Recent Results from thePandaX-II ExperimentAndi Tan (University of Maryland)on behalf of the PandaX-II Collaboration
Osaka University, Dec. 21, 2017

International workshop on “Axion physics and dark matter cosmology”

Outline•

Dark Matter Direct Detection•PandaX at CJPL•

Recent Results from the PandaX-II•

Future of PandaX•Conclusion

2

Dark Matter Detection
Indirect SearchDMAMS

DAMPEDM?

SMSM

Direct Search
3Collider SearchDirect Detection

A = m2/m1•

DM: velocity ~1/1500 c, mass ~100 GeV, KE~ 20 keV•Nuclear recoil (NR): recoiling energy ~10 keV•Electron recoil (ER): 10-4 suppression inenergy, very difficult to detect•

New ideas exist, e.g. Hochberg, Zhao, andZurek, PRL 116, 011301

4

Available Space and Global Racing

Generic WIMPNature Physics 13, 212–216 (2017)•Event rate approaches a few cnts/100kg/y•

Lowest exclusion at ~8×10-47cm2 at ~40GeV/c2•~2 orders away from the neutrino floor
5

Dual Phase Xenon ExperimentS2ee e- e

S1invisible phonons
if nuclear recoil•

Dual phase TPC technology allows:
1. 3D positioning
2. Single Scatter selection•

S1 and S2 ratio allows ER/NR discrimination
6

Need three legs to make a table stable•

World leading bkg level: 0.2×10-3 evt/day/kg/keV•First SR: 1024 kg x 34.2 day, no candidate found•Minimum limit: 7.7 x 10-47 cm2 @ 35 GeV

LUX XENON1T
250 kg, concluded 2016, Largest LXe TPC ever built
Sanford Labcylinder: 96 × 97cm
LZ(7-ton) in preparation active Xe target 2.0t(3.2t total)
248 PMTs (Hamamatsu R11410-21)
7China Jin-Ping underground Lab8

China Jin-Ping underground Lab

9PandaX Collaboration
Started in 2009, ~50 people

10Shanghai Jiao Tong University (2009-)
Peking University (2009-)
Shandong University (2009-)
Shanghai Institute of Applied Physics,CAS (2009-)
University of Science & Technology ofChina (2015-)
China Institute of Atomic Energy(2015-)Sun Yat-Sen University (2015-
Yalong Hydropower Company (2009-)
University of Maryland (2009-)
Alternative Energies & Atomic Energy
Commission (2015-)
University of Zaragoza (2015-)
Suranaree University ofTechnology(2016-)

PandaX Experiments

PandaX-I: 120 kg
DM experiment2009-2014

PandaX-II: 580 kg PandaX-xT:
multi-ton DM
DM experimentexperiment2014-2018Future

PandaX-III: 200 kg to136
1 ton HP gas Xe
0vDBD experiment
FutureParticle and Astrophysical Xenon Experiments
11

The PandaX-II Detector•New inner vessel with clean SS•

New and taller TPC•

More 3” PMTs and improved basedesign with split ±HV power supply•

New isolated skin veto region

12

PandaX-II Run History
Mar. 9 – June 30, low background with
10-fold reduction of Kr (Run9, 79.6
days)

Nov. 2016 – Mar. 2017, 2nd distillation
campaign and recommissioning

2016

2015

Nov. 22 – Dec. 14, Physics commission(Run8, 19.1 days, stopped due to highKrypton background)

2017

Jul – Oct, ER calibration& tritium removal

Run9 =79.6 days, exposure: 26.2 ton-day• Run10 =77.1 days, exposure: 27.9 ton-day• Largest reported DM exposure to date•

13

Apr.22 – July15, dark
matter data taking(Run10, 77.1 days)

Run8+9(33 t-day) SI and SD Results

Minimum elastic SI exclusion:-46
222.5x10 cm @ 40 GeV/c
PRL 117, 121303 (2016)

Minimum -n SD cross section limit:-41
224.1x10 cm at 40 GeV/c
PRL 118, 071301 (2017)141 TeV DM SI Inelastic

PandaX-II Run9 Axion
and Inelastic Results
PRL 119, 181806 (2017)1010−9−1010 TeV DM SI Inelastic

Solar Neutrino

Si(Li)

CoGeNT
10−11

XMASS
SolarNeutrino
EDELWEISS-IICDEX-1

VZ
KS

SZ
DF

10II
-1 SSEX EI
NA e
CD LW
E
RA 57 FEDJO -IIA
aXnd
RedGiant−1310−510−4

XENON100Pa

10−12

LUX

10−12M

PandaX-II

10−310−2

10−1

Axion mass (keV/c2)

110

LUX

−13

PandaX-II
1010CDEX-1
MAJORANA

CDMS

ggAe

10KIMSXENON100−11EDELWEISS-II

Ae10

−10

1234

5 6 7 8 9 10
2Axion mass (keV/c )Among the leading axion search on axionelectron coupling using DD experiments
15

Opened up energy window to accessiniFal-ﬁnal mass diﬀerence up to 300
keV (high mass DM, ~TeV)• Tightest direct constraint on this todate (to be published)•−1420

PRD 96, 102007 (2017)

2017 new data and results

16

New SI DM search results from Run10•

Improved trigger threshold (from 80 to 50 PE≈2 electrons)•Calibration••

Channel-by-channel SPE efficiency ( " ZLE)•

Improved detector ER/NR response model

2.5 times reduction in total background•Kr85 ↓6 times•

Accidental ↓ 3 times•

Xe127 ↓13 times
17Calibration Program•

Gain Calibration•

ZLE Efficiency on S1 and S2•

Uniformity correction and Energy reconstruction•NR Calibration using 241AmBe•

ER calibration using CH3T18

Cali: Overall ZLE efficiency by LED Vs. S11•

Average efficiency measured at 3 PE (S1threshold) was about 80% using blue LED•

PMT Double-PE emission would further
improve the efficiency•Little impact to S2, little impact to position
reconstruction

ZLE efficiency

0.80.6
Run10 LED

0.40.20102

10103

S1 [PE]

19

Cali: Uniformity Correction•

Z correction on S2•••electron absorption toelectronegative impurities

X-Y correction on S2•

geometry•

Field non-uniformity

3D correction on S1•

geometry•

Rayleigh Scatter•

Light attenuation
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Cali: Election Lifetime
1200

1200

1000

800

800

600

0

e- Lifetime [µs]1000Run9400

400

200

0Run10600

Leakage onCirculation loop
Feb.292016

Mar.302016

Apr.292016200May.29
2016

Jun.282016

0Power failure
Apr.242017

May.242017

Jun.23
2017

Jul.232017•Electron lifetime on average 800μs (1.4 m drift distance) in Run10, and generally stable•

Significantly improved from Run921

Cali: Energy Reconstruction
Run10

For this analysis

Run9Run10S2⎛ S1⎞
Eee = W × ⎜+⎟⎝ PDE EEE × SEG ⎠•SEG determined with ZLE efficiency taken into account•Utilized a more careful treatment for the S2 saturation•Resulting best fit <2% with expected energies
22Cali: ER and NR

Neutron calibration:AmBe source deployed•ER calibration using tritiated methane(pioneered by LUX)•

Selected data with electron lifetime~700 s, ~8000 low energy ER events
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Data/model comparison(example)•A tuning of the Nex/Ni (excitation/ionization) parameter was made on
the NEST model, after which data and MC yield good agreement•

Consistent efficiencies obtained using ER/NR calibrations
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Cali: Direct comparison of ER and NR bands
Open Red circles: AmBe data
Soliddots: Tritium
Tritium datadata
Solid BlackBlack dots:
Solid Blue line: Run9+Run10 median

AmBe band median
99.9% NR acceptance from MC•

Events leaked below the NR median: 0.53(8)%25Calibration to Background
Open Red circles: AmBe data
Solid Black dots: Tritium data
Solid Blue line: Run9+Run10 medianGame of Thrones: Background level
AmBe band median
99.9% NR acceptance from MC

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We want keep as clean as possible
in this region and waiting for DM
scattering eventsnd
BKG: 2 distillation campaign
First beneficial occupancy of CJPL-II!

•After ER calibration, realized that thegetter could not remove tritium
background effectively•Suspected tritium attached to wall,
emanation rate balance with removal rate•

2nd distillation campaign (for Kr andtritium)•

Nov. – Mar 2017: recuperate distillation
refill, flush (closed) detector with warmxenon
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Background level
Run9Run10

Xe127

0.42

0.021

Tritium

00.27

Kr85

1.19

0.20

Rn222

0.13

0.12

Rn220

0.01

0.02

Detector ER

0.20

0.20

Solar neutrino

0.01

0.01

Xe136

0.00220.0022

Total

1.96

0.79

Original 127Xe gone, additional
introduced by a bottle ofsurface xenon during distillation
Based on best fit to data (later)
Rest are consistent between
Run9 and Run10
0.8 mDRU ~ 2 events a day!
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BKG: Energy spectrum in Run 10[0,50] keV

DM search range

Data and expected background in good agreement29

BKG: Distribution of events (run10)
ER

Accident
alNeutron

Total
FittedTotal
Observed

Run9376.1

13.50.85

390±50389

Below NRMedian

2.0

0.9

0.35

3.2±0.9

1Run10172.2

3.90.83177±33177

Below NRMedian0.9

0.60.33

1.8±0.5

0Light blue: Data median and 10%/90% quan7leS2>100PEAmBe band median99.9% NR acceptance from MC

Appears to have a downward
fluctuation of background!
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WIMP-nucleon SI result
PRL 119, 181302 (2017)•most stringent limit for WIMP-nucleon cross section for mass >100GeV•

Improved from PandaX-II 2016 limit about 2.5 time for mass>30 GeV•

Lowest exclusion at 8.6×10-47cm2 at 40GeV/c2
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PandaX Future32

PandaX new home: CJPL-II
8 Experimental Halls• 14(H)×14(W)×65(L)m each• Dark matter, 0vDBD,
nuclear astrophysics,
low background CFT•

B2, PandaX site!

33PandaX-xT Experiment•

Preparing new experiments in CJPL-II, hall #B2•

Intermediate stage:•

PandaX-4T (4-ton target) with SI sensitivity~10-47 cm2•

On-site assembly and commissioning:2019-2020•

Eventual goal: G3 xenon dark matter detector(~30T) in CJPL to “neutrino floor” sensitivity34

136
PandaX-III: High pressure Xe TPC•

0DBD signal: two electrons emitting from the same vertex with a summed energy at
the Q value (tracking essential)•TPC: 200 kg, 10 atm, symmetric, double-ended charge readout plane withmicromegas module with cathode in the middle•

Four more upgraded modules for a ton scale experiment•

Published CDR recently: ArXiv:1610.08883

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Conclusion•Searching for WIMPs is far from over•

PandaX experiment since 2012 has been gone throughtwo generations of detectors•PandaX-II has reached and remains at the forefront of theDM search, and will continue data taking till end of 2018•

The collaboration is going forward in preparation for
PandaX-III and PandaX-xT.
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