Improved Measurement of
Electron Antineutrino Disappearance
at Daya Bay
Xin Qian
Caltech
For Daya Bay Collaboration
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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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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
Far/Near νe Ratio
Distances from
reactor
Detector Target Mass
Oscillation deficit
Detector efficiency
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Baseline Survey
Accurate Survey  reduced uncertainties in
reactor flux.
Detailed Survey:
- GPS above ground
- Total Station underground
- Final precision: 2.8 cm
By Total
station
N
Validation:
- Three independent calculations
- Cross-check survey
- Consistent with reactor plant
and design plans
By GPS
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Daya Bay Antineutrino Detector
Automated
calibration system
Prompt positron:
Carries antineutrino energy
Eprompt ≈ Eν – 0.8 MeV
Reflectors at top/bottom
of cylinder
Photomultipliers
Steel tank
Radial shield
Outer acrylic tank
Inner acrylic tank
Delayed neutron capture:
Efficiently tags antineutrino signal
6 “functionally identical”, 3zone detectors reduces
systematic uncertainties
Photosensors:
192 8”-PMTs
Energy resolution:  E  ( 7.5  0.9)%
E
E ( MeV )
(empirical)
total detector mass: ~ 110t
inner: 20 tons Gd-doped LS (d=3m)
mid: 20 tons LS (d=4m)
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outer: 40 tons mineral oil buffer (d=5m)
ACU (Caltech)
• Automated: weekly calibration
• Ge68, LED, Co60, Am-C
MC
MC
0.712 MeV in DYB
> 6 MeV in DYB
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LAB + Gd (0.1%) + PPO (3 g/L) + bisMSB (15 mg/L)
Number of protons: (7.169±0034)
× 1025 p per kg
Gd-loaded liquid scintillator shows
good stability with time
Detector Filling
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 ~15 m @ 430 nm.
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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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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Hall I: AD1/2 Comparison
Data taking began
Aug. 15, 2011
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Hall 2 and Hall 3
Hall 2: Began 1 AD operation
on Nov. 5, 2011
Hall 3: Began 3 AD operation
on Dec. 24, 2011
2 more ADs still in assembly;
installation planned for
2012
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Data Period
• ATwo Detector Comparison:
Sep. 23, 2011 – Dec. 23, 2011
Nucl. Inst. and Meth. A 685 (2012), pp. 78-97
A
Hall 1
• BFirst Oscillation Result:
Dec. 24, 2011 – Feb. 17, 2012
Phys. Rev. Lett. 108, 171803 (2012)
• CUpdated analysis:
Dec. 24, 2011 – May 11, 2012
B
C
Hall 2
To be submitted to Chinese Physics C
– Data volume: 40TB
– DAQ eff. ~ 96%
– Eff. for physics: ~ 94%
Hall 3
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PMT Light Emission (Flashing)
Flashing PMTs: ~ 5% of PMTs
- Easily discriminated with patterns
in charge and time.
Neutrinos
Flashers
2


MaxQ


2


log 10 Quadrant  


 0.45  

Quadrant = Q3/(Q2+Q4)
MaxQ = maxQ/sumQ
Relative PMT charge
Inefficiency to antineutrinos signal:
0.024%  0.006%(stat)
Contamination: < 0.01%
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Further suppressed by
background subtraction.
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AD Energy Calibration (abs/rel+F.V./Point)
F.V. =
Full Volume
6 MeV
Cut
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
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Rn219-Po215-Pb211(Alpha)
6.7+7.4 MeV
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Performance
Energy scale vs. Position
( AD1  AD 2)
Asy  2 
( AD1  AD 2)
LS region
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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
– Water Pool Muon:
• 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.
– AD Shower 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)
• 1 s after AD Shower muon (2.5
GeV).
• Alternative: ~0.4s after AD
Shower muon (3e5 PE~1.8
GeV)
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Ratio
Erec (MeV)
Spill In/Out
Asy
Neutron
thermalization
Δt (us)
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E (MeV)
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
  P(0, 400 R p )  P(0, 200 Rd )
P ( n,  ) 

n
n!
e
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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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
• H/Gd ratio checked with data
– Spallation Neutron
– AmC
Erec (MeV)
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Accidental Background
• Accidental Background: ~1.7% (Near) + ~ 4.6% (Far)
Racc  P (0, 200usR p )  P (1,199us  R p )  Rd  P (0, 200us  Rd )
 199us  R p  P (0,399usR p )  Rd  P (0, 200us  Rd )
 199us  R p  RdM
– 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 NuFact12
Black: All
triggers
Red:
isolated
triggers
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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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β-n decay:
9Li
uncorrelated
Eμ>4 GeV (visible)
Time since last muon (s)
Analysis muon veto cuts control
B/S to ~0.35% Near 0.2% Far NuFact12
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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 10-50 MeV
Analysis muon veto cuts control B/S
to 0.1% (0.13%) of far (near) signal
RPC tagged Fast N background
Validate with fast-n events
tagged by water pool.
Prompt Energy MeV
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Alpha-13C
neutrons
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)
232Th
• Alpha’s rate from data
• <0.02% (Near) + < 0.1% (Far)
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(1ms, 2ms)
238U
(10ms, 160ms)
227Ac
Total
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Background Budgets
Backgrounds
Near
Far + systematic
uncertainties
Accidental
ACU-N
Li9/He8
~1.7%
~0.03%
~0.35%
~4.6% + negligible sys.
~0.3% + 100% relative sys
~0.2% + 50% relative sys
Fast-N
Alpha-N
~0.13%
~0.02%
~0.1% + 30% relative
<0.1% + 50% relative sys
Really small background contamination!
Conservative estimation of systematics for
background contamination!
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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, 054615 (2011)
P. Huber, Phys. Rev. C84, 024617 (2011)
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Flux model has negligible impact on
far vs. near oscillation measurement
(1/20 reduction with ratio)
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IBD rate (/day)
IBD rate (/day)
Antineutrino Rate vs. Time
Detected rate
correlated with reactor
flux expectations.
800
600
400
800
EH2
D1 off
L1 off
600
400
IBD rate (/day)
Predicted (sin22q13 = 0)
Predicted (sin22q13 = 0.089)
Measured
EH1
100
L2 on
L4 off
L1 on
EH3
80
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Predicted Rate:
- Normalization is
determined by data fit.
- Absolute normalization
is within a few percent
of expectations.
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Dec 27
Jan 26
Feb 25
Mar 26
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Run time
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Uncertainty Summary
For near/far oscillation, only
uncorrelated uncertainties
are used.
Largest systematics are smaller
than far site statistics (~0.6%)
Influence of uncorrelated reactor
systematics significantly reduced
by far vs. near measurement.
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Prompt Energy Spectra
~ 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.
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Discovery of a non-zero value of q13
R = 0.940 ± 0.011 (stat) ± 0.004 (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 108 171803
(2012)
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Improved results
R = 0.944 ± 0.007 (stat) ± 0.003 (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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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:
sin 2q13  0.089  0.011
2
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Outlook
• 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.
Expect improvement in
Systematic uncertainties as well
• Y. F. Wang: Daya Bay II, WG1 25th
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Spill in/out
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