17 November 2005 BNL HEP Seminar

1

The Braidwood Neutrino Experiment

• Outstanding questions in neutrino oscillation physics: importance of 13

• Experimental approaches to 13; motivation for a precise reactor experiment

• The Braidwood Experiment

Ed Blucher, Chicago

2Neutrino 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:

1 2 3 1

1 2 3 2

1 2 3 3

e e e eU U U

U U U

U U U

mass eigenstatesflavor eigenstates

• 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

(leptogenesis)

MNSP matrix

( ) ( )N H N H The antilepton excess is converted to a baryon excess throughnonperturbative S.M. B+L violating, but B-L conserving processes.

3

2-Flavor Neutrino Mixing

1

2

cos sin

sin cose

The time evolution of the flavor states is:1 2

1 2

1 2

1 2

cos sin

sin cos

iE t iE te

iE t iE t

e e

e e

For a beam that is pure at t=0,

222 2

2 2 22 1

( ) sin 2 sin ,4

where m

e e

mP L

E

m m

4

What do we know?

Oscillations established with two distinct mass differences

1. Atmospheric: m2~2.5 eV2

Experiments using neutrinos produced by cosmic rays in atmosphere (e.g., SuperK); verified with long-baseline accelerator experiment (K2K).

Super Kamiokande

K2K

5

2. Solar: m2~55 eV2

Series of experiments using neutrinos from the Sun (e.g.,Ray Davis 37Cl experiment, SNO) and KAMLAND experiment using reactors in Japan.

Ray Davis SNO KAMLAND

6

Unconfirmed observation of oscillations with m2~1 eV2 by LSND does not fitinto 3 generation model (with 2 independent mass splittings).

MiniBoone should have results early nextyear.

What about LSND?

7

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 0

sin 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 Big

U U U U Big Big Big

U U U

U

Big Big Big

e

mall

e

S

23s

12 ~ 30° 23 ~ 45°

What is e component of 3 mass eigenstate?

Neutrino mixing and masses

normal inverted

sin2 213 < 0.15 at 90% CL

8

•What is value of 13?

•What is mass hierarchy?

•Do neutrino oscillations violate CP symmetry?

P( e ) P( e ) 16s12c12s13c132 s23c23 sin sin

m122

4EL

sin

m132

4 EL

sin

m232

4 EL

Value of 3 central to these questions; it sets the scale for experiments needed to resolve mass hierarchy and search for CP violation.

Key questions in neutrino mixing

•Why are quark and neutrino mixing matrices so different?

1

~ vs.

?

~ 1

1MNSP CKM

Big Big Small Small

U Big Big Big V

Sm

Small Small

Big Big Big Small Small

all

9Methods to measure sin2213

• Accelerators: Appearance (e) at m22.510-3 eV2

• Reactors: Disappearance (ee) at m22.510-3 eV2 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 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.

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

10

We recommend, as a high priority, a comprehensive U.S. program to completeour understanding of neutrino mixing, to determine the character of the neutrinomass spectrum, and to search for CP violation among neutrinos. This programshould have the following components:

• An expeditiously deployed multi-detector reactor experiment with sensitivity todisappearance down to sin22 = 0.01, an order of magnitude below presentlimits.

• A timely accelerator experiment with comparable sin22 = 0.01 sensitivity andsensitivity to the mass hierarchy through matter effects.

• A proton driver in the megawatt class or above and neutrino superbeam with anappropriate very large detector capable of observing CP violation and measuring the neutrino mass-squared differences and mixing parameters with high precision.

Recommendation 2 (of 3):

11

+/- 0.028

cp— normal— inverted

m2=2.510-3 eV2

Both reactor and accelerator experiments have sensitivity tosin22, but accelerator measurements have ambiguities

Example: T2K. P(e)=0.0045 sin2213=0.028

(5 yr

12

90% CL exluded regions with no osc.signal 90% CL allowed regions with osc.signal

Reactor and accelerator sensitivities to sin22

sin22θ13 = 0.05, δCP=0, Δm2 = 2.5×10-3 eV2

(3 yr reactor, 5 yr T2K)

δCP=0, Δm2 = 2.5×10-3 eV2

(3 yr reactor, 5 yr Nova)

Braidwood

13Resolving the 23 Degeneracy

Green: Nova OnlyBlue: Braidwood Reactor

plus Nova

Red: Double-Chooz plus offaxis disappearance experiments measure sin2223, while P(e)sin223sin2213.

•If 2345, disappearanceexperiments, leave a 2-fold degeneracy in 23 – it can be resolved by combination of a reactor and e appearance experiment. Braidwood (3 yrs) + Nova

Nova only (3yr + 3yr) Double Chooz (3yrs) + Nova

90% CL

Example:

sin22 23 = 0.95 0.01 Δm2 = 2.5×10-3 eV2 sin2213 = 0.05

Δm2 = 2.5×10-3 eV2 sin2213 = 0.05

14

CP

P( e

)

sin2213=0.1

T2K

CP

Nova

CP Violation and the Mass Hierarchy

15P

( e

)

CP

sin22=0.01from reactor

T2K - 5 years

Neutrino, normal hierarchy

Neutrino, inverted hierarchy

Example: Reactor + T2K running

sin2213=0.1

16

Nova and T2K Sensitivity to CP and Mass Hierarchy

If Braidwood does not see an oscillation signal, it will be difficult for

long-baseline “superbeam” experiments to investigate mass hierarchy and

CP violation.

17Reactor Measurements of Neutrino Oscillations

0.00

10.00

20.00

30.00

40.00

50.00

60.00

1.50 2.50 3.50 4.50 5.50 6.50 7.50 8.50 9.50

Enu (MeV)

Arb

itra

ry S

cale

Neutrino Flux Cross Section # of Events

Reactors are copious sources of per second.21: 5 GW 10 e e

Detection of antineutrino by e p e n

( 6 / fission)e

Flux

Cross

secti

onObservable spectrum

(~100 events /GW/ yr / ton at L= 1500 m)

18

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:

Our sensitivity goal: sin22~0.01.Level at which long-baseline accelerator experiments can be used to measure mass hierarchy, CP violation.

19Chooz: Current Best Experiment

L=1.05 km

P=8.4 GWth

D=300mwe

m = 5 tons, Gd-loaded liquid scintillator

sin22< 0.15 for m2=2.5103 eV2

CHOOZ Systematic errors

Reactor flux

Detect. Acceptance

2%

1.5%

Total 2.7%

20

~200 m ~1300 m

How to improve on previous reactor experiments? Add an identical near detector

Eliminate dependence on reactor flux; only relative acceptance of detectors needed

Optimize baseline (1500 m) Larger detectors (5 ton 100 tons) Reduce backgrounds (Go deeper 100m 150 to 300 m; active veto systems)

21

Many sites have been discussed:

•Kraznoyarsk (Russia)•Chooz (France)•Kashiwazaki (Japan)•Diablo Canyon (California)•Braidwood, Byron (Illinois)•Wolf Creek (Kansas)•Brazil•Taiwan• Daya Bay (China)

22

Ref: Marteyamov et al, hep-ex/0211070

Reactor

Detector locations constrained by existing infrastructure

Features - underground reactor - existing infrastructure

~20000 ev/year~1.5 x 106 ev/year

Kr2Det: Reactor 13 Experiment at Krasnoyarsk

23

The Chooz site, Ardennes, France

24Daya Bay, China

25

U.S. Nuclear Power Plants

26

BraidwoodNeutrino

Experiment

Features of Braidwood Site:• 23.6 GW reactors – 7.17 GW maximum power• Flat: flexibility, equal overburden at near and far sites, surface transportation of detectors• Favorable geology (dolomitic limestone): good for excavation, low radioactivity (order of magnitude lower U, Th than granite)

Braidwood

27The Braidwood Collaboration

14 Institutions

70 Collaborators

28

Goals: Flexibility, redundancy, cross checks•4 identical 65 ton fiducial mass detectors; 2 at near site (L=270m), 2 at far site (L=1510m) •“Two zone detectors”: inner zone with Gd-loaded LS and r=2.6 m; outer zone with mineral oil and r=3.5 m.• Movable detectors with surface transport for cross-calibration; vertical shaft access to detector halls•Oscillation measurements using both rate and energy spectrum• Full detector construction above ground; detectorsfilled simultaneously with common scintillator.• Near and far detectors at same depth of 183 (464 mwe) gives equal spallation rates that can be exploited for detector and background checks

Braidwood Baseline Design

29

Near Near DetectorDetector

Far Far DetectorDetector

Braidwood Site

30Bore Hole Project at the Exelon Site

Bore hole project completed in January 2005

– Bore holes drilled to full depth (200m) at near and far shaft positions on Braidwood site.

– Provided detailed information on geology, ground water, radioactivity, etc.

– Confirmed feasibility of detectors down to depths of 460mwe.

– Reduces contingency required for underground construction

– Demonstrated willingness of Exelon to allow construction on their site.

31Braidwood Design Sensitivity

GOALS:

1. Discovery potential (at 3) for sin2213 > 0.01

2. Sensitivity (90% CL) down to the sin2213 = 0.005 level

With cross checks and redundancy to establish signal and check systematic errors

• See signal in both rate and energy spectrum measurements

• Cross calibrate detector pairs at high-rate near site

• Cross calibrate near/far detectors using spallation isotopes like 12B

• Multiple near and far detectors give direct cross checks on detector systematics at 0.05% for the near set and 0.3% for far

• Large detectors allow studies of the radial dependence of the IBD signal and backgrounds.

32

Normalization and spectral information

E (MeV)

E (MeV)

Predicted spectrum 13=0 (from near detector)

Observed spectrum (far detector)sin2213=0.04

Counting analysis: Compare number of events in near and far detectorSystematic uncertainties:• relative normalization of near and far detectors• relatively insensitive to energy calibration

Energy spectrum analysis: Compare energy distribution in near and far detectorsSystematic uncertainties:• energy scale and linearity• insensitive to relative efficiency of detectors

33Detectors and analysis strategy designed to minimize relative acceptance differences

6 meters

Shielding

To reduce backgrounds: depth + active and passive shielding

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) and s releasedfrom n+Gd capture (Evis>6 MeV).

No explicit requirement on reconstructedevent position; little sensitivity to E requirements.

,e p e n Neutrino detection by

n

e+

e

Gd-loaded liquidscintillator

n mGd → m+1Gd s (8 MeV); =20sec

34

– Outer steel buffer oil containment vessel (7m diameter)

• 1000 low activity glass 8” PMTs evenly distributed on inside surface (25% coverage)

– Inner acrylic Gd-loaded scinitillator containment vessel (5.2m diameter)

– Top access port can be used to insert calibration sources

Conceptual Mechanical Design

35Detector With Moveable Veto System and Shielding

36

Known volume of stable, identical Gd-loadedliquid scintillator in each detector

Well understood efficiency of positron and neutron energy requirements

Acceptance Issues

Must know: (relative) number of protons in fiducial region (relative) efficiency for detecting IBD events

37

Reconstructed Positron Energy

~E 0.8 MeV

Reconstructed NeutronCapture Energy

Reconstructed Energy Cuts:

• positron: Evis > 0.5 MeV

• n-Gd capture: Evis > 6 MeV

Monte Carlo Studies

Studies based on hit-levelsimulation with parameterizationsof many detector effects.

Studies using full GEANT4simulation are underway.

n Capture on H

n Capture on Gd

Reconstructed e+ and n-capture energy

38

Energy Scale

Use neutron capture peaks from IBD events to measure energy scale.

In each far detector, E scale can be measured to 0.3% every 5 days. (This calibration averages over detector in exactly the same way as signal events.)

Acceptance uncertainty from energy scale should be ~0.1%.

Calibration fromneutron capture peaks

0.1%uncertainty

39

I

IIIII

I. Gd-loaded liquid scintillatorII. catcher: liquid scintillator (no Gd)III. Non-scintillating buffer

3-zone versus 2-zone detectors

(Braidwood 2-zoneDesign)

40

Acceptance Sensitivity to Energy Scale

41

0.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

0.010

0.011

0.012

10/15/2004 12/4/2004 1/23/2005 3/14/2005 5/3/2005 6/22/2005 8/11/2005 9/30/2005 11/19/2005

Calendar Date

Ab

s a

t 4

30

nm

1.2% Gd in PC 0.2% Gd in PC (mixed samples from Gd16-Gd20)0.14% Gd in PC

0.2% Gd in 20%PC 80% Dodecane0.2% Gd in 40%PC 60% dodecane

338 days

297 days

259 days

198 days

Gd - Liquid Scintillator (Gd-LS)

• Detectors must be filled simultaneously common scintillator;

relative volume measurement with <0.2% uncertainty.

• We plan to use 0.2% Gd + 20% PC + 80% dodecane mixture

developed by BNL Nuclear Chemistry group.

(Dick Hahn, Minfeng Yeh, et al.)– Long-term stability tests in progress

– So far, stable with attenuation length > 18 m.

Stability of Gd-LS(Absorbance of 0.002 corresponds to attenuation Length of ~20 m).

Chooz degradation was 0.4%/day

x - Braidwood scintillator

42Movable Detectors

• Transport is necessary to move detectors from construction/filling area to below ground halls

• Movable detectors allow direct check of relative detector acceptances at

near site

• Possible scenario:

• Possible method: Use climbing jack system with cable to lift and put detectors on multi-wheeled trailer (standard method used in industry).

Goldhofer TrailerMoving 400 tons

Period Near Far

Initial 3 months

3 year data run

Final check

A B

A BC D

A CBD

43

• Unique feature of the Braidwood site: Near and far detectors have equal, well-understood,

substantial overburden

Can use produced 12B events to measure:• Near/far relative target mass from the total rate• Near/far energy calibrations from the relative energy

distribution

• ~50,000 12B beta-decay events per year per detector can be tagged and isolated giving a statistical uncertainty of 0.45%

• Systematic uncertainties related to the knowledge of relative near/far overburden; must be known to few percent from:– Geological survey information (Bore hole data: near/far agreement <1%)– Cosmic muon rates in the near and far locations

12BBeta decays t1/2 = 20ms (can tag to muon)13.4 MeV endpoint

Using Isotope Production to Measure Fiducial Mass

44

Summary of Acceptance Uncertainties

45Backgrounds

Even though near and far shielding is the same, backgrounds donot cancel: signal/background ratios in the near and far detectors are different.

– Uncorrelated backgrounds from random coincidences (not a problem)• Reduced by limiting radioactive materials

• Limestone rock at Braidwood site has low radioactivity

• Directly measured from rates and random trigger setups

– Correlated backgrounds

• Neutrons that mimic the coincidence signal

• Cosmogenically produced isotopes that decay to a beta and neutron

(9Li and 8He).

46Cosmic Muon Rates at Braidwood Depths

• Calculation of muon rate at 464 mwe (600 ft)– Used data from boreholes for density and

material– Average muon flux = 0.213 /m2/sec– Average muon energy = 110.1 GeV

Material Chemical composition Density (g/cm3)

Depth of top of layer (m)

Soil SiO2 1.60 0.0

Mudstone SiO2 2.46 11.3

Mudstone SiO2 2.52 27.1

Limestone CaCO3 2.61 42.7

Limestone CaCO3 2.63 61.0

Mudstone SiO2 2.60 63.1

Dolomitic Limestone 0.63* CaCO3 + 0.37*MgCO3

2.58 82.6

Dolomitic Limestone 0.63* CaCO3 + 0.37*MgCO3

2.62 98.8

Dolomitic Limestone 0.63* CaCO3 + 0.37*MgCO3

2.71 116.4

Dolomitic Limestone 0.63* CaCO3 + 0.37*MgCO3

2.62 135.0

Limestone CaCO3 2.63 142.3

Limestone CaCO3 2.71 157.6

Dolomitic Limestone 0.63* CaCO3 + 0.37*MgCO3

2.66 168.9

47Veto (Tagging) SystemGoal: < 1 n background event/day/detector.Strategy: tag muons that pass near the detector. Use shieldingto absorb neutrons produced by muons that miss the veto system.

Veto Detectors

p

n

n

6 meters

Shielding

Residual n background:

1. Veto inefficiency ─ 99% efficiency → 0.25/detector/day

2. Fast neutron created outside the shielding ─ 0.5/detector/day

With μ rate in the veto system of 21 Hz and the tag window of 100 μs → 0.2% dead time

Muon identification must allow in situ determination of the residual background rate

48Background Simulations

For a veto system with 2 mwe of

shielding, both a GEANT4 and a

MARS calculation give:

• 170 n/ton/day produced in the surrounding rock

• 4500 n/day emerging from the rock

• Background rate of ~0.75 events/ dayafter the veto requirements

Neutrons that reach the vessel wall

Fra

ctio

n o

f N

eu

tro

ns

Untaggedneutrons

De

tect

or

499Li and 8He

Isotopes like 9Li and 8He can be created in μ spallation on 12C and can decay to β+n.

Long lifetimes make veto difficult: 9Li~178ms

KAMLAND found isotope production correlated with muons that shower in the detector. from the thesis of Kevin McKinny

Tagging showering muons and rejecting events in a 0.5 s window eliminates 72% of 9Li and results in 7% deadtime.

Expect 0.078 9Li/ton/day; half decay in β+n modes; 72% are tagged 0.7/detector/day.

More…

50

Background Summary

Background Rate (<) Dead Time

Random 0.4/detector/day none

Fast Neutron (inside) 0.3/detector/day 0.2%

Fast Neutron (outside) 0.5/detector/day none9Li 0.7/detector/day 7%

Total 1.9/detector/day ~7%

Compare to 160 signal/detector/day at the far site (S/N~85)

51Sensitivity and Discovery Potential

• For three years of Braidwood dataand m2 > 2.5 x 10-3 eV2

90% CL limit at sin2213 < 0.005

3 discovery for sin2213 > 0.013

Summary of Uncertainties for 3 yr Data

With two near and two far detectors, thetotal uncertainty in the near/far ratio is 0.33%

5290% CL Sensitivity vs Years of Data

• Information from both counting and shape fits

• Combined sensitivity for sin2213 reaches the 0.005 after three years

0.001

0.010

0.100

1.000

0.1 1 10 100

Running Time (yrs)

sin

22q

13 9

0% C

L li

mit

(1.

28 s

)

count

shape

combine

3 ye

ars

Dm2 = 2.5e-3 eV2

53Braidwood Measurement Capability

For 3 years of data and a combined counting plus shape analysis m2 = 2.5 x 10-3 eV2 and sin2213 = 0.02

54Other Physics: Neutrino Electroweak Couplings

Braidwood experiment can isolate about 10,000e – e events that will allow the measurement of the neutrino gL

2 coupling to ~1%

– This is 4 better than past -e experiments and would give an error comparable to gL

2(NuTeV) = 0.3001 0.0014

Precision measurement possible since:– Measure elastic scattering relative to inverse beta decay

– Can pick a visible energy window (3-5 MeV) away from background

gL2 - gL

2(SM)

55

2004 Engineering/R&D proposal 2005 NuSAG Review2006 Full proposal submission2007 Project approval; construction start2010 Start datataking

Cost Estimate:

Civil Costs: $34M + $8.5M (Cont.)

4 Detectors and Veto Systems: $18M + $5M (Cont.)

Status of Project

Exelon enthusiasticsupporter of project

56

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 – a reactor experiment to measure 13 is a key part of this program.

•A reactor experiment will provide the most precise measurementof 13 or set the most restrictive limit.

•Reactor experiment with sensitivity of sin22~1% will give information needed to understand future roadmap of neutrino program.

•Braidwood offers an ideal site to perform an experiment with the required sensitivity (sin2213 = 0.005 at 90% c.l.)