Transcript Slide 1

Using Reactor Neutrinos to Study
Neutrino Oscillations
Jonathan Link
Columbia University
Heavy Quarks and Leptons 2004
June 1, 2004
Doing Physics With
Reactors Neutrinos
The original Neutrino discovery
experiment, by Reines and Cowan,
used reactor neutrinos…
Reines and Cowan at the Savannah River Reactor
…actually anti-neutrinos. The νe interacts with
a free proton via inverse β-decay:
e+
νe
W
n
p
Later the neutron captures giving a coincidence
signal. Reines and Cowan used cadmium to
capture the neutrons.
The first successful neutrino detector
Nuclear Reactors as a Neutrino Source
• Nuclear reactors are a very intense sources of νe deriving from
the b-decay of the neutron-rich fission fragments.
• The observable ne spectrum is the product
of the flux and the cross section.
Arbitrary
• A typical commercial reactor, with 3 GW thermal power,
From Bemporad, Gratta and Vogel
produces 6×1020 ne/s
Observable n Spectrum
• The spectrum peaks around ~3.6 MeV.
• Visible “positron” energy implies ν energy
Eν = Ee + 0.8 MeV ( =mn-mp+me-1.022)
• Minimum energy for the primary signal is 1.022 MeV from e+e−
annihilation at process threshold.
• Two part coincidence signal is crucial for background reduction.
Uses of Reactor Neutrinos
• Measure cross sections or observe new processes
(e.g. neutral current nuclear coherent scattering)
• Search for anomalous neutrino electric dipole
moment
• Measure the weak mixing angle, sin2θW
• Monitor reactor core (non-proliferation application)
• Measure neutrino oscillation parameters (Δm2’s and
mixing angles)
Observations of Neutrino Oscillations
Reactor neutrinos can probe oscillations in all three observed
Δm2 regions.
Oscillations are observed as a
deficit of νe with respect to
expectation.
Δm2≈3 to 3×10-2 eV2
θ??
Δm2≈2.5×10-3 eV2
θ23 & θ13
Δm2≈7.5×10-5 eV2
θ12
Short Baseline: 10 to 100 meters
Bugey, Gosgen & Krasnoyarsk
Medium Baseline: 1 to 2 km
Chooz, Palo Verde & future
Long Baseline: 100+ km
KamLAND
Short Baseline Oscillation Searches
Experiments like Bugey rule out the low Δm2, large mixing angle
region of the LSND signal.
The Bugey Detector
Neutrons
capture on
Lithium
Reactor
Excluded
Bugey looked for evidence of oscillations between 15 & 45
meters. Gosgen was closer and Krasnoyarsk farther away.
Observations of Neutrino Oscillations
Reactor neutrinos can probe oscillations in all three observed
Δm2 regions.
Oscillations are observed as a
deficit of νe with respect to
expectation.
Δm2≈3 to 3×10-2 eV2
θ??
Δm2≈2.5×10-3 eV2
θ23 & θ13
Δm2≈7.5×10-5 eV2
θ12
Short Baseline: 10 to 100 meters
Bugey, Gosgen & Krasnoyarsk
Medium Baseline: 1 to 2 km
Chooz, Palo Verde & future
Long Baseline: 100+ km
KamLAND
Medium Baseline Oscillation Searches
Experiments like Chooz looked for oscillations in the atmospheric
Δm2. At the time they ran the atmospheric parameters were
determined by Kamiokande, not Super-K (larger Δm2).
1050 m baseline
Chooz Nuclear Reactors, France
Gadolinium
loaded liquid
scintillating
target.
Medium Baseline Oscillation Searches
• No evidence found for ne oscillation.
• This null result eliminated nm→ne as the primary mechanism for
the Super-K atmospheric deficit.
• sin22q13< 0.18 at 90% CL (at
Dm2=2.0×10-3)
• Future experiments should try to
improve on these limits by at least
an order of magnitude.
Down to sin22q13 <~ 0.01
In other words, a
measurement to better than
1% is needed!
Observations of Neutrino Oscillations
Reactor neutrinos can probe oscillations in all three observed
Δm2 regions.
Oscillations are observed as a
deficit of νe with respect to
expectation.
Δm2≈3 to 3×10-2 eV2
θ??
Δm2≈2.5×10-3 eV2
θ23 & θ13
Δm2≈7.5×10-5 eV2
θ12
Short Baseline: 10 to 100 meters
Bugey, Gosgen & Krasnoyarsk
Medium Baseline: 1 to 2 km
Chooz, Palo Verde & future
Long Baseline: 100+ km
KamLAND
Long Baseline Oscillations
The KamLAND experiment uses neutrinos from 69 reactors to
measure the solar mixing angle (θ12) at an average baseline of
180 km.
Scatter plot of energies for
the prompt and delayed
signals
Neutron Capture on
Hydrogen results in
a 2.2 MeV gamma
In 145 days of running they saw 54
events where 86.8±5.6 events where
expected.
The fit energy confirms the oscillation
hypothesis.
Long Baseline Oscillations
KamLAND Results
Eliminates all but the large
mixing angle (LMA) solution.
The best fit sin22θ12= 0.91
The best fit Δm2 = 6.9×10-5 eV2
The Δm2 sensitivity comes
primarily from the solar
measurements
Future Experiments to Search for
a Non-zero Value of sin22θ13
Subject of a lot of interest because of it
relevance to lepton CP violation and
neutrino mass hierarchy.
See Whitepaper: hep-ex/0402041
Sin22θ13 Reactor Experiment Basics
νe
νe
νe
Well understood, isotropic source
of electron anti-neutrinos
Oscillations observed
Eν ≤ 8 MeV
as a deficit of νe
νe
νe
νe
Probability νe
1.0
sin22θ13
Unoscillated flux
observed here
Survival Probability
P( ν e  ν e )  1 - sin 2 2θ13 sin 2 (1.27Dm132 L / Eν )
Distance
1200 to
1800 meters
Proposed Sites Around the World
Power
Baseline
Shielding
Sensitivity
(GWthermal)
Near/Far (m)
Near/Far (mwe)
90% CL
Krasnoyarsk, Russia
1.6
115/1000
600/600
0.03
Kashiwazaki, Japan
24
300/1300
150/250
0.02
Double Chooz, France
8.9
150/1050
30/300
0.03
Diablo Canyon, CA
6.7
400/1700
50/700
0.01
Angra, Brazil
5.9
500/1350
50/500
0.02
Braidwood, IL
7.2
200/1700
450/450
0.01
Daya Bay, China
11.5
250/2100
250/1100
0.01
Site
Status
What is the Right Way to Design the Experiment?
Start with the dominate systematic errors from previous
experiments and work backwards…
CHOOZ Systematic Errors, Normalization
Near Detector
Identical Near and Far Detectors
The combination of these two plus a complex
analysis
gives you
the anti-neutrino
flux
Movable
Detectors,
Source
Calibrations, etc.
CHOOZ Background Error
BG rate
0.9%
Muon Veto and Neutron Shield
(MVNS)
Statistics may also be a limiting factor in the sensitivity.
Backgrounds
There are two types of background…
1. Uncorrelated − Two random events that occur close together
in space and time and mimic the parts of the coincidence.
This BG rate can be estimated by measuring the singles rates,
or by switching the order of the coincidence events.
2. Correlated − One event that mimics both parts of the
coincidence signal.
These may be caused fast neutrons (from cosmic m’s) that
strike a proton in the scintillator. The recoiling proton mimics
the e+ and the neutron captures.
Or they may be cause by muon produced isotopes like 9Li and
8He which sometimes decay to β+n.
Estimating the correlated rate is much more difficult!
Reducing Background
1. Go as deep at you can (300 mwe → 0.2 BG/ton/day at CHOOZ)
2. Veto m’s and shield
neutrons (Big
effective depth)
3. Measure the recoil
proton energy and
extrapolate into the
signal region.
(Understand the BG
that gets through and
subtract it)
Shielding
Veto
Detectors
p
n
n
6 meters
m
m
Isotope Production by Muons
300 mwe
(/ton/day)
450 mwe
(/ton/day)
9Li+8He
0.17 ± 0.03
0.075 ± 0.014
8Li
0.28 ± 0.11
0.13 ±0.05
E ≤ 16 MeV, τ½ = 0.84 s
6He
1.1 ± 0.2
0.50 ± 0.07
E ≤ 3.5 MeV, τ½ = 0.81 s
Source
Comments:
E ≤ 13.6 MeV, τ½ = 0.12 to 0.18 s
16% to 50% correlated β+n
A ½ second63veto
that
± 9 after every
28 ±muon
4
E ≤ deposits
0.96 MeV,more
τ½ = 20that
m
detector should eliminate 70 to 80% of all
10C2 GeV in the
8.0 ± 1.5
3.6 ± 0.6
E ≤ 1.98 MeV, τ½ = 19 s
correlated decays.
9
11C
C
0.34 ± 0.11
0.15 ± 0.05
E ≤ 16 MeV, τ½ = 0.13 s
The vetoed
sample
be used
to make
a background
0.50
± 0.12 can 0.22
± 0.05
E ≤ 13.7
MeV, τ½ = 0.77 s
7Besubtraction
of ±in2.6
a fit to the
spectrum.
16.0
7.2 ±energy
1.1
E ≤ 478 keV, τ½ = 0.53 d
8B
12B
11
3
E ≤ 13.4 MeV, τ½ = 0.02 s
Movable Detector Scenario
The far detector spends about 10% of the run at the near site where the relative
normalization of the two detectors is measured head-to-head.
Build in all the calibration tools needed for a fixed detector system and verify
them against the head-to-head calibration.
1500 to 1800 meters
Reactor Sensitivity
• Sensitivity to sin22θ13 ≤ 0.01 at
90% CL is achievable.
• Combining with off-axis some of
the CP phase, δ, range can be ruled
out.
• Unexpected results are possible &
might break the standard model.
Reactor Sensitivity
Is the mixing angle θ23
is not exactly 45º then
sin2θ23 has a two-fold
degeneracy.
Combining reactor
results with off-axis
breaks this degeneracy.
With the 0.03 precision
of the Double Chooz
experiment the
degeneracy is not
broken.
Aggressive Experiment Timeline
2003
2004
Site Selection
1 year
2005
Proposal
2 years
2006
2007
2008
Construction
2 years
2009
2010
2011
Years
Run
3 years (initially)
Site Selection: Currently underway.
The early work on a proposal is currently underway.
With movable detectors, the detectors are constructed in parallel
with the civil construction
Run Phase: Initially planned as a three year run. Results or
events may motivate a longer run.
Conclusions and Prospects
• Reactor neutrinos are relevant to oscillations in all observed
Δm2 regions.
• The KamLAND experiment has been crucial to resolving the
oscillation parameters in the solar Δm2 region.
• There are many ideas for reactor θ13 experiments around the
world and it is likely that more than one will go forward.
• Controlling the systematic errors is the key to making this
measurement.
• With a 3+ year run, the sensitivity in sin22q13 should reach 0.01
(90% CL) at Dm2 = 2.0×10-3.
• Reactor sensitivities are similar off-axis and the two methods
are complementary.
• The physics of reactor neutrinos is interesting and important.
Question Slides
Why Use Gadolinium?
Gd has a huge neutron capture cross section. So you get faster
capture times and smaller spatial separation. (Helps to reduce
random coincidence backgrounds)
With Gd
Without Gd
~30 μs
With Gd
Without Gd
~200 μs
Also the 8 MeV capture energy (compared to 2.2 MeV on H)
is distinct from primary interaction energy.
Characterizing BG with Vetoed Events
Matching distributions from vetoed events outside the signal
region to the non-veto events will provide an estimate of
correlated backgrounds that evade the veto.
n interactions
Proton recoils
?
From CHOOZ
Other Useful Distributions:
• Spatial separation prompt and
delayed events
Faster neutrons go farther
• Radial distribution of events
BGs accumulate on the
outside of the detector.
Medium Baseline Oscillation Searches
• Homogeneous detector
• 5 ton, Gd loaded, scintillating
target
• 300 meters water equiv. shielding
• 2 reactors: 8.9 GWthermal
• Baselines 1115 m and 998 m
• Used new reactors → reactor off
data for background measurement
Chooz Nuclear Reactors, France
Palo Verde
Palo Verde Generating Station, AZ
• 32 mwe shielding (Shallow!)
• Segmented detector:
Better at handling the
cosmic rate of a shallow site
• 12 ton, Gd loaded, scintillating
target
• 3 reactors: 11.6 GWthermal
• Baselines 890 m and 750 m
• No full reactor off running
Exelon has agreed to work with us
to determine the feasibility of using
their reactors to perform the
experiment.
“We are excited about the
possibility of participating in a
scientific endeavor of this nature”
“At this time we see no
insurmountable problems that
would preclude going forward with
this project.”
They have given us reams of
geological data which we are
currently digesting.
Quantitative Analysis of Movable vs.
Fixed Detectors
Both the Kashiwazaki and Krasnoyarsk proposals assume that they can get the
relative normalization systematic down to 0.8% with fixed detectors.
Double Chooz believes that 0.6% is achievable.
6 GW, 50 tons and 1200 meters Baseline
0.025
Sensitivity
Even if you halve the
relative normalization,
fixed detector are not as
sensitive for a two year
(or longer) run.
Fixed 0.8%
Fixed
Fixed 0.4%
Movable
Movable
0.03
0.02
0.015
All fixed detector
scenario quickly become
systematics limited.
0.01
0.005
0
0
5
10
Years
15
20
Optimal Far Baseline
0.018
0.05
GW,meters
50 tons,
5 years
7 GW, 50 tons,7 200
Near
BL and 3 Years
90%
90% CL
CL Sensitivity
Sensitivity
One must consider
both the location of
the oscillation
maximum (~2200 m
at Δm2=2×10-3) and
statistics loss due to
1/r2 flux.
Δm2=2.0E-3
0.015
0.04
0.012
0.03
Δm2=1.5E-3
Δm2=5.0E-3
Δm2=2.5E-3
Δm2=1.0E-3
0.009
0.02
0.006
0.01
0.003
Kinimatic Phase ≡ 1.27Δm2L/E for E=3.6 MeV
00
600
30
800
40
50 1000 60
120070
1400
80
901600 100 1800110
Far Baseline (meters)
Kinimatic Phase (Degrees)
2000
120
At the preferred Dm2 the optimal region is quite wide. In a
configuration with a tunnel connecting the two detector sites, one
should choose a far baseline that gives the shortest tunnel (1200 to
1400 meters).
Sensitivity Scaling with Systematic Error
For a rate only analysis
The optimal baseline
is very sensitive to the
level of systematic
error.
Optimal Baseline in Kinimatic Phase
(degrees)
90
85
80
Δm2=2.5E-3
Δm2=1.5E-3
75
70
65
60
0
100
200
300
400
500
600
% Systematic Error at Kinimatic Phase of 60 Degrees
700
The standard
assumption of 0.8%
relative efficiency
error for fixed
detectors is ~250% of
the statistical error
after 3 years at
Braidwood.
Comparison of Shape & Rate
Statistics Limited
Systematics Limited
0.03
0.06
0.1
Systematic
SystematicError
Error= =200%
0%
0.025
0.05
0.08
Shape Shape
0.02
0.04
0.03
0.015
0.02
0.01
Sensitivity at 90% C.L.
Sensitivity at 90% C.L.
Counting
Counting
Counting
Shape
0.06
0.04
0.02
0.01
0.005
Systematic Error = 600%
0
00
20
20
70
70
120
120
Kinimatic
KinimaticPhase
Phase(degrees)
(degrees)
20
70
120
Kinimatic Phase (degrees)
The optimal Baseline for the systematics limited shape analysis is ~40º.
The optimal baseline for the systematics limited counting experiment is at
the least optimal spot for a shape analysis!
You better know what regime your working in.
Sensitivity wrt Near Baseline
Ultimately the location of the near detector will be determined
by the reactor owners. The main question here is what can we
live with?
7 GW, 50 tons, 1200 meters Far Baseline and 3 years
0.016
There is a 1/r2
dependence in
statistics (a small
effect) and increasing
oscillation
probability with
distance.
0.015
Sensitivity
0.014
0.013
0.012
0.011
0.01
0.009
0.008
0.007
0
100
200
300
400
500
600
700
Near Baseline
Sensitivity degrades with increasing near baseline.
When Lnear=Lfar the sensitivity is about the same as CHOOZ.