1. Xin Qian Caltech For Daya Bay Collaboration
Improved Measurement of Electron Antineutrino Disappearance at Daya Bay
Xin Qian
Caltech
For Daya Bay Collaboration
NuFact12

2. Design goal for systematic 0.2~0.4% (relative)
The largest, deepest reactor Θ13 experiment in Town, aimed for a precision measurement of sin22θ13 <0.01
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3. Key to reach 0.01
(Stat) Powerful reactors (17.6 GW) + Large Mass (80 ton)
(Sys) Reactor related: using near/far to form ratio + baseline (near ~0.4 km, far ~1.7 km)
(Sys) Detector related: “identical detectors” + “precise detector calibration”
(Sys) Background related: deep underground to reduce cosmic+ active/passive shielding
Distances from
reactor
Far/Near νe Ratio
Oscillation deficit
Detector efficiency
Detector Target Mass
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4. Accurate Survey  reduced uncertainties in reactor flux.
Baseline Survey
Accurate Survey  reduced uncertainties in reactor flux.
Detailed Survey:
- GPS above ground
- Total Station underground
- Final precision: 2.8 cm
Validation:
- Three independent calculations
- Cross-check survey
- Consistent with reactor plant
and design plans
By Total station
By GPS
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5. Daya Bay Antineutrino Detector
Automated
calibration system
Reflectors at top/bottom of cylinder
Prompt positron:
Carries antineutrino energy
Eprompt ≈ Eν – 0.8 MeV
Delayed neutron capture:
Efficiently tags antineutrino signal
Photomultipliers
Steel tank
Radial shield
Outer acrylic tank
Inner acrylic tank
Photosensors: ”-PMTs
Energy resolution: (empirical)
total detector mass: ~ 110t
inner: 20 tons Gd-doped LS (d=3m)
mid: 20 tons LS (d=4m)
outer: 40 tons mineral oil buffer (d=5m)
Three ACUs,
6 “functionally identical”, 3-zone detectors reduces systematic uncertainties
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6. ACU (Caltech) Automated: weekly calibration Ge68, LED, Co60, Am-C MC
0.712 MeV in DYB
> 6 MeV in DYB
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7. Detector Filling Detector target filled from GdLS in ISO tank.
LAB + Gd (0.1%) + PPO (3 g/L) + bis-MSB (15 mg/L)
Number of protons: (7.169±0034) × 1025 p per kg
Gd-loaded liquid scintillator shows good stability with time
Detector target filled from
GdLS in ISO tank.
Load cells measure
20 ton target mass
to 3 kg (0.015%)
A 1-m apparatus yielded attenuation length of ~ nm.
H/C ratio dominate uncertainy. Checmitry,
Xray + Combustion
Fill in pairs, within 2 weeks.
3 fluids filled simultaneously, with heights matched to minimize stress on acrylic vessels
Gadolinium-doped Liquid Scintillator (GdLS)
Liquid Scintillator (LS)
Mineral Oil (MO)
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8. Muon Veto System
Dual tagging systems: 2.5 meter thick two-section water shield and Resistance Plate Chambers (RPC)
Outer layer of water veto (on sides and bottom) is 1m thick, inner layer >1.5m. Water extends 2.5m above ADs
288 8” PMTs in each near hall
384 8” PMTs in Far Hall
4-layer RPC modules above pool
54 modules in each near hall
81 modules in Far Hall
Two Zone ultrapure water Cherenkov detector
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9. Hall I: AD1/2 Comparison
Data taking began
Aug. 15, 2011
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10. Hall 2 and Hall 3 Hall 2: Began 1 AD operation on Nov. 5, 2011
on Dec. 24, 2011
2 more ADs still in assembly;
installation planned for
2012
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11. Data Period ATwo Detector Comparison: Sep. 23, 2011 – Dec. 23, 2011
Hall 1
Hall 2
Hall 3 A B C
Data Period
ATwo Detector Comparison: Sep. 23, 2011 – Dec. 23, 2011
Nucl. Inst. and Meth. A 685  (2012), pp
BFirst Oscillation Result:
Dec. 24, 2011 – Feb. 17, 2012
Phys. Rev. Lett. 108, (2012)
CUpdated analysis:
Dec. 24, 2011 – May 11, 2012
To be submitted to Chinese Physics C
Data volume: 40TB
DAQ eff. ~ 96%
Eff. for physics: ~ 94%
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12. PMT Light Emission (Flashing)
Flashing PMTs: ~ 5% of PMTs
- Easily discriminated with patterns in charge and time.
Neutrinos
Flashers
Quadrant = Q3/(Q2+Q4) MaxQ = maxQ/sumQ
Relative PMT charge
Inefficiency to anti-neutrinos signal:
0.024%  0.006%(stat)
Contamination: < 0.01%
Further suppressed by background subtraction.
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13. AD Energy Calibration (abs/rel+F.V./Point)
6 MeV Cut
F.V. = Full Volume
Sources
Energy (MeV)
Co60 (ACU)
2.5 (abs)
AmC-n (ACU)
8.0
Ge68 (ACU)
1.0
Spallation-n (F.V.)
8.0 /2.2
IBD-n (F.V.)
8.0 / 2.2
K-40
1.3
Tl-208
2.6
Bi214-Po214-Pb210 (Alpha)
7.7 MeV
Bi212-Po212-Pb208(Alpha)
8.8 MeV
Rn219-Po215-Pb211(Alpha)
MeV
PMT gain calibration
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14. Performance
Energy scale vs. Position
LS region
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15. Inverse Beta Decay Selection: Muon Veto on delay signal:
Remove flashers
0.712 MeV prompt event
612 MeV delay event
1200 us time correlation
Multiplicity Cut
No prompt-like event 200us before prompt.
No prompt-like events between prompt and delay.
No prompt-like events 200 us after delay.
Also alternative methods.
Muon Veto on delay signal:
Water Pool Muon:
600 us after WP muon
AD Non-shower Muon:
1 ms after AD muon (20 MeV)
Alternative: 1.4 ms after AD muon (3000 PE~18 MeV)
AD Shower Muon:
1 s after AD Shower muon (2.5 GeV).
Alternative: ~0.4s after AD Shower muon (3e5 PE~1.8 GeV)
20 MeV AD muon, Shiwer 1.8 GeV
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16. Neutron thermalization
Ratio
Erec (MeV)
Spill In/Out
Spill/out RENO
Neutron thermalization
Asy
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Δt (us)
Erec (MeV)

17. Alternative Multiplicity Cut
Goal of multiplicity cut is to remove ambiguity in the prompt energy
One need to sync calculation of
Efficiency
Accidental Background
Livetime Calculation
Decoupled Multiplicity Cut (DMC)
No additional prompt-like events 400 us before delay.
No delay-like events 200 us after delay.
Fixed window Cut  Easy to calculate live time
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18. Study to understand Efficiencies
6 MeV Cut
Timing Cut
Erec (MeV)
Absolute Efficiency
Comparison with MC (6 MeV, Timing, Spill-in/out)
H/Gd ratio checked with data
Spallation Neutron
AmC
Why absolute efficiency is cancelled.
H/Gd Ratio
Erec (MeV)
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19. Accidental Background
Accidental Background: ~1.7% (Near) + ~ 4.6% (Far)
Multiple methods to calculate Rprompt to get systematic uncertainties
RdelayM is a direct measured quantity
Single Spectra are well understood (Po-210, K-40, Tl-208)
Cross check: coincidence vertex, off window coincidence
Black: All triggers
Red: isolated triggers
Single spectrum + single neutron signal, reason
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20. ACU Am-C ACU Am-C Correlated Background (GEANT4 MC)
Anchor Single Neutron rate with Data
~0.03% (Near) + ~0.3% (Far)
n-like singles
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21. β-n decay:
Eμ>4 GeV (visible)
9Li
uncorrelated
Time since last muon (s)
Muon + shower  Li9 and He8
Analysis muon veto cuts control B/S to ~0.35% Near 0.2% Far
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22. RPC tagged Fast N background
Fast neutrons
Fast Neutrons:
Energetic neutrons produced by cosmic rays
(inside and outside of muon veto system)
Mimics antineutrino (IBD) signal:
- Prompt: Neutron collides/stops in target
- Delayed: Neutron captures on Gd
Constrain fast-n rate using
IBD-like signals in MeV
Analysis muon veto cuts control B/S to 0.1% (0.13%) of far (near) signal
RPC tagged Fast N background
Muon interact of neutron
Validate with fast-n events
tagged by water pool.
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Prompt Energy MeV

23. Alpha-13C neutrons Alpha’s rate from data
Potential alpha source:
238U, 232Th, 235U, 210Po:
Each of them are measured in-situ:
U&Th: cascading decay of
Bi(or Rn) – Po – Pb
210Po: spectrum fitting
Combining (α,n) cross-section, correlated background rate is determined.
(1ms, 3ms)
238U
232Th
(10ms, 160ms)
Radioactive contamination alpha
227Ac
Alpha’s rate from data
<0.02% (Near) + < 0.1% (Far)
(1ms, 2ms)
Total
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24. Background Budgets Really small background contamination!
Backgrounds
Near
Far + systematic uncertainties
Accidental
~1.7%
~4.6% + negligible sys.
ACU-N
~0.03%
~0.3% % relative sys
Li9/He8
~0.35%
~0.2% + 50% relative sys
Fast-N
~0.13%
~0.1% + 30% relative
Alpha-N
~0.02%
<0.1% + 50% relative sys
Really small background contamination!
Conservative estimation of systematics for background contamination!
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25. Reactor Flux Expectation
Antineutrino flux is estimated for each reactor core
Flux estimated using:
Isotope fission rates vs. reactor burnup
Reactor operators provide:
- Thermal power data: Wth
- Relative isotope fission fractions: fi
Energy released per fission: ei
V. Kopekin et al., Phys. Atom. Nucl. 67, 1892 (2004)
Antineutrino spectra per fission: Si(Eν)
K. Schreckenbach et al., Phys. Lett. B160, 325 (1985)
A. A. Hahn et al., Phys. Lett. B218, 365 (1989)
P. Vogel et al., Phys. Rev. C24, 1543 (1981)
T. Mueller et al., Phys. Rev. C83, (2011)
P. Huber, Phys. Rev. C84, (2011)
simulation
Flux model has negligible impact on
far vs. near oscillation measurement
(1/20 reduction with ratio)
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26. Antineutrino Rate vs. Time
Detected rate
correlated with reactor
flux expectations.
Predicted Rate:
- Normalization is
determined by data fit.
- Absolute normalization
is within a few percent
of expectations.
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27. Uncertainty Summary For near/far oscillation, only
uncorrelated uncertainties
are used.
Largest systematics are smaller
than far site statistics (~0.6%)
Combined, total efficiency is 79%
Influence of uncorrelated reactor
systematics significantly reduced by far vs. near measurement.
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28. Prompt Energy Spectra Near Site data contains AD1/2 comparison period.
~ 82k
~ 29k
~ 204k
Erec (MeV)
Erec (MeV)
Erec (MeV)
Near Site data contains AD1/2 comparison period.
High-statistics reactor antineutrino spectra.
B/S ratio is 5% (2%) at far (near) sites.

29. Discovery of a non-zero value of q13
R = ± (stat) ± (syst)
sin22θ13=0.092±0.017
A clear observation of far site deficit
5.2 s for non-zero value of q13 with the first 55 days’ data
PRL (2012)
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30. Improved results R = 0.944 ± 0.007 (stat) ± 0.003 (syst)
Increase stat to 7 sigma
R = ± (stat) ± (syst)
sin22θ13=0.089±0.011
With 2.5x more statistics, an improved measurement to q13 Spectral distortion consistent with oscillation
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31. Summary Daya Bay Experiment is well on track.
Six functionally identical ADs enabled a rapid discovery of Θ13
Calibration/Construction of ADs well achieved designed systematic goal.
ADs are stable and backgrounds are well understood.
2 additional ADs are under construction (will deploy in this year).
The Daya Bay reactor neutrino experiment has made an unambiguous observation of reactor electron-antineutrino disappearance at ~2 km:
8 AD full configuration! Completion!
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32. Outlook Y. F. Wang: Daya Bay II, WG1 25th
Good progress in shape analysis.
Understanding the Energy non-linearity.
Measurement of Δm2ee ~ |Δm231|
Measuring sin22θ13 ~ 5% level
Measurement of absolute antineutrino Flux
Precise measurement of antineutrino energy spectrum.
Y. F. Wang: Daya Bay II, WG1 25th
Expect improvement in Systematic uncertainties as well
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33. NuFact12

34. Spill in/out
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