Transcript NOW2004 - Istituto Nazionale di Fisica Nucleare

Overview of Reactor Neutrino Measurements
Michael Shaevitz
Columbia University
NOW2004
• With the recent confirmation by Kamland and isolation of
the Dmsolar2 in the LMA region, the field is turning to:
– Measuring the last mixing angle q13
– Obtaining better precision on q23, q12, Dmsolar2 and Dmatm2
(along with checking LSND)
 Road to measuring n-mass hierarchy and n-CP
violation
• Reactor oscillation measurements unique for:
– Measuring q13
– Constraining q23
1
2
Outline
• Reactor neutrino measurements
– Physics capabilities
– Experimental method:
• How to measure a small disappearance signal
– Possible experimental sites
– Sensitivity studies
• Comparisons and combinations with offaxis experiments
Brief Aside onne Elastic Scattering
• Use Osc. Experiment near
detector for measurement
• Use far detector to measure
background
• A reactor-based weak mixing angle
(sin2qW) measurement
– Probe new physics in neutrino
sector at low Q2
– With errors comparable to APV,
Moller Scattering, and NuTeV
– Also, sensitivity to n magnetic
moment (~x3 better than current).
Moller (e-e-)
E158 – Prelim
3
Weak Mixing Angle Measurement
• Measure the elastic scattering rate
using the Near Detector
• Experiment needs a close detector
(< 200m) with > 300 mwe shielding
ne e–  ne e–
• Detector requirements very similar to
oscillation experiment
 Low cost addition to osc. Program
 Need to obtain reduced backgnds
• Measure the neutrino flux using
Inverse Beta Decay (IBD) events in
the same fiducial volume with same
deadtime.
ne p  e– n
• Reduction of backgrounds
– Only use events in the 3 < Evisible < 5
MeV window
– Veto events with delayed coincident
neutrons
– Veto events with various types of
associated cosmic muons
• Use the Far Detector to monitor the
non-elastic background rate.
 d(sin2qW ) = 0.0019
(compare to NuTeV = 0.0017)
(See J.Conrad, J.Link, and M.S., hep-ex/0403048)
4
• Nuclear reactors are a very intense sources
ofνe with a well understood spectrum
– 3 GW ≈ 2×1021 MeV/s → 6×1020ne/s
– Reactor spectrum peaks at ~3.7 MeV
– Oscillation Max. for Dm2=2.510-3 eV2
at L = 1.8 km
Arbitrary
Reactor Measurements of q13
5
From Bemporad, Gratta and Vogel
Observable n Spectrum
• Osc prob (sin2q13=1.0) = 0.81 for 1.8 km
= 0.55 for 1.05 km
35
Dm = 2.5 10 eV
2
-3
2
30
Observed Events
No Osc.
25
• Disappearance Measurement:
Look for small rate deviation from 1/r2
measured at a near and far baselines
20
1050 m
15
– Counting Experiment
10
1500 m
• Compare events in near and far detector
5
– Energy Shape Experiment
1800 m
0
1.50
2.50
3.50
4.50
5.50
En (MeV)
6.50
7.50
8.50
• Compare energy spectrum in near and far
detector
6
Reactor Neutrino Event Signature
• The reaction process is inverse β-decay followed by neutron
capture
– Two part coincidence signal is crucial for background reduction.
n e p  e n
n capture
• Positron energy spectrum implies the neutrino spectrum
Eν = Evis + 1.8 MeV – 2me
• In undoped scintillator the neutron will capture on hydrogen
n H → D g (2.2 MeV)
• More likely the scintillator will be doped with gadolinium to
enhance capture
n mGd → m+1Gd g’s (8 MeV)
Physics of θ13 at Reactors
• The reactor experiment can make an unambiguous
measurement of the mixing parameter sin22θ13.
Small dependence on:
• mass hierarchy
• CP violation phase, dCP
• sin2q23 and q23 degeneracy
• sin2q12 and Dmsolar2
7
Current Status of q13 Knowledge
• CHOOZ and Palo Verde
Experiments
– Neither experiments found
evidence forne oscillation
– sin22q13< 0.14 at 90% CL
(at Dm2=2.5×10-3 eV2)
8
• Recent global fits including
solar + KamLAND + CHOOZ
– Kamland spectral distortion fixes
Dm2Solar  Compare to solar gives q13
– Gives reduced 90% CL upper limits
especially at smaller Dm132
• sin22q13 <0.09 @ Dm132=2.510-3 eV2
• sin22q13 <0.12 @ Dm132=1.310-3 eV2
Chooz Systematic Uncertainties
CHOOZ Only
Maltoni et al.
hep-ph/0405172
Physics of θ13 at Reactors
• The reactor experiment can make an unambiguous
measurement of the mixing parameter sin22θ13.
• Sensitivity should reach sin22θ13 ≤ 0.01 to 0.03 at 90% CL in
three years of running. Better sensitivity is also possible.
• In conjunction with the off-axis (beam experiments) the
reactor experiment breaks the degeneracy of q23 <45 or >45.
9
The θ23 Degeneracy Problem
10
Disappearance neutrino measurements are sensitive to sin22θ23
Super-K / Minos /
T2K Measures
2


1
.
27
D
m
2
2
23 L

P(n   n x )  sin 2θ 23 sin 
En


But the leading order term in offaxis νμ→νe oscillations is
Offaxis q13 Measures
2


1
.
27
D
m
2
2
2
13 L

P(n   n e )  sin θ 23 sin 2θ13 sin 
En


Example: Measurement of sin22 q23 = 0.95
 q23 = 38 or 52
Prediction for appearance rate  sin2q23
 sin2q23 = 0.38 or 0.62 (x1.6 uncertainty)
Physics of θ13 at Reactors
• The reactor experiment can make an unambiguous
measurement of the mixing parameter sin22θ13.
• Sensitivity should reach sin22θ13≤ 0.01 to 0.03 at 90% CL in
three years of running. Better sensitivity is possible.
• In conjunction with the off-axis (beam experiments) the
reactor experiment breaks the degeneracy of q23 <45 or >45
• Direct knowledge of the mixing angles is important in its
own right! Could be crucial to constructing a theory of flavor.
• The reactor measurement determines the feasibility of CP
violation and mass hierarchy studies in off-axis.
11
12
How Do You Measure a Small Disappearance?
• Use identical near and far detectors to cancel many sources of
systematics.
Sin22θ13 Reactor Experiment Basics
νe
νe
νe
13
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
14
How Do You Measure a Small Disappearance?
• Use identical near and far detectors to cancel many sources of
systematics.
• Design detectors to eliminate the need for analysis cuts that
may introduce systematic error.
Detector Design Basics
• Homogenous Volume
• Viewed by PMT’s
Coverage of 20% or better
• Gadolinium Loaded, Liquid
Scintillator Target
Enhances neutron capture
• Unloaded Scintillator Region
To capture energy from gamma
rays. Eliminates need for fiducial
volume cut.
• Pure Mineral Oil Buffer
To shield the scintillator from
radioactivity in the PMT glass.
Allows you to set an energy cut
well below the 1 MeV e+eannihilation energy.
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16
How Do You Measure a Small Disappearance?
• Use identical near and far detectors to cancel many sources of
systematics.
• Design detectors to eliminate the need for analysis cuts that
may introduce systematic error.
• Detector cross calibration may be used to further reduce the
near/far normalization systematic error.
 Use common sources to cross calibrate
 Move far detectors to near site for cross calibration
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How Do You Measure a Small Disappearance?
• Use identical near and far detectors to cancel many sources of
systematics.
• Design detectors to eliminate the need for analysis cuts that
may introduce systematic error.
• Detector cross calibration may be used to further reduce the
near/far normalization systematic error.
• Reduce background rate and uncertainty
Backgrounds
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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 by fast neutrons (from cosmic ’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!
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How Do You Measure a Small Disappearance?
• Use identical near and far detectors to cancel many sources of
systematics.
• Design detectors to eliminate the need for analysis cuts that
may introduce systematic error.
• Detector cross calibration may be used to further reduce the
near/far normalization systematic error.
• Reduce background rate and uncertainty
─ Go as deep as you can
─ Veto
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Veto Background Events
Fast neutrons
9Li
Veto ’s and shield neutrons
and 8He
E ≤ 10.6 MeV
τ½ = 0.18 to 0.12 s
0.075 produced/ton/day (450 mwe)
50% to 16% correlated β+n
Shielding
Veto
Detectors
KamLAND
Data
p
n
n
6 meters


A ½ second veto after every
muon that deposits more that 2
GeV in the detector should
eliminate 70 to 80% of all
correlated decays.
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How Do You Measure a Small Disappearance?
• Use identical near and far detectors to cancel many sources of
systematics.
• Design detectors to eliminate the need for analysis cuts that
may introduce systematic error.
• Detector cross calibration may be used to further reduce the
near/far normalization systematic error.
• Reduce background rate and uncertainty
─ Go as deep as you can
─ Veto
─ Use vetoed events to measure the background
• Redundant measurements to give convincing signal
─ Multiple detectors at each site
─ See osc. signal in both rate and spectral distortion
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Scales of Experiments and Sensitivities
small: sin22q13~0.03
– Goal: fast experiment to explore region x3-4 below the Chooz limit.
– Sensitivity through rate mainly
– Example: “Double-Chooz” experiment (300 GW-ton-yrs)
medium: sin22q13~0.01
–
–
–
–
Make a discovery of q13 in region of interest for the next 10-20 year program
Sensitivity enough to augment the physics of offaxis measurements
Sensitivity both to rate and energy shape
Example: Braidwood, Daya Bay (3000 GW-ton-yrs)
large: sin22q13~0.002-0.004??
– Measurement capability comparable to second generation offaxis
experiments
– Sensitivity mainly through energy shape distortions
– MiniBooNE/Kamland sized detector (20,000 GW-ton-yrs)
Counting (Rate) vs Energy Shape Measurement
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Small detectors give a rate comparison near to far
Large detectors can show a energy shape distortion near to far
sin22θ13 Sensitivity
medium
From Huber et al.
hep-ph/0303232
large
small
Exposure (GW·ton·years)
The location of the transition from rate to shape depends on the level of systematic
error  snorm = relative normalization of the two detectors
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Proposed Reactor Oscillation Experiments
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
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300/1300
150/250
0.02
Double Chooz, France
8.4
150/1050
30/300
0.03
Diablo Canyon, CA
6.7
400/1700
50/700
0.01
On Hold
Angra, Brazil
5.9
500/1350
50/500
0.02
Go
Braidwood, IL
7.2
200/1500
450/450
0.01
Daya Bay, China
11.5
250/2100
250/1100
0.01
Site
Many Sites have been investigated as potential hosts
to a reactor neutrino experiment.
This is appropriate since getting the cooperation of the reactor
company and host country is a main challenge.
See Talks this afternoon by G. Mention and M. Goodman
Stop
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
Kashiwazaki,
Japan
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300/1300
150/250
0.02
Double Chooz, France
8.4
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
200/1500
450/450
0.01
Daya Bay, China
11.5
250/2100
250/1100
0.01
Site
Status
far
near
near
Kashiwazaki-Kariwa, Japan (
• 7 Reactors, 24 GWth
6m Diameter Shaft – 200 ~ 300m depth
• Three ~8.5 ton detectors
• Two near detectors at baselines of ~400 m
• One far detector at 1.3 ~ 1.8 km
21 different baselines but not a problem
• Sensitivity of sin22θ13≤0.025 in 2 years
Fast! Reaches systematics limit quickly
• Currently working its way through the
Japanese system
(2 yr R&D budget approved)
far
near
near
)
Minakata, Sugiyama, Yasuda, Inoue, 27
and Suekane hep-ph/0211111
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,
Double
Chooz, France
France
8.4
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
200/1500
450/450
0.01
Daya Bay, China
11.5
250/2100
250/1100
0.01
Site
Status
Double CHOOZ , France
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• Use old far detector hall at ~1050 meters
• Near detector at 125-250 meters (~50 mwe)
• 11 ton Gd loaded detectors.
• Sensitivity of sin22θ13≤0.03 in 3 years
Fast and Inexpensive
• Has scientific approval
• Released an LOI (hep-ex/0405032)
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Double – CHOOZ Experiment
• Cost estimate:
• Experimental Goals:
– Statistical error = 0.4%
– Background error = 1%
– Relative detector error = 0.6%
– Detectors: ~ $7M
– Civil Construction: ~ $10M
• Schedule:
Now : Securing approvals from
agencies and company
Feb. 05: If approved, complete
design and put out bids
May 07: Complete far detector
Early 08: Complete near detector
• Advantages:
– Infrastructure exists for far site
– Reactor company would build
near site if approved
– Shallow near detector site
(50-100 mwe)
• Deadtime = 500 s / muon
 30 - 60% Deadtime
– Far baseline only 1km which
limits sensitivity especially for
low Dm2
sin22q13 limit
• Disadvantages:
1.5×10-3 eV2
2.0×10-3 eV2
3.0×10-3 eV2
1
2
3
4
5
6
7
Exposure in Years
8
9
10
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
8.4
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
Braidwood, IL
7.2
200/1500
200/1700
450/450
0.01
Daya Bay, China
11.5
250/2100
250/1100
0.01
Site
Status
Braidwood, Illinois
• Four 65 ton detectors
• Near detectors at 200 meters & 450 mwe
• Two far detectors located at 1500 meters,
450 mwe
• Sensitivity of sin22θ13≤0.01 in 3 years
• High level of cooperation with utility
Braidwood
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Braidwood Experiment
• Experimental Goals:
– Statistical error = 0.15%
– Background error = 0.4%
– Relative detector error = 0.6%
• Advantages:
– Deep near site allows other reactor
physics measurements
– Favorable geology and low bkgnd
– Redundancy and cross checks
• Both rate and spectral distortion
analysis
• Surface transport of detectors for cross
calibration
• Multiple near and far detectors
• Disadvantages:
– Infrastructure costs high due to new
undeveloped site
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• Cost estimate:
Civil: ~ $35M
Detectors: $15M
• Schedule:
2005: R&D proposal submission.
2006: Full proposal submission
2007: Project approval; start const.
2009: Start data collection
Braidwood
3 yrs
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
8.4
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
200/1500
450/450
0.01
Daya
Bay, China
China
Daya Bay,
11.5
250/2100
250/1100
0.01
Site
Status
Daya Bay, China
35
• 4 Reactors, 11.5 GWth
• Several ~8 ton detectors
• Near detectors at baseline of 300
and 400 meters, 200 to 250 mwe
• Far detectors at baselines of 1800
and 2400 meters, 1100 mwe
• Sensitivity of sin22θ13≤0.01 in 3
years
• Utility/government approval is likely
• China would support civil
construction, but foreign support is
needed for detectors
36
Daya Bay Experiment
• Experimental Goals:
– Statistical error = 0.2%
– Background error = 0.3%
– Relative detector error = 0.4%
• Advantages:
– Horizontal access tunnel approach
•
•
•
•
Large overburden reduces bkgnd
Flexibility to change baseline
Easy to service detector
Cross calibration by moving
detectors to near site
• Disadvantages:
– Uncertainties associated with the
Chinese approval system
• Cost estimate:
~ $25M
• Schedule:
2005: R&D, engineering design, and
secure funding
2006-2007: Construction
2008: Start data collection
Reactor Sensitivity Studies
(Comparing and Combining with Offaxis Measurements)
37
(K. McConnel and M. Shaevitz – hep-ex/0409028)
•
Try to do estimates including all effects:
– Dm232 = 2.510-3 eV2 and allowed to
vary with s = 0.110-3 eV2
– Include both mass hierarchies
Dm232 > 0 and Dm232 < 0
– sin22q23 = 0.95 and allowed to vary
with s = 0.01
• Reactor experimental inputs:
– Small scale: d(sin22q13) = 0.03
– Medium scale: d(sin22q13) = 0.01
– Large scale: d(sin22q13) = 0.005
(90% CL upper limit sensitivities
at Dm2 = 2.510-3 eV2)
• Offaxis experimental inputs:
– JPARC to SuperK (T2K) exp.
– Include ambiguity of q23 < 45 or
> 45
– Dm122 and q12 fixed at current values
– dCP allowed to vary between 0 to
360
• Rates for n andnfrom LOI
• with / without upgrade x5 rate
– Offaxis NuMI (Nova) exp.
• Rates for n andnnew LOI appendix
• with / without proton driver upgrade
(x5 rate)
Comparisons of Reactor and Off-axis Sensitivities
38
large medium small reactor
×5 beam
rate
90% CL upper limits for underlying null
sin22θ13 (= 0)
Green: Offaxis exp. (5yr n only)
Blue: Combined Medium Reactor plus Offaxis
combine
with med.
reactor
White: Offaxis Only (x5 rate)
A medium scale reactor experiment,
gives a more stringent limit on sin22θ13 in
several regions than off axis, even with
proton driver like statistics (×5 beam rate).
combine
with med.
reactor
5 yrs n-only
After a medium scale reactor limit, only a
small window of opportunity exists for an
observation of νμ to νe with an off-axis.
Comparison of Reactor to Off-axis Signal Meas.
Chooz-like, small scale
Braidwood-like medium scale
90% CL regions for sin22θ13 = 0.05,
δCP=0 and Δm2 = 2.5×10-3 eV2
Green: Offaxis exp. Only
Blue: Combined Medium Reactor plus Offaxis
Red: Combined Small Reactor plus Offaxis
A medium scale reactor experiment
makes a significantly better measurement
of sin2q13 than the offaxis experiments.
5 yrs n-only
39
Resolving the θ23 Degeneracy
40
Example: sin22q23 = 0.95  q23 = 38 or q23 = 90-38  = 52
• For sin2q13= 0.05,
combining with offaxis ν andν running
(3 yrs n + 3 yrsn )
– Medium scale
reactor breaks the
θ23 degeneracy.
– Small reactor is not
sufficient to break
the degeneracy.
(q23TRUE= 38)
Regions where q23 Degeneracy Resolved at 2s
(Is q23 < 450 or > 450 ?)
3 yrs n + 3 yrsn
3 yrs n + 3 yrsn
41
Constraining the CP Violation Parameter, dCP
and Determining the Mass Hierarchy
• Use Medium Reactor (d(sin22q13) = 0.01)
can predict the neutrino prob.
P(nne)
• Oscillation probability vs dCP for offaxis exp
(Dm2 = 2.5x10-3 eV2 , sin22q13 = 0.05)
42
T2K n - 5 years
sin22q13=0.1
Neutrino, normal hierarchy
Neutrino, inverted hierarchy
dCP
43
Reactor Contribution to CP Violation
• For large sin2q13, can
restrict dCP with
reactor and n-only
offaxis data.
(3 yrs)
• For smaller sin2q13,
need offaxisn data to
restrict dCP.
• In all cases, precision
on determining sin2q13
comes from reactor
data.
5 yrs n-only
3 yrs n + 3 yrsn
CP Violation 3s Discovery Regions
large medium small reactor
44
large medium small reactor
Reactor measurement
does not contribute
much to measuring CP
violation…
with proton driver
with x5 upgrade
To the right of the curve,
dCP=0 or p is excluded by
at least three standard
deviations
with x5 upgrades
Dm2 = 2.5×10-3 eV2
sin22q23 = 0.95  0.01
3 yrs n + 3 yrsn
But a null reactor
measurement, even at
the Double Chooz
sensitivity, can mostly
rule out accessible
regions for T2K / Nova.
Determining Reach in Mass Hierarchy
To the right of the curve, mass hierarchy is
resolved by at least two sigma
large medium small reactor
large medium small reactor
45
Reactor measurement
does not contribute much
to resolving the mass
hierarchy…
Again, a null reactor
measurement, even at the
Double Chooz sensitivity,
can mostly rule out
accessible regions for
T2K and Nova.
with proton driver
with x5 upgrades
Dm2 = 2.5×10-3 eV2
sin22q23 = 0.95  0.01
3 yrs n + 3 yrsn
Far Future  Reactor plus Hyper-K + Nova(x10)
• Hyper-K alone makes poor
determination of CP violation
phase, dCP
– q23 degeneracy limits accuracy
– Mass hierarchy introduces wide
range
 Medium scale reactor data
can lift q23 degeneracy
(black dashed curves)
 Combination with Nova can
determine hierarchy
Bottom Line:
Combination of reactor plus
Hyper-K plus Nova can make
precise determination
(black dashed curves)
46
Conclusions
• A reactor experiment is “the” prime and only unambiguous
measurement of q13
– q13 is a important physics parameter
• Needed to constrain the models of lepton mixing matrix
• If very small, probably indicates a new symmetry
– q13 is key for planning future long-baseline experiments to measure
CP violation and the mass hierarchy
• If sin22q13 is > ~0.02, T2K and Nova make a nice program
• If sin22q13 is < ~0.01, need other techniques to access the physics
(1st,2nd max. measurements; Superbeam exps, Neutrino Factory….)
• Reactor measurements are important for sorting out the q23 ambiguity
(q23 vs 900- q23)
– Again this is an important, fundamental physics parameter (like q13)
– May be important for CP violation and mass hierarchy measurements
• Reactor experiments are being pursued at many sites and it is likely
that several will be approved and go forward.
47