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Neutron Electric Dipole Moment Experiment Jen-Chieh Peng

International Workshop on “High Energy Physics in the LHC Era” Valparaiso, Chile,

December 11-15, 2006

University of Illinois at Urbana-Champaign

• Physics of neutron EDM• Proposal for a new neutron EDM experiment at

SNS (Spallation Neutron Source)• Results of R&D and future prospect

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Neutron Electric Dipole Moment

sdxdxxd nn ˆ)( 3

Non-zero dn violates both P and T symmetry

Under a parity operation: Under a time-reversal operation:

EEss

,ˆˆ EEss

,ˆˆ

EdEd nn

EdEd nn

Consider the energy nd E

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Physics Motivation for Neutron EDM Measurement

• Time Reversal Violation • CP Violation (in the light-quark baryon sector) • Physics Beyond the Standard Model

– Standard Model predicts dn ~ 10-31 e•cm – Super Symmetric Models predict dn ≤ 10-25 e•cm

• Baryon Asymmetry of universe – Require CP violation beyond the SM

SM Prediction Experiment

e 10-38 e•cm 2×10-27 e•cm

μ 10-35 e•cm 1×10-19 e•cm

n 10-31 e•cm 3×10-26 e•cm

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History of Neutron EDM Measurements

Current neutron EDM upper limit: < 3.0 x 10-26 e•cm (90% C.L.)

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List of Neutron EDM Experiments

B = 1mG => 3 Hz neutron precession freq.

Ex. Type <v>(m/cm) E (kV/cm) B (Gauss) Coh. Time (s) EDM (e.cm) year

Scattering 2200 1025 -- 10-20 < 3 x 10-18 1950

Beam Mag. Res. 2050 71.6 150 0.00077 < 4x 10-20 1957

Beam Mag. Res. 60 140 9 0.014 < 7 x 10-22 1967

Bragg Reflection 2200 109 -- 10-7 < 8 x 10-22 1967

Beam Mag. Res. 130 140 9 0.00625 < 3 x 10-22 1968

Beam Mag. Res. 2200 50 1.5 0.0009 < 1 x 10-21 1969

Beam Mag. Res. 115 120 17 0.015 < 5 x 10-23 1969

Beam Mag. Res. 154 120 14 0.012 < 1 x 10-23 1973

Beam Mag. Res. 154 100 17 0.0125 < 3 x 10-24 1977

UCN Mag. Res. <6.9 25 0.028 5 < 1.6 x 10-24 1980

UCN Mag. Res. <6.9 20 0.025 5 < 6 x 10-25 1981

UCN Mag. Res. <6.9 10 0.01 60-80 < 8 x 10-25 1984

UCN Mag. Res. <6.9 12-15 0.025 50-55 < 2.6 x 10-25 1986

UCN Mag. Res. <6.9 16 0.01 70 < 12 x 10-26 1990

UCN Mag. Res. <6.9 12-15 0.018 70-100 < 9.7 x 10-26 1992

UCN Mag. Res. <6.9 4.5 0.01 120-150 < 6.3 x 10-26 1999

EdBH

d = 10-26 e•cm, E = 10 KV/cm => 10-7 Hz shift in precession freq.

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Neutron EDM Experiments

Limitations: • Short duration for observing the precession • Systematic error due to motional magnetic

field (v x E)

Both can be improved by using ultra-cold neutrons

Ramsey’s Separated Oscillatory Field Method

Neutron precession frequency will shift by 2 / d E

(d = 10-26 e•cm, E = 10 KV/cm => 10-7 Hz shift )

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Ultra-Cold Neutrons (UCN)• First suggested by Fermi

• Many material provides a repulsive potential of ~ 100 nev (10 -7 ev) for neutrons

• Ultra-cold neutrons (velocity < 8 m/s) can be stored in bottles (until they decay).

• Gravitational energy is ~ 10-7 ev per meter

• UCN can be produced with cold-moderator (tail of the Maxwell distribution)

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Neutron EDM Experiment with Ultra Cold Neutrons

• Use 199Hg co-magnetometer to sample the variation of B-field in the UCN storage cell

• Limited by low UCN flux of ~ 5 UCN/cm3

A higher UCN flux can be obtained by using the “superthermal” down-scattering process in superfluid He

ILL Measurement

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UCN Production in Superfluid 4He

Incident cold neutron with momentum of 0.7 A-1 (10-3 ev) can excite a phonon in 4He and become an UCN

(Golub and Pendlebury)

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Kinematics of n - 4He Scattering

E(Q) is the phonon dispersion relation

||);(22

2222

fifi KKQQE

m

K

m

K

200nev

(typical wall potential)

θ is neutron’s scattering angleFor 1 mev neutron beam, σ(UCN)/σ(tot) ~ 10-3 for 200 nev wall potential Mono-energetic cold neutron beam with ΔKi/Ki ~ 2%

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UCN Production in Superfluid 4He

Magnetic Trapping of UCN(Nature 403 (2000) 62)

560 ± 160 UCNs trapped per cycle (observed)

480 ± 100 UCNs trapped per cycle (predicted)

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A proposal for a new neutron EDM experiment

Collaborating institutes:

Arizona State, UC Berkeley, Caltech, Duke, Hahn-Meitner, UIUC, Indiana, Kentucky, Leiden, LANL, MIT, NCSU, ORNL,

Simon-Fraser, Tennessee, Yale

( Based on the idea originated by R. Golub and S. Lamoreaux in 1994 )

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How to measure the precession of UCN in the Superfluid 4He bottle?

• Add polarized 3He to the bottle• n – 3He absorption is strongly spin-dependent

Total spin σabs at v = 5m/sec

J = 0 ~ 4.8 x 106 barns

J = 1 ~ 0

KeVtpHen 7643

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Neutron EDM Measurement Cycle

• Fill cells with superfluid 4He containing polarized 3He • Produce polarized UCNs with polarized 1mev neutron beam • Flip n and 3He spin by 90o using a π/2 RF coil • Precess UCN and 3He in a uniform B field (~10mG) and a

strong E field (~50KV/cm). (ν(3He) ~ 33 Hz, ν(n) ~ 30 Hz) • Detect scintillation light from the reaction n + 3He p + t

• Empty the cells and change E field direction and repeat the measurement

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1 1( ) { [1 cos( )]}tott

n rN t Ne P P t

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Two oscillatory signals

-15

-10

-5

0

5

10

15

603.4 603.6 603.8 604 604.2 604.4 604.6

Time (sec)

Am

plit

ud

e

3 0

03

3

3 [2( ) 2 ]1) Scintillation light from with

2) SQUID signal from the precession of

/

[2 ] with /

He n n

He

n H Bp

He

d

B

e t E

SQUID signal

Scintillation signal

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Status of SNS neutron EDM

• Many feasibility studies and measurements (2003-2006 R&D)

• CD-0 approval by DOE: 11/2005– Construction Possible: FY07-FY10– Cost: 15-18 M$

• CD-1 approval anticipated at end of 2006

• Collaboration prepared to begin construction in FY07

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3He Distributions in Superfluid 4He

Beam FWHM = 0.26 cm

0

5000

10000

15000

20000

25000

30000

35000

40000

-6.00 -4.00 -2.00 0.00 2.00 4.00 6.00

Position (cm)

n-3

He

No

rma

lize

d C

ou

nts

Neutron Beam

Position

4He

TargetCell

3He

Preliminary

T = 330 mK

Dilution Refrigerator atLANSCE Flight Path 11a

Physica B329-333, 236 (2003)

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3He Diffusion Coefficient in 4He

Europhysics Letters 58, 781 (2002);

Phys. Rev. Lett. 93, 105302 (2004)

n-3He Captures

0

20000

40000

60000

80000

100000

120000

140000

160000

-3 -2 -1 0 1 2 3

Position (cm)

Co

un

ts

Heater Off 0.84 K

Heater 27.1 mW 1.08K

175k -Li Glass

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Polarized 3He Atomic Beam Source

1 K cold head

Injection nozzle

Polarizerquadrupole

Spin flip region

Analyzerquadrupole

3He RGAdetector

Produce polarized 3He with 99.5% polarization at a flux of 2×1014/sec and a

mean velocity of 100 m/sec

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Dressed Spin in Neutron EDM

• Neutrons and 3He naturally precess at different frequencies (different g factors)

• Applying an RF field perpendicular to the constant B field, the effective g factors of neutrons and 3He will be modified (dressed spin effect)

• At a critical dressing field, the effective g factors of neutron and 3He can be made identical !

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Critical dressing of neutrons and 3HeCrossing points equalize

neutron and 3He g factors:

neutron 3He

3 1n 1n 0 3 0

0 0

c c

g g

BBJ J

J x J x

1.1127

3Heneutron

cx

n 1 /x B

Effective dressed g factors:

Reduce the danger of B0 instability between

measurements

1.19

3.86

6.77

9.72

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Los Alamos Polarized 3He Source

1 K cold head

Injection nozzle

Polarizerquadrupole

Spin flip region

Analyzerquadrupole

3He RGAdetector

B1 dressing

B0 static

Polarizer Analyzer RGA

36 in

3He Spin dressing experiment

Ramsey coils

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Polarized 3He source at LANLMapping the dressing field

source

analyzer

RGA

Spin-flip coils and dressing coils added inside the solenoid.

Cold head

Quad separatorSolenoid

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Observation of 3He dressed-spin effect

3He Larmor Frequency

26.2

26.4

26.6

26.8

27.0

27.2

27.4

27.6

0 2 4 6 8 10

Dressing Coil Current [A]

3He

Larm

or F

requ

ency

[k

Hz]

Esler, Peng and Lamoreaux, Preprint (2006)

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Polarized 3He relaxation time measurements

H. Gao, R. McKeown, et al, arXiv:Physics/0603176

T1 > 3000 seconds in 1.9K superfluid

4He

Acrylic cell coated with dTPB

Additional test is being done at

600mK

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High voltage tests Goal is 50 kV/cm

200 liter LHe. Voltage is amplified with a variable capacitor

90 kV/cm is reached for normal state helium. 30 kV/cm is reached below the λ-point

J. Long et al., arXiv:physics/0603231

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SNS at ORNL

First proton beam was delivered in April 2006First proton beam was delivered in April 2006

1.4 MW Spallation Source (1GeV proton, 1.4mA)1.4 MW Spallation Source (1GeV proton, 1.4mA)

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SNS Target Hallp beam

FNPB-Fundamental Neutron Physics Beamline

FNPB construction

underway

Cold beam available

~2007

UCN linevia LHe~2009

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FNPB Beamline

Double monochrometer

Selects 8.9 neutronsfor UCN via LHe

o

A

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Dilution Fridge

Helium InsulationVolume

LHe ReservoirIntermediateshield

4 K Shield

Cryovessel

Neutron EDM Detector

Conceptual Design Report is being prepared

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n-EDM Sensitivity vs Time

2000 2010

dn<1x10-28 e-cm

EDM @ SNSEDM @ SNS

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Summary• Neutron EDM measurement addresses

fundamental questions in physics (CP violation in light-quark baryons).

• A new neutron EDM experiment uses UCN production in superfluid helium and polarized 3He as co-magnetometer and analyser.

• The goal of the proposed measurement is to improve the current neutron EDM sensitivity by two orders of magnitude.

• Many feasibility studies have been carried out. Construction is expected to start in FY2007.