1. Electric Dipole Moment of Neutron and Neutrinos
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
Physics of neutron EDM
Status of neutron EDM measurements
Proposal for a new neutron EDM experiment at SNS
Neutrino EDM

2. Neutron Electric Dipole Moment
Non-zero dn violates both P and T symmetry
Under a parity operation:
Under a time-reversal operation:

3. 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 ~ e•cm
Super Symmetric Models predict dn ≤ 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

4. SUSY Prediction of Neutron versus Electron EDM
Barbieri et al.

5. History of Neutron EDM Measurements
Current neutron EDM upper limit: < 6.3 x e•cm (90% C.L.)

6. Neutron EDM Experiments
(d = e•cm, E = 10 KV/cm => 10-7 Hz shift )
Ramsey’s Separated Oscillatory Field Method
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

7. 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)

8. Neutron EDM Experiment with Ultra Cold Neutrons
Most Recent ILL Measurement
• 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

9. 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

10. 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)

11. A proposal for a new neutron EDM experiment
( Based on the idea originated by R. Golub and S. Lamoreaux in 1994 )
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

12. 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

13. 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

14. Two oscillatory signals
SQUID signal
Scintillation signal

15. Status of SNS neutron EDM
Many feasibility studies and measurements ( R&D)
CD-0 approval by DOE: 11/2005
Construction Possible: FY07-FY10
Cost: M$
CD-1 approval anticipated around 10/2006
Collaboration prepared to begin construction in FY07

16. 3He Distributions in Superfluid 4He
Dilution Refrigerator at
LANSCE Flight Path 11a
Position
Target
Cell
3He
Neutron Beam
4He
T = 330 mK
Preliminary
Physica B , 236 (2003)

17. Neutron Tomography of Impurity-Seeded Superfluid Helium
Phys. Rev. Lett. 93, (2004)

18. Critical dressing of neutrons and 3He
Dress field can modify neutron and 3He g factors:
Reduce the error caused by B0 instability between measurements
Effective dressed g factors: B1
1.19
0.408
3.86
1.324
6.77
3.333
9.72
4.348
neutron
3He

19. Los Alamos Polarized 3He Source
3He RGA
detector
Spin flip region
Injection nozzle
1 K cold head
Analyzer
quadrupole
Polarizer
quadrupole
3He Spin dressing experiment
36 in
B0 static
Polarizer
Analyzer
RGA
Ramsey coils
B1 dressing

20. Observation of 3He dressed-spin effect
Esler, Peng and Lamoreaux (2006)

22. Polarized 3He relaxation time measurements
T1 > 3000 seconds in 1.9K superfluid 4He
H. Gao, R. McKeown, et al, arXiv:Physics/

23. UIUC Test Apparatus for Polarized 3He Relaxation at 600 mK
Work carried out by UIUC and students from Hong Kong (CUHK)

24. SNS at ORNL
1.4 MW Spallation Source

25. n EDM Experiment at SNS

26. n-EDM Sensitivity vs Time
SNS
dn<1x10-28 e-cm
2000
2010

27. 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)
Another dedicated neutrino experiment is required at Daya Bay to improve the sensitivity on the neutrino EDM

28. 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.