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Electric Dipole Moment of Neutron and Neutrinos

• Physics of neutron EDM• Status of neutron EDM measurements• Proposal for a new neutron EDM

experiment at SNS• Neutrino EDM

Jen-Chieh Peng

University of Illinois at Urbana-Champaign

Workshop on Future PRC-U.S. Cooperation in High Energy Physics, IHEP, June 11-18, 2006

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Neutron Electric Dipole Momentexdxqn

213 10)1.14.0()(

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 10-40 e•cm 10-27 e•cm

μ 10-38 e•cm 10-19 e•cm

n 10-31 e•cm 10-25 e•cm

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SUSY Prediction of Neutron versus Electron EDM

Barbieri et al.

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History of Neutron EDM Measurements

Current neutron EDM upper limit: < 6.3 x 10-26 e•cm (90% C.L.)

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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 potential 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 much higher UCN flux can be obtained by using the “down-scattering” process in superfluid 4He

Most Recent 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

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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:

UC Berkeley, Caltech, Duke, Hahn-Meitner, Harvard, Hungarian Academy of Sciences, UIUC, ILL, Indiana,

Leiden, LANL, MIT, NIST, NCSU, UNM, ORNL, Simon-Fraser

( 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

33

0

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

2) SQUID signal from the precession of

/

wi ]h [ /t

He n n

He

n H B d Ee p t

He B

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 around 10/2006

• 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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Neutron Tomography of Impurity-Seeded Superfluid Helium

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

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Critical dressing of neutrons and 3HeDress field can modify

neutron and 3He g factors:

3neutron

He

3 1n 1n 0 3 0

0 0

c c

g g

BBg J g J

J x J x

1.1127

3Heneutron

B1

1.19 0.408

3.86 1.324

6.77 3.333

9.72 4.348

cx

n 1 /x B

Effective dressed g factors:

Reduce the error caused by B0 instability between

measurements

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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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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 (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

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UIUC Test Apparatus for Polarized 3He Relaxation at 600 mK

Work carried out by UIUC and students from Hong Kong

(CUHK)

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SNS at ORNL1.4 MW Spallation Source1.4 MW Spallation Source

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n EDM Experiment at SNS

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n-EDM Sensitivity vs Time

2000 2010

dn<1x10-28 e-cm

EDM @ SNSEDM @ SNS

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Neutrino electric dipole moment • For Majorana neutrinos, CPT invariance ensures zero electric

and magnetic dipole moments• For Dirac neutrinos, non-zero EDM is possible (CP-violation)

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21

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2 22 2

2

Bounds on neutrino EDM ( )

1 / (| |

| | 5.2 10

| |

From

|

2 10 (MUNU, TEXONO)

| | 1.4 10 (LSND)

| | 7.8

width

From -e scattering

| )

e

d

e

T Edd

dT m

d e cm

d e cm

d e c

d

T

m

18

22

10 (DONUT)

| | 2.5 10 (PL 128B (1983) 43

From cosmo g

1)

lo y

e cm

d e cm

Another dedicated neutrino experiment is required at Daya Bay to improve the sensitivity on the neutrino EDM

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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.