkSide-50 with the SCENE calibration. Note that most of the systematic errors are correlated between the DarkSide-50 200 V/cmand zero-field light yields.

Experiment Drift field Light yield

DarkSide-50 200 V/cm (7.0 ± 0.3) PE/keVDarkSide-50 Zero (7.9 ± 0.4) PE/keV

SCENE (Jun 2013 Run) Zero (6.3 ± 0.3) PE/keVSCENE (Oct 2013 Run) Zero (4.8 ± 0.2) PE/keV

the 83mKr light yield comes from the systematic uncertainties on the mean SPE response of the PMTs. Table 1 summarizes the values for the light yield from 83mKr with and without drift field. Note that the value of the light yield has a larger uncertainty at zero field, where it is neither possible to account for non-uniformities in the 83mKr distribution in the active volume nor to correct for the z-dependent light collection variations described in Section 5. Both effects are accounted for with the drift field on, where z-position information is available.

To obtain the best calibration of the response in S1 and S2 for nuclear recoils needed for the DarkSide program, members of the collaboration and others performed an experiment called SCENE [25,26]. The SCENE experiment measured the intrinsic scin-tillation and ionization yield of recoiling nuclei in liquid argon as a function of applied electric field by exposing a small LAr TPCto a low energy pulsed narrowband neutron beam produced at the Notre Dame Institute for Structure and Nuclear Astrophysics. Liquid scintillation counters were arranged to detect and identify neutrons scattered in the TPC, determining the neutron scattering angles and thus the energies of the recoiling nuclei. The use of a low-energy narrowband beam and of a very small TPC allowed SCENE to measure the intrinsic yields for single-sited nuclear re-coils of known energy, which is not possible in DarkSide-50.

The measurements performed in SCENE were referenced to the light yield measured with a 83mKr source at zero field. The use of the same 83mKr in DarkSide-50 allows us to use the relative light yields of the two experiments (see Table 1) to determine, from the SCENE results, the expected S1 and S2 signals of nuclear re-coils in DarkSide-50. Table 2 summarizes the expected DarkSide-50S1 yields derived with this method and thus provides nuclear re-coil energy vs. S1 for DarkSide-50. For the analysis reported here, we interpolate linearly between the measured SCENE energies and

83mKr + 39ArLight Yield:

7.9 ± 0.4 PE/keV @ null field7.0 ± 0.3 PE/keV @ 200 V/cm

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)

11Map data ©2015 Google, INEGI 500 mi

AAr vs. UAr - null field

12

LY unchanged from AAr to UAr

[PE]LateS10 2000 4000 6000 8000 100001200014000160001800020000

s]

× kg

×Events / [50 PE

8−10

7−10

6−10

5−10

4−10

3−10

2−10

1−10AAr Data (0 V/cm)

UAr Data (0 V/cm)

13

S1 [PE]0 2000 4000 6000 8000 10000 12000

s]

× kg

×Events / [50 PE

8−10

7−10

6−10

5−10

4−10

3−10

2−10

1−10AAr (200 V/cm)

UAr (200 V/cm)

AAr vs. UAr - 200 V/cm

14

S1 [PE]0 2000 4000 6000 8000 10000 12000

s]

× kg

×Events / [50 PE

8−10

7−10

6−10

5−10

4−10

3−10

2−10

1−10AAr (200 V/cm)

UAr (200 V/cm)

AAr (200 V/cm, LSV Anti-coinc.)

UAr (200 V/cm, LSV Anti-coinc.)

AAr vs. UAr - 200 V/cm

Slight excess at 39Ar endpoint

15

S1 [PE]0 2000 4000 6000 8000 10000 12000

s]

× kg

×Events / [50 PE

8−10

7−10

6−10

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4−10

3−10

2−10

1−10AAr (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)

Kr (Global Fit)85

Ar (Global Fit)39

saturation

39Ar depletion

MC fit prefers 85Kr component to explain excessSee P. Agnes talk for MC

16

39Ar depletion

[PE]LateS10 2000 4000 6000 8000 100001200014000160001800020000

s]

× kg

×Events / [50 PE

8−10

7−10

6−10

5−10

4−10

3−10

2−10

1−10 AAr Data

UAr Data

UAr MC Total

Kr85MC

Ar39MC

Bi214

keV 609(C+P)

Co60

MeV 1.17(C+F)

Co60

MeV 1.33(C+F)

K40

MeV 1.46(P) Bi214

MeV 1.77(C+P) Tl208

MeV 2.62(P)

C: Cryostat

P: PMTsF: Fused Silica

Fitted 85Kr activity in UAr: 2.05 ± 0.13 mBq/kg Fitted 39Ar activity in UAr: 0.73 ± 0.11 mBq/kg

39Ar activity in AAr: 1000 mBq/kg

39Ar reduction factor: 1400

85Kr delayed coincidences

17

85Kr: 0.4% BR to 85mRb (T1/2 = 1 μs, 514 keV γ)Signature: two S1s (β+γ) in delayed coincidence

RatesObserve: 33.1 ± 0.9 events/d

From spectral fit: 35.3 ± 2.2 events/d

s]µt [Δ0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2

s]µ

Even

ts /

[0.

1 0

20

40

60

80

100

120Decay time

sµ 0.12 ±1.64

Dark Matter search I

18

Dark Matter search with UAr begins immediately after turning on fields.

~1.5 Hz trigger rate, predominantly ER events

Accumulated livetime with UAr

First WIMP search with UAr

83mKr calibration

AmC calibration

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

19

90 f0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

1

10

210

310

410

S1: [60.00, 70.00] PE

90 f0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

1

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310

410

S1: [250.00, 260.00] PE

AArUAr

AArUAr

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

20S1 [PE]0 50 100 150 200 250 300 350 400 450 500

NR Acceptance

0

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Total Cut Efficiency

NR Acceptance90f

Overall NR Acceptance

]nrEnergy [keV20 40 60 80 100 120 140 160 180 200

Veto efficiency• Veto neutrons via thermalization or capture in LSV• >99.1% efficiency to veto neutrons from capture alone

(AmBe + simulation) arXiv:1512.07896• Will increase efficiency using neutron thermalization signal• Analysis in progress using new AmC source data

(Dec ’15 - Jan ’16)

21

prompt LSV event with DM box cut

early thermalization signal

late thermalization signal

Set new Rois

I remove events have early thermalization signal

I only interested in the events have latethermalization

I use roi lsv charge to get rid of earlythermalization events

I prompt o↵set ns = -6580ns

I roi time range is based on TPC trigger time

I roi lsv charge[7] time range [-100ns, 17ns]

I roi lsv charge[7] is used in the following studyHao Qian 12 / 21

prompt LSV event with DM box cut

early thermalization signal

late thermalization signal

Set new Rois

I remove events have early thermalization signal

I only interested in the events have latethermalization

I use roi lsv charge to get rid of earlythermalization events

I prompt o↵set ns = -6580ns

I roi time range is based on TPC trigger time

I roi lsv charge[7] time range [-100ns, 17ns]

I roi lsv charge[7] is used in the following studyHao Qian 12 / 21

prompt LSV event with DM box cut

roi energy vs cluster time

I when only require DMbox cut (24755events)

I remove event withroi lsv charge[7]>0.2PE

I require DMbox cut(11749 events left):our clean NR sample

NR before roi lsv charge[7] cut

NR after roi lsv charge[7] cut

Hao Qian 13 / 21

AmC

Dark Matter search II

22

S1 [PE]50 100 150 200 250 300 350 400 450

90

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50%90% 99%

WIMP Search Region

]nr

Energy [keV20 40 60 80 100 120 140 160 180 200

70.9 live-days, 36.9 kg fiducial volume

No events in the WIMP search region.

NR acceptance contours from AmBe in DS50 + SCENE

39Ar + 85Kr + γ

Expect <0.15 ER leakage events

Dark Matter search III

23

]2 [GeV/cχM10 210 310 410

]2 [cm

σ

46−10

45−10

44−10

43−10

42−10

41−10

DarkSide

-50 (AAr

, 2014)

DarkSide

-50 (UAr

, 2015)

DarkSide

-50 (com

bined)

WARP (20

07)

LUX (201

5)

XENON100 (

2012)

PandaX-I (

2014)

CDMS (2015)

PICO (2015)

Combined limit of UAr and AAr exposures in DS50: minimum at 100 GeV/c2: 2 x 10-44 cm2

arXiv:1510.00702

DS50 3 yr projection

24

]2 [GeV/cχM10 210 310 410

]2 [cm

σ

46−10

45−10

44−10

43−10

42−10

41−10

DarkSide

-50 (AAr

, 2014)

DarkSide

-50 (UAr

, 2015)

DarkSide

-50 (com

bined)

DarkSide

-50 (3 y

r proj.)

WARP (20

07)

LUX (201

5)

XENON100 (

2012)

PandaX-I (

2014)

CDMS (2015)

PICO (2015)

Summary

25

• DarkSide-50 performed first ever dark matter search using Underground Argon.

• Measured 39Ar 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.

Backup

26

50d AAr DM search

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50%90% 99%

]nrEnergy [keV20 40 60 80 100 120 140 160 180 200

1422 ± 67 kg-day exposure

S2/S1

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S1 [PE]50 100 150 200 250 300 350 400 450

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50%90% 99%

]nrEnergy [keV20 40 60 80 100 120 140 160 180 200

S2/S1 cut calibrated on AmBe data in DS5050% NR acceptance in S2/S1

Should we ever see a potential WIMP signal, S2/S1 cut is powerful additional handle.