1. Reactor Working Group Report
Future reactor experiments to measure sin2213
What do we learn by combining reactor and accelerator measurements?
Beyond 13
Conclusions and Recommendations
Erin Abouzaid, Kelby Anderson, Gabriela Barenboim, Bruce Berger, Ed Blucher, Tim Bolton,
Janet Conrad, Joe Formaggio, Stuart Freedman, Dave Finley, Peter Fisher, Moshe Gai,
Maury Goodman, Andre de Gouvea, Nick Hadley, Dick Hahn, Karsten Heeger, Boris Kayser,
Josh Klein, John Learned, Manfred Lindner, Jon Link, Bob McKeown, Irina Mocioiu,
Rabi Mohapatra, Donna Naples, Jen-chieh Peng, Serguey Petcov, Jim Pilcher, Petros Rapidis,
David Reyna, Mike Shaevitz, Robert Shrock, Noel Stanton, Ray Stefanski, Richard Yamamoto
29 June 2004
APS Neutrino Study

2. Neutrino physics at nuclear reactors
: The key parameter for next generation of neutrino
oscillation experiments. Its value sets scale of experiments
needed to study CP violation, mass hierarchy.
The reactor experiment offers only way to measure this mixing
angle free of degeneracies.
In combination with accelerator measurements, can resolve
2 degeneracy, and provide early information about CP
violation, mass hierarchy.
Strong consensus in working group that experiment with
sensitivity of sin2213=0.01 should be our goal.
Beyond : sin2W, neutrino magnetic moment, m122 and 12,
sterile neutrinos, SN physics, CPT tests + worldwide reactor
monitoring, searching for a reactor at the center of the earth?

3. Methods to measure sin2213
Accelerators: Appearance (nmne)
Use fairly pure, accelerator produced  beam with a detector a long distance
from the source and look for the appearance of e events
T2K: <E> = 0.7 GeV, L = 295 km
NOA: <E> = 2.3 GeV, L = 810 km
Reactors: Disappearance (nene)
Use reactors as a source of e (<E>~3.5 MeV) with a detector 1-2 kms away
and look for non-1/r2 behavior of the e rate
Reactor experiments provide the only clean measurement of sin22:
no matter effects, no CP violation, almost no correlation with other parameters.

4. Reactor Measurements of
13: Search for small oscillations at
1-2 km distance (corresponding to
Past measurements:
Pee
Distance to reactor (m)

5. Chooz: Current Best  Experiment
P=8.4 GWth
CHOOZ Systematic errors
Reactor  flux
Detect. Acceptance 2%
1.5%
Total
2.7%
L=1.05 km
D=300mwe
m = 5 tons, Gd-loaded liquid scintillator
Neutrino detection by
sin22< 0.2 for m2=2103 eV2

6. How can Chooz measurement be improved?
Add near detector: eliminate dependence on reactor flux calculation; need to understand relative acceptance of two
detectors rather than absolute acceptance of a single detector
+ optimize baseline, larger detectors, reduce backgrounds
~200 m
~1500 m
Issues affecting precision of experiment:
Relative uncertainty on acceptance
Relative uncertainty on energy scale and linearity
Background (depth)
Detector size
Baseline
Reactor power

7. Detectors and analysis strategy designed to minimize relative
acceptance differences
Identical near and far detectors
Central zone with Gd-loaded
scintillator surrounded by buffer
regions; fiducial mass determined
by volume of Gd-loaded scintillator
Events selected based on coincidence
of e+ signal (Evis>0.5 MeV) and
s released from n+Gd capture
(Evis>6 MeV). No position
reconstruction; little sensitivity to
E requirements.
Shielding
6 meters
To reduce backgrounds:
depth + active and passive
shielding

8. Study has focused on three scales of experiments:
Small sin2213 ~ 0.03 (e.g., Double-Chooz, KASKA)
Medium sin2213 ~ 0.01 (e.g., Braidwood, Diablo Canyon,
Daya Bay)
Large sin2213 ~ (e.g.,Angra)
(sensitivities at 90% confidence level)
For each scenario, understand scale of experiment required
and physics impact.

9. Statistics only norm= 0
Sensitivity Using Rate and Energy Spectrum (Huber et al. hep-ph/ )
Shape only norm= 
norm= 0.8%
Statistics only norm= 0
Dm2 = 3×10-3 eV2

10. Statistics only norm= 0
Sensitivity Using Rate and Energy Spectrum (Huber et al. hep-ph/ )
Shape only norm= 
norm= 0.8%
Statistics only norm= 0
Small
Medium
Large
Dm2 = 3×10-3 eV2

11. Different Scales of Experiments
Small: sin2213 ~ 0.03 (e.g., Double-Chooz, KASKA)
Double-Chooz: 10 ton detector at L-1.05 km.
Rate only, non-optimal baseline, shallow near detector, few cross checks
Cost: ~$20 M; start datataking in 2008
Medium: sin2213 ~ 0.01 (e.g., Braidwood, Diablo Canyon, Daya Bay)
ton detectors, optimized baseline, optimized depths, rate and shape info, perhaps movable detectors to check calibration, multiple far detector modules for additional cross checks
Cost:~$50 M (for US sites); start datataking in 2009
Large: sin2213 ~ (e.g., Angra)
~500 ton fiducial mass; sensitivity mainly through E spectrum distortion

12. JPARC to SuperK (T2K) Offaxis NuMI (Nova)
Reactor Sensitivity Studies: Comparing and Combining with Offaxis Measurements
(M. Shaevitz)
Experimental Inputs
JPARC to SuperK (T2K)
n: 102 signal / 25 bkgnd 5 yrs; n: 39 signal / 14 bkgnd 5 yrs
plus upgrade 5 rate
Offaxis NuMI (Nova)
n: 175 signal / 38 bkgnd 5 yrs n: 66 signal / 22 bkgnd 5 yrs
plus Proton Driver upgrade 5 rate
for sin22=0.1
for sin22=0.1
Oscillation parameters

13. combine with med. reactor combine with med. reactor
Setting Limit on sin2213
large medium small reactor
×5 beam rate
T2K
90% CL upper limits for an underlying sin22θ13 of zero
A medium scale reactor experiment sets a more stringent limit on sin22θ13 than off- axis, even with proton driver like statistics (×5 beam rate).
combine with med. reactor
NOνA
combine with med. reactor
Green: Offaxis exp. Only Blue: Combined Reactor plus Offaxis White: Offaxis Only (x5 rate)

14. Determining Value of sin2213
Chooz-like, small scale
Braidwood-like medium scale
90% CL regions for sin22θ13 = 0.05, δCP=0 and Δm2 = 2.5×10-3 eV2
In the case of an observation, even a small-scale reactor measurement makes a better determination of sin22θ13 than off- axis experiments
T2K
NOνA
Green: Offaxis exp. Only Blue: Combined Medium Reactor plus Offaxis Red: Combined Small Reactor plus Offaxis

15. Importance of Multiple Measurements
The reactor measurement may not agree with the results of the off-axis experiments.
With a 1% LSND-like oscillation
For example:
The reactor experiment is blind to an LSND-like oscillation, but it shows up in off-axis as an unexpectedly large νe appearance. The combination of the two experiments can resolve the effect.
δCP = 180º
sin22θ13 = 0.02

16. Resolving the 23 Degeneracy
Green: Offaxis exp. Only Blue: Combined Medium Reactor
plus offaxis experiment
If 2345,  disappearance
experiments, which measure
sin2223, leave a 2-fold degeneracy
in 23 – it can be resolved
by combination of a reactor and
e appearance experiment.

17. Resolving the 23 Degeneracy
Green: Offaxis exp. Only Blue: Combined Medium Reactor
plus offaxis experiment
Red: Double-Chooz plus offaxis
If 2345,  disappearance
experiments, which measure
sin2223, leave a 2-fold degeneracy
in 23 – it can be resolved
by combination of a reactor and
e appearance experiment.
The Double-Chooz sensitivity
is insufficient to resolve degeneracy

18. Constraining the CP Phase
Oscillation probability vs dCP (m2 = 2.5x10-3 eV2 , sin2213 = 0.05)
Reactor measurement defines allowed bands:

19. Reactor Role in Determining CP
For δCP = 270º the reactor measurement eliminates some of the range in CP phase when combined with off-axis ν only running.
Off-axis anti-neutrino running resolves the CP phase on its own, after an additional 3 to 5 years.
Green: Offaxis exp. Only Blue: Combined Medium Reactor plus Offaxis Red: Combined Small Reactor plus Offaxis

20. CP Constraints from Off-Axis + Reactor
Dashed – without Reactor
Solid – with medium scale Reactor
To the right of the curve, this value of  may be excluded by at least two sigma
large medium small reactor
large medium small reactor
Reactor measurement does not add much to CP reach of  +  offaxis,
but a sin22 limit from reactor can largely rule out the possibility of a CP measurement at Nova or T2K.
Nominal Beam Rates
×5 Nominal Beam Rates
m2 = 2.5×10-3 eV2

21. Resolving the Mass Hierarchy
Reactor
(+/- 0.01)
normal
dCP
inverted
NOnA
(5 yr n)
m2=2.5x10-3 eV2

22. Resolving the Mass Hierarchy
Dashed – without Reactor
Solid – with medium scale Reactor
To the right of the curve, mass hierarchy is resolved by at least two sigma
large medium small reactor
large medium small reactor
Reactor measurement does not contribute much to resolving the mass hierarchy …
but a sin22 limit from even a small reactor experiment can largely rule out the possibility of determining sign(m232) at Nova and T2K.
Nominal Beam Rates
×5 Nominal Beam Rates
m2 = 2.5×10-3 eV2

23. Beyond 13: Weak Mixing Angle
Studies indicate that a measurement of sin2W with precision comparable to NuTeV could be performed using e – e scattering (normalized with inverse  decay).
(Conrad, Link, Shaevitz, hep-ex/ )

24. Beyond 13 (cont.)
CPT tests: comparing measurements at reactor experiments
with solar neutrinos and accelerator neutrinos
SN Physics: Like all scintillator experiments, a reactor experiment
will detect SN neutrinos of all flavors (with -p elastic scattering),
providing a test of SN models.
Solar parameters: A detector 70 km from an isolated reactor
complex will allow improved measurements of the solar
Parameters.

25. Conclusions
The worldwide program to understand  oscillations and determine the mixing
parameters, CP violating effects, and mass hierarchy will require a broad range
of measurements.
Our group believes that a key element of this program is a two-detector reactor
experiment (with baselines of 200m and 1.7 km) with sensitivity of 0.01 for sin2213.
It will provide a measurement of  free of ambiguities and with better precision
than any proposed experiment, or will set limits indicating the scale required
for future experiments.
In combination with accelerator experiments, it can resolve the degeneracy in
23, and may give early indications of CP violation and the mass hierarchy.
It can also provide interesting measurements of the weak mixing angle, as well
as neutrino magnetic moments, CPT tests, and supernova physics.

26. Highest priority recommendation
We recommend the rapid construction of a two-detector
reactor experiment with a sensitivity of 0.01 for sin22.

27. Other recommendations:
To help accomplish our highest priority recommendation, we recommend R&D support necessary to prepare a full proposal.
We recommend continued support for the KAMLAND experiment. KAMLAND has made the best determination of m122 to date, and will provide the best measurement of m122 for the foreseeable future. As the deepest running reactor experiment, it also provides critical information about cosmic-ray related backgrounds for future experiments.
We recommend the exploration of potential sites for a next-generation experiment at a distance of 70 km from an isolated reactor complex to make high precision measurements of 12 and m122.
We recommend support for development of future large-scale reactor 13 experiments that fully exploit energy spectrum information.