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Neutron Electric Dipole Moment

Neutron Electric Dipole Moment. Sham Chun Hong. Spin-Dressing Effect. Sham Chun Hong. Abstract. Part 1: Basic knowledge Part 2: The measurement of n eutron e lectric d ipole m oment (nEDM) Part 3: My work

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Neutron Electric Dipole Moment

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  1. Neutron Electric Dipole Moment Sham Chun Hong

  2. Spin-Dressing Effect Sham Chun Hong

  3. Abstract • Part 1: Basic knowledge • Part 2: The measurement of neutron electric dipole moment (nEDM) • Part 3: My work • Default audience: Any undergraduates in CUHK

  4. Part I Basic knowledge

  5. Basic knowledge • What is electric dipole moment? • Under an electric field, the EDM subjects to a torque given by

  6. Basic knowledge • What is magnetic dipole moment? • A magnetic dipole moment feels the torque

  7. Basic knowledge • Particles carry intrinsic angular momentum. It is called spin. • The gyromagnetic ratio is an intrinsic property of a particle. The value can be obtained from standard tables.

  8. Basic knowledge • Under a constant magnetic field, the spin of a particle would precess due to the presence of magnetic dipole moment. • Same story for electric field.

  9. Part II Measurement of nEDM

  10. Why nEDM? • A non-zero electric dipole moment of any elementary particle is an evidence of CP violation and T violation. • No one has ever found the EDM of any elementary particle.

  11. How to measure nEDM? • If nEDM exists, neutron spin would precess in electric field. • Obtain nEDM by measuring the frequency of spin precession. • But how?

  12. Measurement of neutron spin precession • Spin-dependent reaction rate between He3 and neutron. • Measure the scintillation light from the reaction. • Difficulty: spin relaxation

  13. Add a magnetic field • Purpose: maintain the spin • Under this magnetic field, the spin of He3 would be “chasing” the spin of neutron. • Error would be caused by the instability of this magnetic field.

  14. Solution to the difficulty • Do “magic” to equalize the effective gyromagnetic ratio of He3 and neutron. • My work is a to demonstrate this “magic”.

  15. Part III My work

  16. Theory of spin-dressing effect • Effective gyromagnetic ratio is modified under the action of a sideway oscillatory magnetic field. • This is called spin-dressing effect. • My work is to demonstrate this effect experimentally.

  17. The experimental work • Spin-dressing effect was demonstrated in particle beams. We demonstrate the effect in a cell. • How to demonstrate?

  18. Optical pumping • Purpose: obtain a preferential direction for spin to point

  19. Holding field • Holding field is provided by a tower of Helmholtz coils, which will be shown in next slide.

  20. More about the setup

  21. The measurement cycle • Optical pumping, stop pumping, pi/2 burst, apply dress field, observe precession signal from He3, optical pumping again,…

  22. How do we determine the precession frequency? • Technique of beat oscillation

  23. Result • Only a preliminary result. Limited by the stability of dress field.

  24. Last day’s result

  25. Conclusion • Spin-dressing effect as a trick to improve the sensitivity of nEDM experiment. • Preliminary demonstration of spin-dressing effect in a cell. • If the spin-dressing measurement is going to continue, work is probably needed to enhance and stabilize the dress field.

  26. Acknowledgements • Physics department of CUHK • Supervisor: Jen-Chieh Peng • Coworkers: Jacob Yoder, Pinghan Chu and Steve Williamson

  27. THE END Q&A Session!

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