29 June 2004 APS Neutrino Study

Reactor Working Group Report

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

• Future reactor experiments to measure sin2213

• What do we learn by combining reactor and accelerator measurements?

• Beyond 13

• Conclusions and Recommendations

Neutrino physics at nuclear reactors

Beyond : sin2W, 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?

: The key parameter for next generation of neutrino oscillation experiments. Its value sets scale of experimentsneeded 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 resolve2 degeneracy, and provide early information about CPviolation, mass hierarchy.

Strong consensus in working group that experiment withsensitivity of sin2213=0.01 should be our goal.

Methods to measure sin2213

• Accelerators: Appearance (e)

• Reactors: Disappearance (ee) 2

2 2 1313( ) 1 sin 2 sin very small terms

4e e

m LP

E

22 2 2 213

23 13 13( ) sin sin 2 sin not small terms ( , ( ))4e CP

m LP sign m

E

Use fairly pure, accelerator produced beam with a detector a long distancefrom the source and look for the appearance of e events

T2K: <E> = 0.7 GeV, L = 295 km NOA: <E> = 2.3 GeV, L = 810 km

Use reactors as a source of e (<E>~3.5 MeV) with a detector 1-2 kms awayand 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.

2atmm 2

solarm2 2

2 2 2 213 1213 12( ) 1 sin 2 sin sin 2 sin

4 4e e

m L m LP

E E

Reactor Measurements of ( )e eP

13: Search for small oscillations at

1-2 km distance (corresponding to 2 ).atmm

Distance to reactor (m)

Pee

2 3 213

213

2.5 10

sin 2 0.04

3.5

m eV

E MeV

Past measurements:

Chooz: Current Best Experiment

L=1.05 km

P=8.4 GWth

D=300mwe

m = 5 tons, Gd-loaded liquid scintillator

sin22< 0.2 for m2=2103 eV2

CHOOZ Systematic errors

Reactor flux

Detect. Acceptance

2%

1.5%

Total 2.7%

,e p e n Neutrino detection by 8 of s; ~ 30 secn Gd MeV

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

Detectors and analysis strategy designed to minimize relative acceptance differences

6 meters

Shielding

Identical near and far detectors

Central zone with Gd-loadedscintillator surrounded by bufferregions; fiducial mass determinedby volume of Gd-loaded scintillator

Events selected based on coincidenceof e+ signal (Evis>0.5 MeV) ands released from n+Gd capture(Evis>6 MeV). No positionreconstruction; little sensitivity to E requirements.

To reduce backgrounds: depth + active and passiveshielding

Study has focused on three scales of experiments:

•Small sin2213 ~ 0.03 (e.g., Double-Chooz, KASKA)

•Medium sin2213 ~ 0.01 (e.g., Braidwood, Diablo Canyon, Daya Bay)

•Large sin2213 ~ 0.005 (e.g.,Angra)

For each scenario, understand scale of experiment requiredand physics impact.

(sensitivities at 90% confidence level)

norm= 0.8%

Sensitivity Using Rate and Energy Spectrum(Huber et al. hep-ph/0303232)

m2 = 3×10-3 eV2

Shape only norm =

Statistics only norm = 0

norm= 0.8%

m2 = 3×10-3 eV2

Sensitivity Using Rate and Energy Spectrum(Huber et al. hep-ph/0303232)

Shape only norm =

Statistics only norm = 0

Small Medium Large

Small: sin2213 ~ 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 checksCost: ~$20 M; start datataking in 2008

Medium: sin2213 ~ 0.01 (e.g., Braidwood, Diablo Canyon, Daya Bay)50-100 ton detectors, optimized baseline, optimized depths, rate and shape info, perhaps movable detectors to check calibration, multiple far detector modules for additional cross checksCost:~$50 M (for US sites); start datataking in 2009

Large: sin2213 ~ 0.005 (e.g., Angra)~500 ton fiducial mass; sensitivity mainly through E spectrum distortion

Different Scales of Experiments

Reactor Sensitivity Studies:Comparing and Combining with Offaxis Measurements

• Experimental Inputs

JPARC to SuperK (T2K)

• : 102 signal / 25 bkgnd 5 yrs; : 39 signal / 14 bkgnd 5 yrs

• plus upgrade 5 rate

Offaxis NuMI (Nova)

• : 175 signal / 38 bkgnd 5 yrs : 66 signal / 22 bkgnd 5 yrs

• plus Proton Driver upgrade 5 rate

• Oscillation parameters

for sin22=0.1

for sin22=0.1

(M. Shaevitz)

large medium small reactorlarge medium small reactor

T2K

NOνA

combinewith med.

reactor

combinewith med.

reactor

××5 beam5 beamraterate

Setting Limit on sin2213

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).

Green: Offaxis exp. OnlyBlue: Combined Reactor plus OffaxisWhite: Offaxis Only (x5 rate)

T2K

NOνA

Chooz-like, small scaleChooz-like, small scale

Braidwood-like medium scaleBraidwood-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

Determining Value of sin2213

Green: Offaxis exp. OnlyBlue: Combined Medium Reactor plus OffaxisRed: Combined Small Reactor plus Offaxis

Importance of Multiple Measurements

The reactor measurement may not agree with the results of the off-The reactor measurement may not agree with the results of the off-axis experiments. axis experiments.

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.

With a 1% LSND-like oscillationWith a 1% LSND-like oscillation

δCP = 180º

sin22θ13 = 0.02

Resolving the 23 Degeneracy

Green: Offaxis exp. OnlyBlue: Combined Medium Reactor

plus offaxis experiment•If 2345, disappearanceexperiments, which measuresin2223, leave a 2-fold degeneracyin 23 – it can be resolvedby combination of a reactor ande appearance experiment.

Resolving the 23 Degeneracy

Green: Offaxis exp. OnlyBlue: Combined Medium Reactor

plus offaxis experiment

Red: Double-Chooz plus offaxis•If 2345, disappearanceexperiments, which measuresin2223, leave a 2-fold degeneracyin 23 – it can be resolvedby combination of a reactor ande appearance experiment.

•The Double-Chooz sensitivityis insufficient to resolve degeneracy

Constraining the CP Phase

• Oscillation probability vs CP (m2 = 2.5x10-3 eV2 , sin2213 = 0.05)

• Reactor measurement defines allowed bands:

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.

Reactor Role in Determining CP

Green: Offaxis exp. OnlyBlue: Combined Medium Reactor plus OffaxisRed: Combined Small Reactor plus Offaxis

CP Constraints from Off-Axis + Reactor

Dashed – without ReactorSolid – with medium scale Reactor

mm22 = 2.5 = 2.5××1010-3-3 eV eV22

To the right of the curve, this value of may be excluded by at least two sigma

Nominal Nominal Beam RatesBeam Rates

××5 Nominal 5 Nominal Beam RatesBeam Rates

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.

large medium small reactorlarge medium small reactor large medium small reactorlarge medium small reactor

NOA(5 yr )

Reactor(+/- 0.01)

CP

normal

inverted

m2=2.5x10-3 eV2

Resolving the Mass Hierarchy

Dashed – without ReactorDashed – without ReactorSolid – with medium scale ReactorSolid – with medium scale Reactor

Nominal Nominal Beam RatesBeam Rates

××5 Nominal 5 Nominal Beam RatesBeam Rates

To the right of the curve, mass To the right of the curve, mass hierarchy is resolved by at least two hierarchy is resolved by at least two sigmasigma

Resolving the Mass Hierarchy

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(m23

2) at Nova and T2K.

mm22 = 2.5 = 2.5××1010-3-3 eV eV22

large medium small reactorlarge medium small reactor large medium small reactorlarge medium small reactor

Beyond 13: Weak Mixing Angle

Studies indicate that a measurement of sin2W with precision comparable to NuTeV could be performed using e – e scattering (normalized with inverse decay).Use the antineutrino-electron elastic scattering

e

e

ZW

ddT

G2m 2 {(CV+CA)2 +(CV-CA)2 (1- )2 + (CA

2-CV2) mT

E T E2=

CV = ½ + 2 sin2 W

CA = ½

T = electron KE energyE = neutrino energym= mass of electronThis assumes =0

The total rate for this process is sensitive to sin2 W

(ES)

}

e

e

(Conrad, Link, Shaevitz, hep-ex/0403048)

CPT tests: comparing measurements at reactor experimentswith solar neutrinos and accelerator neutrinos

SN Physics: Like all scintillator experiments, a reactor experimentwill 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 reactorcomplex will allow improved measurements of the solar Parameters.

Beyond 13 (cont.)

Conclusions

•The worldwide program to understand oscillations and determine the mixingparameters, CP violating effects, and mass hierarchy will require a broad rangeof 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 sin2213.

•It will provide a measurement of free of ambiguities and with better precisionthan any proposed experiment, or will set limits indicating the scale requiredfor future experiments.

•In combination with accelerator experiments, it can resolve the degeneracy in23, and may give early indications of CP violation and the mass hierarchy.

•It can also provide interesting measurements of the weak mixing angle, as wellas neutrino magnetic moments, CPT tests, and supernova physics.

We recommend the rapid construction of a two-detector

reactor experiment with a sensitivity of 0.01 for sin22.

Highest priority recommendation

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 m12

2.

•We recommend support for development of future large-scale reactor 13 experiments that fully exploit energy spectrum information.