16 April 2004 SAGENAP

A reactor experiment to measure θ13E. Blucher, Chicago

• APS neutrino study• Importance of θ13

• Unique role of reactor experiments• Conclusion

APS ν Study: Identify key questions of neutrino physics and evaluatemost promising experimental approaches to answering them.

written report in summer 2004

Working groups formed to explore particular experimental approaches:Solar/atmospheric, accelerators, reactors, neutrino factories, 0νββ decay, cosmology/astrophysics

Reactor working group: explore possibilities for neutrino physics with nuclear reactors

Broad participation from community:Erin Abouzaid, Kelby Anderson, Gabriela Barenboim, Andrew Bazarko, Eugene Beier, 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, Byron Roe, Mike Shaevitz, Robert Shrock, Noel Stanton, Ray Stefanski (+ Thierry Lasserre, Hervé de Kerret)

Neutrino physics at nuclear reactors

θ13 + several additional possibilities: sin2θW, solar ∆m2, neutrino magnetic moment, SN physics, CPT tests

E.g., early studies indicate that a measurement of sin2θW with precision comparable to NuTeV could be performed using νe – e− scattering.

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

νe

e−

ZW

dσdT

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

2-CV2) mT

E T E2=

CV = ½ + 2 sin2 θW

CA = ½

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

}

νe

e−

APS reactor study builds on work presented in series of international workshops, andwritten up in whitepaper.

International Workshops:Alabama, June 2003Munich, Germany October 2003Niigata, Japan, March 2004Paris, France, June 2004

APS reactor group meetings:Argonne, December 2003Chicago, February 2004May 2004

Neutrino Oscillations

• During last few years, oscillations among different flavors of neutrinos have been established; physics beyond the S.M.

• Mass eigenstates and flavor eigenstates are not the same (similar to quarks):

1 2 3 1

1 2 3 2

1 2 3 3

e e e eU U UU U UU U U

µ µ µ µ

τ τ τ τ

ν νν νν ν

=

mass eigenstatesflavor eigenstates MNSP matrix

• Raises many interesting questions including possibility of CP violation in neutrino oscillations.

• CP violation in neutrino sector could be responsible for the matter-antimatter asymmetry.

What do we know?1 2

1 2 3

1 2 3

12 12 13 13

12 12 23 23

13

3

13 23

cos sin 0 cos 0 sin 1 0 0sin cos 0 0 1 0 0 cos sin

0 0 1 sin 0 cos

?

0 sin co

CP

CP

ee e

i

i

U U Big BigU U U U Big Big Big

U U U

U

Big Big Big

e

mall

e

S

µ µ µ

τ τ τ

δ

δ

θ θ θ θθ θ θ θ

θ θ θ

−

= =

= − × ×

− − 23sθ

θ12 ~ 30° θ23 ~ 45°sin2 2θ13 < 0.2 at 90% CL

What is νe componentof ν3 mass eigenstate?

normal inverted

Key questions

•What is value of θ13?

•What is mass hierarchy?

•Do neutrino oscillations violate CP symmetry?P(ν µ → ν e ) − P(ν µ → ν e ) = −16s12c12s13c13

2 s23c 23 sinδ sin ∆m122

4EL

sin ∆m13

2

4EL

sin ∆m23

2

4EL

•Why are quark and neutrino mixing matrices so different?

1~ vs.

?~ 1

1MNSP CKM

Big Big Small SmallU Big Big Big V

SmSmall Small

Big Big Big Small Small

all

Value of θ13 central to these questions; it sets the scale for experiments needed to resolve mass hierarchy and search for CP violation.

Methods to measure sin22θ13

• Accelerators: Appearance (νµ→νe)2

2 2 2 21323 13 13( ) sin sin 2 sin not small terms ( , ( ))

4e CPm LP sign mEµν ν θ θ δ∆

→ = + ∆

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

T2K: <Eν> = 0.7 GeV, L = 295 km NOνA: <Eν> = 2.3 GeV, L = 810 km

• Reactors: Disappearance (νe→νe)2

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

4e em LPE

ν ν θ ∆→ = − +

Use reactors as a source of νe (<Eν>~3.5 MeV) with a detector 1-2 kms awayand look for non-1/r2 behavior of the νe rate

Reactor experiments provide the only clean measurement of sin22θ13:no matter effects, no CP violation, almost no correlation with other parameters.

Reactor Measurements of P( )e eν ν→

2atmm∆ 2

solarm∆2 2

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

4 4e em L m LPE E

ν ν θ θ∆ ∆→ ≈ − −

θ13: Search for small oscillations at 1-2 km distance (corresponding to 2 ).atmm∆

P ee

2 3 213

213

2.5 10

sin 2 0.043.5

m eV

E MeVν

θ

−∆ = ×

==

Past measurements:

Distance to reactor (m)

Chooz: Current Best θ13 Experiment

L=1.05 km

P=8.4 GWth

D=300mwe

m = 5 tons, Gd-loaded liquid scintillator

2.7%Total

2%1.5%

Reactor ν fluxDetect. Acceptance

CHOOZ Systematic errors

,e p e nν ++ → +Neutrino detection bysin22θ13< 0.2 for ∆m2=2×10−3 eV28 of s; ~ 30 secn Gd MeV γ τ µ+ →

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

Study has focused on three scales of experiments:• Small sin22θ13 ~ 0.03-0.04 (e.g., Double-Chooz)

• Medium sin22θ13 ~ 0.01 (e.g., Braidwood, Diablo Canyon,Daya Bay)

• Large sin22θ13 < 0.01

Ref. hep-ph/0403068

For each scenario, understand cost, timescale, and physics impact.

Strong consensus in working group that experiment withsensitivity of sin22θ13~0.01 should be our goal.

• If sin22θ13 < 0.01, it will be difficult for long-baseline “superbeam” experiments to investigate mass hierarchy and CP violation.

Reactor experiment with sensitivity of 0.01 will indicatescale of future experiments needed to make progress.

• If sin22θ13 > 0.01, a precise measurement will be needed tocombine with accelerator experiments.

Both reactor and accelerator experiments have sensitivity tosin22θ13, but accelerator measurements have ambiguities

Example: T2K. ∆P(νµ→νe)=0.0045 ∆sin22θ13=0.028

+/- 0.028

— normal— inverted

∆m2=2.5×10-3 eV2

(5 yr ν)

δcp

Reactor and accelerator sensitivities to sin22θ13

3σ Limits

Reactor with sensitivity of sin22θ13~0.01 at 90% c.l. (3σ~0.018)

NOνA

Value of θ13 sets scale of experiment needed to resolve mass hierarchy and study CP violation.

Example: FNAL Off-Axis (NOνA)Possible reactor limitssin22θ13<0.04sin22θ13<0.01

2σ limits for resolutionof mass hierarchy for3 years of ν and 3 yearsof ν running

Complementarity of reactor and accelerator experiments

NOνA(5 yr ν)

Reactor(+/- 0.01)

normal

inverted

∆m2=2.5x10-3 eV2

δCP

Searching for CP violation

δCP

P(ν µ

→ν e

)

sin22θ13=0.1

T2K

Example: Reactor + T2K ν runningP(

ν µ→

ν e)

T2K ν - 5 years

Neutrino, normal hierarchyNeutrino, inverted hierarchy

sin22θ13=0.1

∆sin22θ13=±0.01from reactor

δCP

δCP Measurement (with / without Reactor)

ν +νJHF+NuMI

ν +νJHF+NuMI+Reactor

ν +νJHF

δ = 270°

ν +νJHF+Reactor

sin22θ13=0.06

Conclusions

• Extremely exciting time for neutrino physics!

• Value of sin22θ13 sets the scale for experiments needed to study mass hierarchy and CP violation

• Reactor experiment has potential to be fastest, cheapest, and cleanest way to establish value of θ13

• Reactor experiment with sensitivity of sin22θ13~1% will giveinformation needed to understand future roadmap of neutrinoprogram

• Reactor and accelerator experiments are complementary:reactor information improves sensitivity of acceleratorexperiments to CP violation and mass hierarchy

δCP

P(ν µ

→ν e

)

At FNAL: Large matter effects will make it difficult to look forCP violation until mass hierarchy is resolved.