Electric Dipole Moment of Neutron and Neutrinos

1. Electric Dipole Moment of Neutron and Neutrinos Jen-Chieh Peng University of Illinois at Urbana-Champaign • Physics of neutron EDM • Status of neutron EDM measurements • Proposal for a new neutron EDM experiment at SNS • Neutrino EDM Workshop on Future PRC-U.S. Cooperation in High Energy Physics, IHEP, June 11-18, 2006

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~ 10-31 e•cm • Super Symmetric Models predict dn ≤ 10-25 e•cm • Baryon Asymmetry of universe • Require CP violation beyond the SM

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 10-26 e•cm (90% C.L.)

6. Neutron EDM Experiments (d = 10-26 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

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 (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

16. Position 3He Distributions in Superfluid 4He Dilution Refrigerator at LANSCE Flight Path 11a Target Cell 3He Neutron Beam 4He T = 330 mK Preliminary Physica B329-333, 236 (2003)

17. Neutron Tomography of Impurity-Seeded Superfluid Helium Phys. Rev. Lett. 93, 105302 (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: 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 B0static RGA Polarizer Analyzer 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/0603176

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 2000 2010 EDM @ SNS dn<1x10-28 e-cm

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.