1. science
2. physics

Recent Results
from the Daya Bay Experiment
Bei-Zhen Hu
On behalf of the Daya Bay collaboration
2015/3/30
March 30th, 2015 @ National Taiwan University
1
Neutrino Oscillation
1968 - Homestake (Raymond Davis Jr. and John N. Bahcall)
The first experiment to detect the electron neutrino from Sun
What’s
wrong???
Only 1/3 of prediction
Neutrino
oscillation
John N. Bahcall
Pontecorvo
1998
Super-Kamiokande Collaboration
announced the first evidence of neutrino
oscillations, consistent with the theory that
the neutrino has non-zero mass.
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Phys. Rev. Lett. 89, 011301 (2002)
Phys. Rev. Lett. 92, 181301 (2004)
2
Neutrino Mixing
The neutrino flavor eigenstates are linear combination of the mass eigenstates
3
n a = åUai n i
How they interact
Flavor states
i=1
How they propagate
Mass states
Pontecorvo–Maki–Nakagawa–Sakata matrix (PMNS matrix)
æ
0
ç 1
U PMNS = ç 0 cosq 23
çç
è 0 -sinq 23
öæ cosq
0
13
֍
sinq 23 ֍
0
ç
÷
id
cosq 23 ÷øç -e sinq13
è
θ23 ≈ 45o
Atmospheric ν
Accelerator ν
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ö
0 e-id sin q13 ־ cosq12
ç
÷
ç -sinq12
1
0
֍
0
cosq13 ֍
0
øè
θ13 < 10o (2003, CHOOZ)
Short-Baseline Reactor ν
Accelerator ν
sin q12
cosq12
0
0 ö
÷
0 ÷
÷
1 ÷ø
θ12 ≈ 35o
Solar ν
Long-Baseline Reactor ν
3
Detection Strategy
1) Accelerator-based experiments:  Principle: look for ne appearance in a nm beam.
ne?
nm
(Far Detector can be
slightly off-axis)
Near Detector
Neutrino beam
Far Detector
 Need to use much longer baselines (hundreds of kms).
 There are currently three experiments: MINOS, NOnA and T2K.
2) Reactor-based experiments:
 Principle: look for electron anti-neutrino disappearance
ne
Near Detector
Far Detector
 There are currently three experiments: Daya Bay, Double CHOOZ, and RENO.
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4
Reactor-Based Experiment
Antineutrino survival probability:
æ Dmee2 L ö
æ 2 ö
4
2
2 Dm21L
P n e ® n e = Pee =1- sin 2q13 sin ç
÷ - cos q13 sin 2q12 sin ç
÷
è 4E ø
è 4E ø
(
)
2
sin 2 (Dmee2
2
L
2 L
2 L
) º cos2 q12 sin 2 (Dm31
) + sin 2 q12 sin 2 (Dm32
)
4E
4E
4E
2
Dm31
º m32 - m12
2
Dm32
º m32 - m22
2
2
Dm31
~ Dm32
2
Dm31
= 2.32 ´10 -3 eV 2
P. Adamson et al. (MINOS Collaboration),
Phys. Rev. Lett. 106, 181801(2011).
2
Dm21
º m22 - m12 = 7.59 ´10-5 eV 2
sin 2 2q12 = 0.861+0.026
-0.022
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θ13 revealed by deficit of reactor antineutrinos at ~2 km.
5
Keys to a precise measurement of θ13
 Baseline Optimization
 High statistics: powerful reactors, larger detectors, long run-time
 Reduction of systematic errors:
æ
öæ
– Reactor-related: Far/ near relative measurement
– Detector-related: functionally identical detectors
Nf
N p, f Ln ö æ e f öé Psur (E, L f , q13 ) ù
÷ç ÷ ç ÷ê
=ç
ú
N n çè N p,n ÷øçè L f ÷ø è e n øë Psur (E, Ln , q13 ) û
2
 Low background
Deeper overburden (m.w.e):
250 (EH1), 265(EH2), 860(EH3)
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6
Daya Bay Experiment
North America (17)
Brookhaven Natl Lab, CalTech, Illinois Institute
of Technology, Iowa State, Lawrence Berkeley
Natl Lab, Princeton, Rensselaer Polytechnic,
Siena College, UC Berkeley, UCLA, Univ. of
Cincinnati, Univ. of Houston,
UIUC, Univ. of Wisconsin, Virginia Tech,
William & Mary, Yale
Europe (2)
Charles University, JINR Dubna
South America (1)
Catholic Univ. of Chile
Asia (21)
National Chiao Tung Univ., National Taiwan Univ., National United Univ.
Beijing Normal Univ., CGNPG, CIAE, Dongguan Polytechnic, ECUST, IHEP, Nanjing Univ., Nankai Univ.,
NCEPU, Shandong Univ., Shanghai Jiao Tong Univ., Shenzhen Univ., Tsinghua Univ., USTC, Xian Jiaotong
Univ., Zhongshan Univ., Chinese Univ. of Hong Kong, Univ. of Hong Kong,
An International Effort: 230 Collaborators from 40 Institutions
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7
The Daya Bay Experiment
EH3
EH2
• 17.4 GWth power
• 8 operating detectors
• 160 t total target mass
LA2
LA1
EH1
DYB
DYB: Daya Bay
LA1: Ling Ao 1 power plant
LA2: Ling Ao 2 power plant
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DYB Near Hall (EH1):
363 m from DYB
98 m overburden
LA Near Hall (EH2):
481 m from LA1
526 m from LA2
112m overburden
Far Hall (EH3):
1615 m from LA1
1985 m from DYB
350 m overburden
8
Detection Method
Inverse β-decay (IBD):
n e + p ® e+ + n
|+H ® D + g
|+Gd ® Gd * ® Gd + g 's
2.2 MeV 200 m s
8MeV 30 m s
Prompt positron:
Carries antineutrino energy
Ee+ ≈ Eν – 0.8 MeV
Delayed neutron capture:
Efficiently tags antineutrino signal
Prompt + Delayed coincidence provides distinctive signature
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9
Antineutrino Detector (AD) Design
3 zone cylindrical vessels
Liquid
Mass
Function
3m
Inner
acrylic
Gd-LS
20 ton
Antineutrino
target
4m
Outer
acrylic
LS
20 ton
Gamma
Catcher
5m
Stainless
steel
MO
40 ton
Radiation
shielding
192 8” PMTs in each detector
Top and bottom reflectors
3 Automatic calibration units (ACU)
Gd-LS: 0.1% Gadolinium-loaded liquid scintillator
LS: liquid scintillator
MO: mineral oil
PMT: Photomultiplier tubes (Hamamatsu)
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10
Anti-neutrino
detectors
ADs
were
assembled in clean-room
Stainless Steel Vessel (SSV)
in assembly pit
Install top reflector
Close SSV lid
Install lower reflector
Install PMT ladders
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Install
Acrylic Vessels
11
Install calibration units
Muon Veto System
Dual veto systems: 2.5 meter thick two-zone water cherenkov and RPCs
• Water Cherenkov detector:
• Veto cosmic ray muons
• Shield neutrons and gammas from rock
• 4-layer RPC:
• Covers water pool to provide muon tagging.
• Goal efficiency: > 99.5% with
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uncertainty
<0.25%
Two-zone ultrapure water cherenkov detector
12
Fill pool with purified water (~1 wk)
Near Hall (EH1) Installation
Install filled AD1 and AD2 in pool
Data taking started on 15 Aug 2011
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Roll RPC over cover
Place cover over pool
13
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14
Detector Calibration
Automatic Calibration Units (ACUs)
• Three different Z axis:
A. Central (R=0)
B. GdLS edge (R=1.35 m)
C. LS (R=1.7725 m)
• 3 sources for each z axis
1. LED diffuser ball (500 Hz) for time calibration
2. 10 Hz 68Ge (0 KE e+ = 20.511 MeV ’s)
3. 0.75 Hz 241Am-13C neutron source (3.5 MeV n without
) + 100 Hz 60Co gamma source (1.173+1.332 MeV s)
• Temporary special calibration sources:
1. : 137Cs (0.662 MeV), 54Mn (0.835 MeV), 40K (1.461 MeV)
2. n: 241Am-9Be, 239Pu-13C
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15
Overview of the Energy Response Model
True energy Etrue to reconstructed kinetic energy Erec
Not discussed in this talk
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• Minimal impact on oscillation measurement
• Crucial for measurement of reactor spectra
16
Energy Nonlinearity Calibration
(Beta+gamma)
Daya Bay
Daya Bay
Daya Bay
• The semi-empirical model has been constructed
and checked with various sources.
• Energy model is constrained with gamma and
electron sources.
~1% uncertainty (correlated among detectors)
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17
Antineutrino Candidate Selection
Inverse Beta Decay ( IBD ): n e + p ® e+ + n
|+H ® D + g
|+Gd ® Gd * ® Gd + g 's
nH
nGd
Reject PMT Flashers*
Water Pool
0.4 μs
0.6 μs
AD (>20 MeV)
0.8 μs
1 μs
AD shower
(>2.5 GeV)
1s
1s
Prompt Energy [MeV]
[1.5, 12]
[0.7, 12]
Delayed Energy [MeV]
3σ
[6, 12]
(σ~0.14MeV)
Capture time [μs]
[1, 400]
[1, 200]
e+ n Distance Cut [mm]
500
N/A
Muon
veto
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nH
2.2 MeV 200 m s
8MeV 30 m s
nGd
18
Backgrounds
Backgrounds
Uncorrelated
Accidental (from singles) :
1.4 %(near)/2.3%(far)
Fast neutron:
Cosmic Muon
0.4 %(near)/0.4%(far)
9Li/8He:
0.03 %(near)/0.2%(far)
Correlated
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Calibration
source
AmC (n-Fe):
0.1 %(near)/0.1%(far)
Intrinsic
radioactivity
13C(α,n)16O
:
0.01 %(near)/0.1%(far)
Mimic Process
Prompt
Delayed
Any two triggers within the time window
Neutron
scatters/stops in
target
β-decay
Neutron inelastic
scattering
Neutron
captures
Neutron scatters/
16O* decay with e+e19
Antineutrino Rate vs. Time
Collected more than 1 million antineutrino events
• 621 days of data, including more than one year in full 8-AD configuration
• 4 times more statistics than previously published result
• Detected rate strongly correlated with reactor flux expectations
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20
Far vs. Near Comparison
(nGd analysis new preliminary, data set: 24-Dec-2011 to 30-Nov-2013 )
The observed relative rate deficit and relative spectrum distortion are
highly consistent with oscillation interpretation
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21
Results of Oscillation Parameter
(nGd analysis new preliminary, data set: 24-Dec-2011 to 30-Nov-2013 )
• Most precise measurement of sin22θ13, precision reached<6%
• Most precise measurement of in the electron neutrino disappearance channel
sin 2 2q13 = 0.084+0.005
-0.005
-3
2
| Dmee2 |= 2.44+0.10
´10
(eV
)
-0.11
c 2 / NDF = 134.7 /146
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22
Results of Oscillation Parameter
(nGd analysis new preliminary, data set: 24-Dec-2011 to 30-Nov-2013 )
sin 2 2q13 = 0.084+0.005
-0.005
-3
2
| Dmee2 |= 2.44+0.10
´10
(eV
)
-0.11
c 2 / NDF = 134.7 /146
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23
Results of nH Analysis
Phys. Rev. D 90, 071101 (2014)
Benefits: Larger target mass, independent statistics
Challenges: Greater accidental background and more energy leakage
10
104
8
103
6
After accidental background subtraction_1
12
103
10
8
102
6
Delayed Energy [MeV]
10
5
12
103
10
8
102
6
102
4
4
2
0
0
2
4
6
8
10
12
Prompt Energy [MeV]
10
2
1
0
0
10
4
10
2
2
4
6
8
10
12
Prompt Energy [MeV]
1
0
0
2
4
6
8
10
12
Prompt Energy [MeV]
Accidental background made by singles_1
40000
Delayed Energy [MeV]
Delayed Energy [MeV]
12
Delayed Energy [MeV]
After Distance Cut. Before Accidental Subtraction
Before Distance Cut
35000
30000
25000
20000
nGd (6~12 MeV) signals
Ep <3MeV and Ed <3MeV
nH (Ep >3.5 MeV) signals
15000
10000
5000
0
0
500 1000 1500
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2000 2500 3000 3500 4000 4500 5000
Distance [mm]
12
103
10
8
102
6
4
10
2
0
0
2
4
6
8
10
12
Prompt Energy [MeV]
1
24
1
Results of nH Analysis
Phys. Rev. D 90, 071101 (2014)
Benefits: Larger target mass, independent statistics
Challenges: Greater accidental background and more energy leakage
Best fit:
sin 2 2q13 = 0.083± 0.018, c 2 / ndf = 4.6 / 4
Consistent with:
Combined with previous 6AD Gd
capture results ( sin 2 2q13 = 0.089 ± 0.009 )
Phys. Rev. Lett. 112, 061801 (2014)
sin 2 2q13 = 0.089 ± 0.008
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25
Sterile Neutrino Search
Phys. Rev. Lett. 113, 141802 (2014)
Survival Probability for 3+1:
Illustration
2015/3/30
æ Dmee2 L ö
æ 2 ö
2
2 Dm41 L
P(n e ® n e ) @1- cos q14 sin 2q13 sin ç
÷ - sin 2q14 sin ç
÷
è 4En ø
è 4En ø
4
2
2
26
Sterile Neutrino Search
Phys. Rev. Lett. 113, 141802 (2014)
• Result from 217 days of 6AD data
• No significant signal observed
2
• Set most stringent limit at 10-3 eV2 < Dm41
< 0.1 eV2
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27
Summary
• Daya Bay has new preliminary measurements
from 24-Dec-2011 to 30-Nov-2013
sin 2 2q13 = 0.084+0.005
-0.005
-3
2
| Dmee2 |= 2.44+0.10
´10
(eV
)
-0.11
• Independent nH rate analysis has measured
sin 2q13 = 0.083± 0.018, c / ndf = 4.6 / 4
2
2
• Sterile neutrinos: Set most stringent limit at
2
10-3 eV2 < Dm41
< 0.1 eV2
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28
Summary
• Data taking planned to continue to 2017
Precision of oscillation parameters will be improved in the future.
Both sin22θ13 and Δm2ee will below 3%
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29
Thank you very much!
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30
List
of
Recent
θ
Measurements
13
List of all θ Measurements
13
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19 31
Signal and Background Summary
(nGd analysis)
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32
Detection Strategy
Relative
Relative measurement
measurement with 8 functionally identical detectors
The number of detected antineutrinos :
N det =
Np
4p L2
N p : the number of proton
ò e × s × Psur (E, L,q13 )× S dE
The ratio of number of detected antineutrinos :
L: detector baseline
e : detection efficiency
s : Invers Beta Decay (IBD) cross section
IBD process: n e + p ® e+ + n
S: anti-neutrino flux
N f æ N p, f öæ Ln ö æ e f öé Psur (E, L f , q13 ) ù
÷÷çç ÷÷ ç ÷ê
= çç
ú
N n è N p,n øè L f ø è e n øë Psur (E, Ln , q13 ) û
2
 Reduction of systematic errors: reactor flux uncertainty and detector systematics
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33
PMT light-emission (flashing)
Flashing PMTs
Neutrinos
Flashers
- Instrumental background from ~5% of PMTs
- ‘Shines’ light to opposite side of detector
- Easily discriminated from normal signals
Relative PMT charge
Quadrant = Q3/(Q2+Q4)
MaxQ = maxQ/sumQ
(contains ‘hottest’ PMT)
34
2015/3/30
Inefficiency to antineutrinos signal:
0.024% ± 0.006%(stat)
Contamination: < 0.01%
Background: Fast neutrons
Constrain fast-n rate using
IBD-like signals in 10-50 MeV
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
Analysis muon veto cuts control B/S to
0.06% (0.1%) of far (near) signal.
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Validate with fast-n events
tagged by muon veto.
35
Background:
241Am-13C
A subtle background from our calibration neutron source
Single n-like z distribution in physics run

241Am-13C
source produces ~0.75 Hz
neutrons via 13C(α,n)16O.
 Neutrons interact with steel to produce fake
(prompt,delayed) pair
241Am-13C
n-Fe captures
(23040/day/AD)
12B/12N
bottom
Reconstructed z (m)
top
 Correlated background expected
small  0.2~0.3/day/module
(MC)
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 Constrained with data, special
calibration
36
Background: β-n decay
This background is directly measured by
fitting the distribution of IBD candidates vs.
time since last muon.
9Li/8He
τ½ = 178 ms, Q = 13. 6 MeV
8He: τ = 119 ms, Q = 10.6 MeV
½
9Li:
- Generated by cosmic rays
- Long-lived
- Mimic antineutrino signal
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uncorrelated
Analysis muon veto cuts control
B/S to ~0.3±0.1%.
37
Near Hall Installation
 Installation in the first Near Hall:
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38
The search for q13
 One of the main priorities in the field of neutrino physics is to make a
precise measurement of q13:
 q13 was the last unknown mixing angle in the PMNS matrix
 A non-zero q13 opens the door to answering many other questions:
o Why is there more matter than anti-matter in our universe?
Is there CP
violation in the
lepton sector?
10,000,000,001
Matter
10,000,000,000
Anti-matter
o What is the mass hierarchy of the neutrino sector?
 Until quite recently, all we knew is that q13 was small, but we didn’t know how
small:
sin2(2q13) < 0.15 (90 C.L.)
from PDG 2011
 An aggressive experimental program has been set in motion to measure this
angle
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39
Questions
Which is the right mass
hierarchy?
Is θ13=0 or just very small?
Focus of the
rest of the talk
2
m12
2
m23
Is q23 exactly p/4?
What is the rest mass
of neutrinos?
And also:
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Do neutrinos obey CP, CPT?
Are neutrinos their own antiparticles?
Are there more than 3 neutrinos (sterile, heavier than Z)?
Physics of neutrino-nucleon low energy interactions?
40
Detection Strategy
 There are currently two strategies being used for going after q13:
1) Accelerator-based experiments:
ne?
nm
Neutrino beam
(Far Detector can be
slightly off-axis)
Near Detector
Far Detector
 Principle: look for ne appearance in a nm beam.
o Probability is sensitive to mass hierarchy and CP-violating phase d.
 Need to use much longer baselines (hundreds of kms).
o Need very large detectors (>10 ktons)
o Need very powerful beams (~1MW)
 There are currently three experiments that operate under this principle under construction
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41
and/or in operation: MINOS, NOnA and T2K.
Detection Strategy
2) Reactor-based experiments:
ne/MeV/fisson
ne
Far Detector
Near Detector
Flux × L2
reactor
(at optimal energy)
< 15%
2(2q )
sin
13
Energy (MeV)
 Principle: look for electron anti-neutrino disappearance
o Anti-neutrinos are produced in copious amounts in nuclear reactors.
o Look for disappearance at ~1-2km from reactor.
o Need detectors in the order of several hundreds of tons.
 Looking for a small effect:
o Key is to keep the systematics under control.
 There are currently three experiments that operate under this principle: Daya Bay, Double
CHOOZ, and RENO.
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42
Detector Response: Acrylic Vessels
Energy loss in acrylic causes small distortion of energy spectrum
If antineutrino interacts in or near
acrylic vessel, a portion of the kinetic
energy of inverse beta positrons will
not be detected
True versus visible MC e+ energy
e+ traversed
acrylic
Annihilation gammas with longer
range can also deposit energy in the
vessels
e+ stopped
in acrylic
Simulation
IBD in target
Generated 2D distortion matrix from MC to correct
predicted positron energy spectrum
IBD in acrylic
(~1.3%)
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Uncertainties from varying acrylic vessel
thicknesses and MC statistics incorporated into
analysis.
43
Scintillator Response Model
Electron response
2 parameterizations to model quenching effects and Cherenkov radiation:
1) 3-parameter purely empirical model:
2) Semi-emp. model based on Birks' law:
Gammas + positrons
• Gammas connected to electron model
through MC:
Simulation of individual e-, e+ energies
due to gamma interaction in scintillator.
• Positrons connected to electron model
through MC:
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44
Electronics Non-Linearity Model
PMT readout electronics introduces additional biases
Electronics does not fully capture
late secondary hits
• Slow scintillation component missed
at high energies
• Charge collection efficiency decreases
with visible light
PMT readout electronics introduces additional biases
• Effective model as a function of total visible energy
• 2 empirical parameterizations: exponential and quadratic
• Total effective non-linearity f from both scintillation and electronics effects:
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45