1. 杨勇 上海交通大学 On Behalf of the Collaboration 粒子物理数据分析基础和前沿研讨会,2016/9/13
PandaX-II 物理数据分析
杨勇 上海交通大学
On Behalf of the Collaboration
粒子物理数据分析基础和前沿研讨会,2016/9/13
2018/12/4
杨勇，上海交通大学

2. Run history Series of Engineering runs
Distillation and re-started (Run9, 80 days)
2015
2016
Detector installation
at CJPL
Nov 22 – Dec 14, Physics commission (Run8, 19 days, stopped due to high Krypton background)
The pandax-2 detector was installed in the lab in Oct 2014, and we had a couple of engineering runs in in the end of last year, we had one physics commissioning run about 19 days of DM data . We call this run as run8. it was stopped because we found an air leakage which leads to a high krypton contamination. So we paused the data taking and send the detector back to shanghai for krypton distillation. After that we resumed the data taking. Till the end of June, we have collected about 80 days of data. This is what we call run9.
Run8+Run9=98.7 days
Exposure:3.3x104 kg-day
Phys. Rev. D. 39, (2016)
2018/12/4
杨勇，上海交通大学

3. Analysis chains overview
RAW data:
(PMT pulse)
Reconstruction:
S1, S2 and
Vertex, Energy
Calibration:
Electron recoil
Nuclear recoil
Selections:
Trigger
Fiducial volume
S1&S2
Results:
Background and signal estimate,
Limit calculation
2018/12/4
杨勇，上海交通大学

4. Calibration program Internal/external ER peaks:
Detector uniformity corrections
Light/charge collection parameters
Low rate AmBe neutron source:
 Simulate DM NR signal
CH3T injection: tritium beta decays
 Simulate ER background
2018/12/4
杨勇，上海交通大学

5. Single electron gain Identify smallest S2 in the data
S2 <100 pE (mainly from one electron) fit with F-D fuction x two Gaussian
Identify smallest S2 in the data
Fitted with two Gaussians (with S2 rec. efficiency taken into account)
SEG: 24.4 0.7 PE/e
2018/12/4
杨勇，上海交通大学

6. Extracting detector parameters
W = 13.7 eV
Gaussian fits to all ER peaks in data
Linear fit in S1/E vs S2/E to extract PDE and EEE
2018/12/4
杨勇，上海交通大学

7. Comparison with NEST-0.99
NEST (Noble Element Simulation Technique): simulation of the excitation, ionization, and corresponding scintillation and electroluminescence processes in liquid noble elements.
NEST ER model
Reasonable agreement observed between NEST ER model and values extracted from our data
2018/12/4
杨勇，上海交通大学

8. Energy nonlinearity and resolution
< 2% nonlinearity achieved for [33, 1332] keV
129Xe (n,)
131Xe (n,)
Resolution approaching high energy
127Xe
2018/12/4
杨勇，上海交通大学

9. NR calibration
Data medians ±2s
MC medians ±2s
ER median
NR 99.9% acceptance
S2>100 PE
Ambe calibration data in log_10(S2/S1) vs S1. Medians of data: red solid circles : MC simulation: green square.
2-sigma boundaries, red (data) and green (MC)…..
162.4 hours of AmBe data taken, with ~3400 low energy single scatter NR events collected
NR median curve and NR detection efficiency determined
2018/12/4
杨勇，上海交通大学

10. ER calibration with CH3T
ER median
NR median
Events
18.0 hours of tritium data taken, with ~2800 low energy ER events collected. (<E>=5.7keV)
9 events leaked below NR median, (0.32  0.11)%
Consistent with Gaussian expectation
2018/12/4
杨勇，上海交通大学

11. Event Selections Summary
Notes
Trigger
Mostly triggering on s2 signal, threshold about 80 PE
Data quality
S1, S2 Identification, bad files removal, etc
Skin Veto
Suppress ambient gamma background
Single S2
Remove double-scattering events
Fiducial volume
Suppress ER&NR background
BDT cut
Suppress accidental background
3<S1<45 PE, S2(raw)>100 && S2<10000 PE
Low energy selection
2018/12/4
杨勇，上海交通大学

12. Background Overview Background Run8 Run9 Estimate Method
Electron Recoil
Mostly Kr-85
Mostly Kr-85, Xe-127
Normalization from data, distribution from MC&Data (eff. applied)
Accidental
Single S1 and single S2 random coincidence
From Data
Nuclear Recoil
Neutrons in detector materials
Neutron spectra calculated with SOURCES4A package,
distri. from MC (eff. applied)
Neutron background mostly from PTFE
2018/12/4
杨勇，上海交通大学

13. Krypton-85 background
krypton is intrinsically present on the ppb (parts per billion level=10−9 Kr/Xe ) in commercially available xenon. (In atmosphere, ppm=10-6 level)
Kr85/Kr = 2x10-11
85mRb ms
2018/12/4
杨勇，上海交通大学

14. KITPC WORKSHOP, BEIJING
85Kr
Estimated from delayed b-g coincidence analysis
Uniformly distributed
Significantly reduced after distillation
Waveform: one b
one g
Commissioning
run (Run8)
1st Physics run (Run 9)
2018/12/4
KITPC WORKSHOP, BEIJING

15. ER background Normalization of Xe127&Kr-85 extracted from data
127Xe (408 keV)
127Xe/129mXe
127Xe from the 408 keV
( keV) peak
Normalization of Xe127&Kr-85 extracted from data
~0.4 mDRU(10-3/keV/kg/day) for <10 KeVee
2018/12/4
杨勇，上海交通大学

16. Accidental background
Isolated S1 before an S1
Isolated S2
pair
Isolated S1 and S2 were selected and randomly paired to simulate accidental events
Significantly reduced using a multivariate technique (BDT).
x3 rejection for background
90% efficiency for signals
2018/12/4
杨勇，上海交通大学

17. BDT discrimination power
Accidental S1 and S2 lack intrinsic correlations
Single S2 likely originated from the gate grid (small width, large asymmetry)
2018/12/4
杨勇，上海交通大学

18. Overall background rate in run9
Events selected with energy <10 keV
~2 mDRU on average (15.3 mDRU in Run 8)
Decrease over time due to 127Xe decay
2018/12/4
杨勇，上海交通大学

19. WIMP-Xenon Recoil Energy and Rate
Total recoiling events
/ ton xenon / year
sc-p=10-45 cm2
sc-p=10-45 cm2
Xenon
2018/12/4
杨勇，上海交通大学

20. WIMP signal efficiency
> 80% efficiency for recoil energy keV
DM detection efficiency
10 GeV, 100 GeV
2018/12/4
杨勇，上海交通大学

21. Results Run8 Run9 Below NR med. ER Accidental NR Total bkg. Data Run8
ER Median
NR Median
NR 99.9% acceptance
S2>100 PE
Below
NR med. ER
Accidental NR
Total bkg.
Data
Run8
2.0
0.33
0.09
2.4 ±0.8 2
Run9
1.2
0.84
0.35
2.4 ±0.7 1
2018/12/4
杨勇，上海交通大学

22. Final candidates in Run9
Gray: all
Red: below NR median
Green: below NR median and in FV
389 total candidates found in the FV
1 below NR median
Outside FV, edge events more likely to lose electrons, leading to S2 suppression
2018/12/4
杨勇，上海交通大学

23. Selected Data/Bkg in 1-D comparison
log(s2/s1) distribution
for all s1 bins
Run8+9
Good agreement between data and
background prediction
2018/12/4
杨勇，上海交通大学

24. CLs method for upper limit
Derive a upper limit on signal strength m, given a desired CL.
(1) Construct Likelihood function
binned
unbinned
(2) Construct test statistics (profiled likelihood ratio)
(3)
(4) Calculate two p-values
2018/12/4
杨勇，上海交通大学

25. CLs method for upper limit
Derive a upper limit on signal strength m, given a desired CL.
(5) Calculate
(6) Adjust m until CLs(m) = 0.05, for 95% CL , or 0.1 for 90% CL
2018/12/4
杨勇，上海交通大学

26. WIMP-nucleon SI upper limits
90% CL Spin-independent limits calculated with CLs method
X10 X2
2018/12/4
杨勇，上海交通大学

27. Comparison with LUX latest result
PandaX-II 33 ton –day, LUX 33.5 ton-day
2018/12/4
杨勇，上海交通大学

28. Summary and outlook
PandaX-II has reached the forefront of the DM search!
79.6 live-day of dark matter data with much suppressed background (152 mDRU)
In combination with commissioning run (19.1 day), ~33,000 kg-day exposure in total. No DM signals are observed
The WIMP-nucleon elastic scattering cross sections are constrained to 2.5x10-46 cm2 at 40 GeV DM mass.
Continue PandaX-II data taking till end of In preparation for PandaX-xT!
2018/12/4
杨勇，上海交通大学

29. 谢谢！
2018/12/4
杨勇，上海交通大学