Daya Bay 実験における反電子ニュートリノ消失の観測中島 康博
Lawrence Berkeley National Laboratory
Kyoto University
April 27, 2012

Contents
Introduction
Neutrino mixing
Measurement of θ13
using reactor neutrinos
Daya Bay Experiment
Data Set for Oscillation
Analysis
Event selection
Background estimation
Oscillation Analysis Results

2

nniness of the mass, and thus we can make following approximations

Neutrino Mixing

vor eigenstate of neutrino, |⇥ ⌃ ( = e, µ, ⌅, s), can be express
The probability for ⌅ ⌅ ⌅⇥ oscillation is given as
ABC9#0175>#-3#131.-D8#)-#E1-37"t ⇤ L,
tates, |⇥
⌃
(i
=
1,
2,
3,
·
·
·
);
i
Neutrino flavor eigenstates and Mass eigenstates are mixed
2
m
⇥
i
2
2
⇤
Ei = pi + mi ⇤ pi +2, 2 (Ei E j )t
Mass eigenstate
Weak eigenstate.
|⇥ P(⌅⌃ =⌅ ⌅⇥ )U= sin⌃ ,sinii |⇥2⇤i2p

6#5(D537)1)57H(α = e, μ, τ)(i =21, 2, 3)i+#&&,0#.2he distance traveled and pi is themomentumof ⇤i . Since ⇤ is produce
MNS
mixing matrix

2p, all of⇤’s
masseigenstates
haveacommonmomentum.
Makingan
approximation
of
E
⇤
p
+
m
/2p andincluding theThus,
facto
i
Neutrinos change their flavor as they travel (neutrino
oscillation)ithat ⇤⇥ isobserved
after⇤
travels
distancethedistance
L
is
giv
the
probability
is formulated
asthe
Naturalinterferometerto
explorefundamental
natureofneutrinos
(
(
(
+
,
,
,
)
,
$%
$
%
Two neutrino
case:

2

2

2

m [eV ]L[km]2 ⇥ 1.27ipL

P (⇤ ⌅ ⇤⇥ ) = | ⌃⇤P(⌅
Usin⌅ ⌅|⇥ ) == sin 2⇤i U⇥i e⇥ |⇤(t)⌥i

22 m2i Li 2pe

E[GeV]⇥,⇤
θ: mixing angle ⇥
L
2
⇥
⇥
2
2
Δm
:
mass
squareddiﬀerence
2
2⇥
24
=
Re(U
1.27L ismthe
i U⇥i U j U⇥j ) sin andij fli
where
m
⇥
m
m
is
the
mass-squareddi⇥erence
L
[km]
:
the
distance
traveled
j
i
?K⇤ M
E
i>j
E (GeV) : the energy of neutrino
neutrino.
⇥⇤

disappearanceof ν⇥ ⇥together
2α their eigenvalues
2 L
If the neutrino
mass
statesmix
andIm(U i U⇥i U j U⇥j ) sin 2.54 mij+2?K⇤⇥M
that is ⇤ 0 and i>j
m2 0, neutrinos can change their flavor E
duringof νβan evidence for the fin
the observation of neutrino appearance
oscillation
gives
3

Mixing Parameters

θ23	
  ≈	
  45o
Atmospheric	  ν	
  Accelerator	
  ν

θ13	
  <	
  10o	  (as	  of	
  2011)
Short-­‐Baseline	
  Reactor	  ν	
  Accelerator	
  ν

θ13 was the only angle not firmly observed
CP-violating phase δ is also not known.

4

θ12	
  ≈	
  35o
Solar	  ν	
  Long-­‐Baseline	
  Reactor	
  ν

experimenta little eas2
parameters it is possible T2K
to produce
an approximate
rather difficult, as it contains three unknown
quantities(δ, θeasier
the sign of ∆m2
13 , andT2K experimenta little
to explain:νe )P (νµ →
s into the matter effect parameter a), as well as all the other oscillation constants,whic2→alreadyνe ) ≈ known
sin2 2θlimits
2θ23thP (νµtheve uncertainties in their measured values. However given
13 sin on
t is possible
to produce
an approximateof this formula that makesthe2θpointofcosth± α sin13 sin δ
Lookfor νe appearance
from νformµ beamment a little easier to explain:− NOvAα sin 2θ13 cos δ cos
Accelerator long-baseline experiments: T2K, MINOS,+ α2 cos2 θ223 sin2 2θ
1
2
2
2
P (νµ → νe ) ≈ sin 2θ13 sin 2θ23 sin ∆where α ≡ ∆m21 /∆m231
3
2KamLAND,
2
a
α 2θ
≡12∆m
∆mSuper-K± α sin 2θ13 sin δ coswhere
θ13 sinsin2212θ/∆m∆ ∆ = and23 sin31 and
31 L/4E.
2
showsthat sindata2θ13show<∼ th0
KamLAND,
andSuper-K
atmospheric− α sin 2θ13 cos δ cos θ13 sin 2θ12 sin
2θ
cos
∆
sin
2∆
23 which will be used in the T
2
shows
that
sin
2θ
13 <∼ 0.2, and of course the val
22
22∆ be used in thealso+ α cos θ23 sin 2θwhich
beexperiment
noted that thisexpa
12 sin will
T2K
all theoapproximation.
abe noted
that this expansion
sets the Looking
matter term
masshierarchy
2
2 Sensitive to θ13
2 , δ andalso∆m21 /∆m31 and ∆ = ∆m31 L/4E.approximation.
The existing limits
from
joint
to the
sola
the
last
onefits(whichisformuin fa
Lookingattheapproximate−2and Super-K
data show that
α
≈
3
×
10
+ solardat
Lookatmosphericfor νe disappearance
onneutrino
α2 andformisthe last one (which. isCHOOZ
inthisfactdepends
theapproximate
magnitudeof
its Bay,
currentup
in2 2θ13 <∼ Reactor
0.2, and basedof course
the valueof(~O(1)δ is completely
unknown.
For
the value
short-baseline
this depends
onkm)
α2 experiments:
and isthereforeDaya
unobservableinoscillatoryof νebe used in theRENO,
T2K experiment
all the othertermsofinitsthis
expansion
are appearance∼the1.first
It shoul
Double Chooz
magnitudecurrent
upper limit,
term
That
then
suggests
the p
2 to zero,2
2eT2K
d that thisPexpansion
sets
thematterterms
however
for
that
is
an
excelleninν
beam.
oscillatory
appearanceofνµ(⌫e ! ⌫e ) ⇡ 1 sin 2✓13 sin ( msitivity) at least an⌫by
31 L/4Etoθ
13
thedoesn’t
proper target
for future
on. Looking at the approximate formula,That
the then
onlysuggests
term that
dependon θ 13ex
experiments
producsitivityto θoscillations
an orderof should
magnitude,anSensitive
only to θ13form(which is in fact
the approximate
for theKamLANDsees),
bu
13 by at leastthat
nitude.
At
the
same
time
me
experiments
producesomeimprovementin oo
on α2 andBothis therefore
inT2K. areForshould
valuesof sin2θ
withinanorderkindsunobservableof experiments
neededtodetermine
13(inparticular,
thedeviation2
nitude.
At
the
same
time
measuring
θ
and
∆m
23
23 w
of its current upper limit,
first term dominates,
describesarelatively
simpl
CPtheviolationand masswhichhierarchy
as being
afrom
necessary
for
ex
(in
particular,
thedeviation
of
θ
π/4
is
a
m
23
5
ppearance of νe in νµ beam.
measurements.

How to Measure θ13

Measurement of θ13 withreactor neutrinos

SourceDetection

νe from n-rich fission products

inverse beta decay νe + p → e+ + n

observed spectrum

ea

r
ed
lat
lcu m
ca ectrusp
ctor~ 200 MeV per fission~ 6 νe per fission
mean energy of νe: 3.6 MeV~ 2 x 1020 νe/GWth-sec
6

Measurement of θ13 withreactor neutrinos
θ13	
  revealed	
  by	
  a	
  deﬁcit	
  of	
  reactor	
  anCneutrinos	
  at	
  ~2	
  km.

KamLAND
Previously
unknown

Measured7

Measurement of θ13 withreactor neutrinos
To measure a small deficit, reducing
systematic uncertainty is the key.
Put “identical” detectors at near and
far distances, and make a relative
measurements.
Uncertainties for absolute reactor
flux (largest error in previous
measurements) and absolutedetector efficiency cancel.
Far/Near	  νe	
  RaCoNear	
  detector(s)constrain	
  ﬂux
Far	
  detector(s)measure	
  oscillaCon

Distances	
  from	  reactor

Detector	
  Target	
  Mass8

OscillaCon	
  deﬁcit

Detector	
  eﬃciency

Indications of non-zero θ13
T2K
2011

PRL	
  107,	
  041801	
  (2011)

MINOS
PRL.	
  107,	
  181802	
  (2011)

Double	  Choozsin2(2θ13)=0.086	
  ±	
  0.041(stat)	
  ±	
  0.030(syst)Y.	
  Abe	  et	
  al.	
  PRL	  108	  131801	
  (2012)	
  Many hints of non-zero value of θ13 in 2011No θ13=0 exclusion with > 2.5σ significance before Daya Bay (and RENO)
9The Daya Bay Experiment10

Daya Bay Collaboration
Asia (20)
IHEP, Beijing Normal Univ., Chengdu
Univ. of Sci and Tech, CGNPG, CIAE,
Dongguan Polytech, Nanjing Univ.,
Nankai Univ., NCEPU, Shandong Univ.,
Shanghai Jiao Tong Univ., Shenzhen
Univ., Tsinghua Univ., USTC,
Zhongshan Univ., Univ. of Hong Kong,
Chinese Univ. of Hong Kong, National
Taiwan Univ., National Chiao Tung
Univ., National United Univ.

North America (16)Brookhaven Nat’l Lab, Cal Tech,
Cincinnati, Houston, Illinois
Institute of Technology,
Iowa State, Lawrence Berkeley
Nat’l Lab, Princeton, Rensselaer
Polytech, UC Berkeley, UCLA,
Wisconsin, William & Mary, Virginia
Tech, Illinois, Siena College

Europe (2)Charles Univ., Dubna
11~230 Collaborators

Location of Daya Bay

Daya Bay Nuclear
Power Station~55 kmHong Kong
International Airport
12

Daya Bay Reactor Complex
Ranked among the top 5 most powerful complexes in the world,
producing 17.4 GWth
Adjacent to mountains
Easy to construct tunnels to reach underground labs with sufficient
overburden to suppress cosmic rays
Ling Ao II NPP
2 × 2.95 GWth
Ling Ao NPP
2 × 2.95 GWthDaya Bay NPP
2 × 2.95 GWth
13

6 Antineutrino Detectors (ADs)in 3 underground halls

Far
Overburden: ~860 mwe
Weighted baseline: ~1650 m

Overburden: ~265 mwe
Weighted baseline: ~500 m

TunnelSurface
Assembly
Building(SAB)

Control
Building

Ling Ao
near
Water
Hall

Construction
tunnelLing Aoreactors

LS
Hall

Entrancem

Ling Ao IIreactors

Daya Baynear

Daya Bay
reactors
14

Overburden: ~250 mwe
Weighted baseline: ~360 m

Detection Method

Antineutrinos are detected via
inverse beta decay reaction~ 30 µsec
w/ 0.1% Gd

Edelayed ~ 8 MeV
Ee+ ~ Eν - 0.8 MeV

Prompt + Delayed coincidence
Prompt positron: carries antineutrino energy
Delayed neutron capture: efficiently tag
antineutrino signal.
15

Antineutrino Detector
Calibration
systemThree zone cylindrical modules:
Zone
Inner	  acrylic	
  vesselOuter	  acrylic	
  vesselStainless	
  steel	
  vessel

Mass
20	
  t

20	
  t

40	
  t

LiquidPurposeGd-­‐doped	
  AnA-­‐neutrino	
  liquid	  targetscinCllator
Gamma	
  Liquid	  catcher	  (from	
  scinCllator
target	  zone)Mineral	
  Oil	
  Stainless
Steel
Vessel (SSV)

RadiaCon	  shielding

192 8-inch
PMTs

Mineral oil
Liquid Scint.
20-t Gd-LS

5m

Reflectors at top/bottom of cylinder
are used to increase light yield.
Energy resolution: (7.5/√E + 0.9)%5m
16

Calibration System
Three Automated Calibration Units(ACUs) per detector.
Deploy sources along the z-axis
Three sources + LED in each ACU,
on a turn table.
68Ge (1.02 MeV)
Energy60Co (2.506 MeV)
calibration
241Am-13C (8MeV)
LED -- Timing and gain calibration｝

17

R=1.7725	
  m

R=0

R=1.35mMuon Tagging System
2.5 meter thick two-section
water shield
Cherenkov detector totag cosmic ray muons.
Shield for neutrons andgammas fromsurrounding materials.
RPC
Covers water pool toprovide further muon
tagging.
Detectors are incommissioning.
Not used for “muonveto” in the first
analysis, but used forbackground studies.
18Antineutrino Detector Assembly
Antineutrino Detectors are assembled at
the Surface Assembly Building

Stainless)Steel)Vessel)(SSV))in)assembly)pit)

Install)top)reﬂector)

Install)lower)reﬂector)

Close)SSV)lid)

Install)PMT)ladders)Install)Acrylic)Vessels)

19

Install)calibra=on)units)Liquid Scintillators

185-ton 0.1% Gd-LS storedin five 40-t tanks@ 430 nm

The (Gd-)LS is produced in theunderground LS hall at Daya Bay.
Gd (0.1%) + PPO (3 g/L)+
bis-MSB(15mg/L) + LAB
Produced 185-ton Gd-LS + 196-ton LS
Completed early 2011
Amount needed for all 8-detectors
are produced at the same time toensure identical detectorsA	
  1-­‐m	
  apparatus	
  yielded	
  aaenuaCon	
  lengthof	
  ~15	  m	  @	
  430	  nm	  (Gd-­‐LS)

20

Detector Filling
The detectors are filled withliquid in the underground liquid
scintillator hall.
Target mass is measured with:
ISO
tank

Monitored with in-situ sensors
Internal	
  camera	
  (during	
  ﬁlling)

21

GdLS tank

Coriolismass flow
meters(1) 4 load cells supporting 20-ton ISO tank(2) Coriolis mass flow meters

Uncertainty ~ 3kg in 20t (0.015%)

LS/MO tank

Near Hall (EH1) Installation
Install filled AD in pool

Fill pool with purified water

Data taking started on 15 Aug 2011

Roll RPC over cover

Place cover over pool
22

Far Hall (EH3)
Installation

23

My work at Daya Bay
In the past year (from April 25, 2011 to April 25, 2012)
Stayed in China for ~150 days
Assembling and testing of the PMT systems of Antineutrino
Detectors
Monitoring Liquid Scintillator optical properties.
Commissioning of the antineutrino detectors in theexperimental halls.
Analyses
Developing neutrino interaction vertex reconstruction
Studying energy scale calibration non-uniformity andsystematic uncertainty
Independent oscillation analysis within LBNL group24

Data Set for
Oscillation Analysis

25

Data Taking Period
Two Detector Side-by-Side Comparison
Sep. 23, 2011 - Dec. 23, 2011
Side-by-side comparison of 2
detectors in EH1Demonstrated detector systematics
better than requirements.

EH1

Daya Bay Collab.
arXiv:1202:6181 (2012)

Current Oscillation Analysis
Dec. 24, 2011 - Feb. 17, 2012
All 3 halls (6 ADs) operating
DAQ uptime > 97%
Antineutrino data: ~89%

EH2

Daya Bay Collab.
Phys. Rev. Lett. 108, 171803 (2012)26

EH3

Data Analysis Approach
Multiple independent analyses are performed to cross-check the results
Difference between analyses
Energy calibration & reconstruction
Calibration source (60Co, 2.5 MeV)
Spallation neutron (8-MeV Gd-capture peak)
Antineutrino selection
Different muon vetoDifferent multiplicity cut
Background studies
Oscillation fit method
Blind analysis
Target mass, baselines and reactor flux information were blinded.
Cross-checked between analyses before unblinding.
All analysis gave a consistent results

Only results from one analysis are presented here
27

Energy Calibration
PMT Gain(PE/ADC)PMT charge vs. time(by low intensity LED)

Energy scale(MeV/PE)
Energy Calibration

Non-uniformity
correction
Energy vs. position

60Co at center

Energy resolution

28

PMT Light Emission (Flashing)
Instrumental background from ~5% of PMTs.
‘Shines’ light to opposite side of the detector
Easily discriminated from normal signals
If we don’t apply the flasher
rejection, This is the dominant
event near “8-MeV”
Relative PMT charge

Quadrant = Q3/(Q2+Q4)
MaxQ = maxQ/sumQ29
the hottest PMT is locatedInefficiency to antineutrinos signal:0.024% ± 0.006%(stat)
Contamination: < 0.01%

Energy Spectrum of All Events
Singles spectrum after muon
veto and flasher cut

Measured rate:~65 Hz in each detector(>0.7 MeV)
Dominated by low-energy
radioactivity
U/Th chains: Stainlesssteel, PMT, scintillator
40K: PMT
Radon: Scintillator
Around “8-MeV”:
IBD neutron
Muon spallation products
Am-C neutron in ACU
positron from IBD

Daya Bay Collab.
arXiv:1202:6181 (2012)

asymmetry

Task: Select IBD from these30

Antineutrino Selection
Prompt + Delayed coincidence
Prompt: 0.7 MeV < Ep < 12 MeV
Delayed: 6.0 MeV < Ed < 12 MeV
Capture time: 1 µs < Δt < 200 µs
Reject flashers
Muon veto
Pool muon: Reject 0.6 ms
AD Muon (>20 MeV): Reject 1 ms
AD Shower Muon (>2.5 GeV):
Reject 1s
Multiplicity :No other signal > 0.7 MeV
in -200 µs to 200 µs of IBD
Clear separation of antineutrino
events from backgrounds
31

Signal region

Prompt and Delayed Energy
0.01
Po215 alpha

0.008
Asymmtry w.r.t. AD1

Uncertainty of prompt energy
cut efficiency: negligible
Uncertainty of delayed energy
cut efficiency: 0.12%
Largest detector related
uncertainty.

0.006

Po214 alpha0.004

Spa n-Gd0.002
0-0.002Asym = (EAD1 – EADn)/<E>-0.004-0.006-0.008-0.01

32

IBD n-Gd

Po212 alpha‘Intrinsic’	
  energy	
  peak	  variaCon:	
  ~0.3%1

2

3
4
AD number

56Spacial Distribution of
IBD candidates
Prompt signal

IBD candidates are
uniformly distributed
in Gd-LS volume

3m-IAV (GdLS)

Delayed signal

33

4m-OAV (LS) EH1 AD1

Neutron Capture Time
Consistent IBD capture time measured in all detectors
Capture time cut:
1μs to 200μs

Relative H/Gd capture efficiencies between ADs differ by less than 0.1%
34

Backgrounds
Uncorrelated background
Accidentals: two uncorrelated events accidentally happened withshort time separation and mimic IBD event.
Correlated (from largest to smallest)
Muon spallation products
β-n decay isotopes (9Li/8He)
Fast neutrons
Correlated signal from 241Am-13C calibration source
13C(α,n)O reaction

35

Backgrounds: Accidental
The largest background
Rate and spectrum can be accurately measured from data
data

data-­‐taking

EH1-AD1 EH1-AD2 EH2-AD1 EH3-AD1 EH3-AD2 EH3-AD3
Accidental 9.82±0.06 9.88±0.06 7.67±0.05 3.29±0.03 3.33±0.03 3.12±0.03rate(/day)
B/S

1.37%

1.38%

1.44%
36

4.58%

4.77%

4.43%

Background: β-n decay
Generated by cosmic rays
Long lived
Mimics antineutrino signal=	
  178	
  ms,	  Q	  =	  13.	
  6	
  MeV
=	
  119	
  ms,	  Q	  =	  10.6	
  MeV

9Li:	
  τ
½
8He:	
  τ
½

Eμ	
  >	
  4	
  GeV	  (visible)

Analysis	
  muon	  veto	  cuts	  control	
  B/S	  to	
  ~0.2%	
  (0.4%)	
  of	
  far	  (near)	
  signal

Time since muon37

Background: Fast Neutron
Energetic neutrons produced bycosmic-rays
Mimic an IBD signal:
Prompt: Neutron recoils/stops
in target
Delayed: Neutron capture on Gd

Constrain fast-n rate using
IBD-like signals in 10-50 MeV

Validate with fast-n events
tagged by muon veto.

Aqer	
  muon	  veto	  cuts,	
  B/S	
  is	
  	
  	
  	
  	
  
~0.06%	
  (0.1%)	
  of	
  far	  (near)	
  signal.	
  	
  

38

241
13
Background: Am- C neutrons
Weak (0.5 Hz) neutron source in ACU can also mimic IBD via inelastic
scattering and capture on iron.

EsCmaCon	  Procedure:	  -­‐	
  Measure	
  uncorrelated	
  gamma	  rays	
  from	  ACU	  in	
  data	  -­‐	
  EsCmate	
  raCo	  of	
  correlated/uncorrelated	
  rate	  using	
  simulaCon	
  -­‐	  Assume	
  100%	
  uncertainty	
  from	  simulaCon

Constrain	
  far	
  site	  B/S	  to	
  0.3±0.3%	
  39

Background:
Potential alpha source:
238U, 232Th, 235U, 210Po:
Each of them are measured in-situ:
U&Th: cascading decay ofBi(or Rn) – Po – Pb
210Po: spectrum fitting

Example alpha rate
in AD1

238U

232Th

235U

210Po

Bq

0.05

1.2

1.4

10

Near Site: 0.04±0.02 per day,Far Site: 0.03±0.02 per day,B/S (0.006±0.004)%
B/S (0.04±0.02)%
40

Combining (α,n) cross-section,correlated background rate isdetermined from MC.

Data Summary
Antineutrino candidates
DAQ live time (day)

AD1

AD2

AD3

AD4

AD5

AD6

28935

28975

22466

3528

3436

3452

49.5530

49.497148.9473

Efficiency
Accidentals (/day)

0.80199.82±0.06

0.79899.88±0.06

0.83637.67±0.05

0.95473.29±0.03

0.95433.33±0.03

0.95383.12±0.03

Fast neutron (/day)

0.84±0.28

0.84±0.28

0.74±0.44

0.04±0.04

0.04±0.04

0.04±0.04

8He/9Li (/day)

Am-C corr. (/day)13C(α, n)16O (/day)
Antineutrino rate (/day)

3.1±1.6
0.04±0.02

0.04±0.02

714.17±4.58

717.86±4.60

1.8±1.1

0.16±0.11

0.2±0.2
0.035±0.02
0.03±0.02
532.29
±3.82

71.78
±1.29

0.03±0.02

0.03±0.02

69.80
±1.28

70.39
±1.28

Consistent antineutrino rate for the detectors in the same hall
Uncertainty is currently dominated by statistics
41

Oscillation Analysis

42Flux Prediction
Reactor neutrino spectrum:

Reactor operators provide:
Thermal power data: Wth
Relative isotope fission fraction 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)43

IBD rate (/day)

IBD rate (/day)

Daily Antineutrino Rate
D2 off

800

D2 on

600
EH1

400
Sep28

Oct28

Dec27

Jan26

L1 off

700
600

EH2

500
400

IBD rate (/day)

Nov27

80

L3 on

EH3

60
40

L2 off

L2 on

L1 on

Predicted
MeasuredSep 28

Oct 28

Nov 27

Dec 27

Jan 26
Run time
44

Predicted rate:
Assumes nooscillation
Normalization isdetermined by fit to
the data.
Measured rate tracks thereactor power variation.

Prompt (Positron) Spectra
EH1

EH2

22466	
  signal
candidates

57910	
  signal
candidates

B/S	
  @	
  EH1/2

B/S	
  @	
  EH3

Accidentals

~1.4%

~4.5%

Fast	
  neutrons

~0.1%

0.06%

8HE/9Li

~0.4%

~0.2%

Am-­‐C~0.03%

~0.3%

α-­‐n

~0.01%~0.04%

Total

~2%

~5%

EH3

10416	
  signal
candidates

45

Reactor
Correlated
Energy/fission
IBD reaction/fission
Combined

Uncorrelated
0.2%
Power
3% Fission fraction
Spent fuel3%
Combined

0.5%
0.6%
0.3%
0.8%

46
TABLE III. Summary of absolute efficiencies, and correlated
and

IBD rate (/day)

D2 off
For800near/far oscillation
D2 onanalysis,the correlated
600
uncertainties
cancel.
EH1
400
Only uncorrelated
uncertainties are used.
Sep28

Oct28

Nov27

700

EH2
600
Largestsystematics
are500
smallerthan far sitestatistics
(~1%)
400
L3 on

IBD rate (/day)

Detector
Efficiency
Correlated Uncorrelated
Target Protons
0.47%
0.03%
Flasher cut
99.98%
0.01%
0.01%
Delayed energy cut
90.9%
0.6%
0.12%
Prompt energy cut
99.88%
0.10%
0.01%
Multiplicity cut
0.02%<0.01%
Capture time cut
98.6%
0.12%
0.01%
Gd capture ratio
83.8%
0.8%
<0.1%
Spill-in
105.0%
1.5%
0.02%
Livetime
100.0%
0.002%<0.01%
Combined
78.8%
1.9%
0.2%

IBD rate (/day)

Systematic Uncertainties

80

L2 off

EH3

Predicted
Influenceof uncorrelated
60
Measuredreactor systematics
further
40 reduced by making
Sep 28
Oct 28 Nov 27
near vs. far measurement.

FIG. 3. Daily average measured IBD r

Compare the He
far/nearbackgrounds were sitemeasured rate and spectra.
same hall and uncorrelated among different halls, wethe best-fit value unchanged while the systematic un-

4 shows the measured numbers of events in each de. The
Mn: measured rates after backgroundrate deficit is obvious for EH3 in comparison with thesubtraction and efficiency correction.
. The

Entries / 0.25MeV

Far vs. Near Comparison
Near halls (weighted)

600

200

Far / Near (weighted)

Clear observation of far site
deficitSpectrum distortion
consistent with oscillation

Far hall400

αi,βi : Weights determined frombaselines and reactor fluxes.
is shown in the

R = 0.940 ± 0.011(stat) ± 0.004(sys)

800

47

0
No oscillation
Best Fit

1.2
1
0.8
0

5

10
Prompt energy (MeV)100.0%78.8%

0.002%
1.9%<0.01%
0.2%

IBD rate (/day)Livetime
Combined

Rate Analysis
ReactorCorrelated
Energy/fission
IBD reaction/fission

Uncorrelated
0.2%
Power
3% Fission fraction
Spent fuel3%
Combined

0.5%
0.6%
0.3%
0.8%

80

EH3

Predicted

6040

Sep 28

MeasuredOct 28

Nov 27

Dec 27

Jan 26
Run timesin2(2θ13) = 0.092 ± 0.016(stat) ± 0.005(sys)Combined

1.15

35

1.1
1.05
1
EH1
0.95
0.9
0

19] (3.1% uncertainty) had little impact
5σ on the results. The
30
thermal energy released per fission is given in Ref. [20]. Non25
equilibrium corrections
for long-lived isotopes were appliedfollowing Ref. [18].
20 Contributions from spent fuel [21, 22](∼0.3%) were included
as an uncertainty.
3σ
15
Thermal-power data provided by the power plant carry an
10
σ [23–25]. The fisuncorrelated uncertainty of 0.5% per1core
5
sion fractions were also provided for each fuel cycle as a func0 a ∼5% uncertainty from validation of thetion of burn-up, with
0
0.05
0.1
0.15
2model was constructedsimulation [26, 27]. A DRAGON [28]sin 2θ
to study the correlation among the fission13rates of isotopes.
The uncertainties of the fission fraction simulation resulted
in a 0.6% uncorrelated uncertainty of the ν e yield per core.
EH2
The
baselines have been surveyed with GPS and Total Station to a precision of 28 mm. The uncertainties in the baseline and the spatial distribution of the fission fractions in thecore had a negligible effect to the results. Fig. 3 presents the
EH3
background-subtracted and efficiency-corrected
IBD rates inthe three EHs. Relative reactor flux predictions are shown forcomparison.
The ν e0.8rate in thepredicted
a weighted
0.6
1 far hall
1.2 was1.4
1.6with 1.8
2
combination of the two near hall measurements assuming no
Weighted Baseline [km]oscillation. The weights were determined by the thermal
power of each reactor and its baseline to each AD. We observed a deficit in the far hall, expressed as a ratio of observed
48
to expected events,

χ2

N detected / Nexpected

TABLE III. Summary of absolute efficiencies, and correlated anduncorrelated systematic uncertainties.

0.2

0.4

FIG. 3. Daily average measured IBD rates per AD in the three eperimental halls as a function of time. Data between the two vertic
dashed lines were used in this analysis. The black curves represe
no-oscillation predictions based
13on reactor flux analyses and detect
simulation for comparison. The predictions have been corrected wi
the best-fit normalization parameter in determining sin2 2θ13 .

Value of θ extracted byusing measured rates ineach detector.
The value of sin 2θ was determined with a χ co2
Uses
structed
with pullstandard
terms accounting χ
for the correlation of thsystematic errors [29],approach.
%
&
"
#
$
2

2

χ =

6
!
Md − Td 1 + ε +
d=1

+

d
r ωr αr + εd

Md
(
2

6 ' 2
!
η
εd
r
+
+ 2d
2
2
σr
σd
σB
d=1

! α2
r

2

13

+ ηd

2

,

(2

where Md are the measured IBD events of the d-th AD wibackgrounds subtracted, Td is the prediction from neutrin
flux, MC, and neutrino oscillations [30], ωrd is the fractioof IBD contribution of the r-th reactor to the d-th AD dete
mined by baselines and reactor fluxes. The uncertainties a
listed in Table III. The uncorrelated reactor uncertainty is σ
(0.8%), σd (0.2%) is the uncorrelated detection uncertaintand σB is the background uncertainty listed in Table II. Th
corresponding pull parameters are (αr , εd , ηd ). The detecto
and reactor-related correlated uncertainties were not include
in the analysis; the absolute normalization ε was determine
from the fit to the data. The best-fit value isConsistent results
obtained by independent
analyses, different reactor
flux models

sin2(2θ13) = 0
excluded at 5.2σ2Current Landscape
RENO also
released theirresults (~ a monthafter us)arXiv:1204.0626v2

49

Summary
The Daya Bay reactor neutrino experiment has observed reactor electron antineutrino disappearance at ~2 km:

R = 0.940 ± 0.011(stat) ± 0.004(sys)Interpretation of disappearance as neutrino oscillation yields:sin2(2θ13) = 0.092 ± 0.016(stat) ± 0.005(sys)sin2(2θ13) = 0 excluded at 5.2σ
Of course, this is not an end for us....
Improved analysis including spectrum shape information is in progress
Install remaining the final pair of antineutrino detectors this summer.
Continue making precision measurement of θ13 with (much) more statistics.
50