Neutron Electric Dipole Moment Sham Chun Hong SpinDressing

Neutron Electric Dipole Moment Sham Chun Hong

Spin-Dressing Effect Sham Chun Hong

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

Part I Basic knowledge

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

Basic knowledge n n What is magnetic dipole moment? A magnetic dipole moment feels the torque

Basic knowledge n n 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.

Basic knowledge n n 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.

Part II Measurement of n. EDM

Why n. EDM? n n 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.

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

Measurement of neutron spin precession n Spin-dependent reaction rate between He 3 and neutron. Measure the scintillation light from the reaction. Difficulty: spin relaxation

Add a magnetic field n n n Purpose: maintain the spin Under this magnetic field, the spin of He 3 would be “chasing” the spin of neutron. Error would be caused by the instability of this magnetic field.

Solution to the difficulty n n Do “magic” to equalize the effective gyromagnetic ratio of He 3 and neutron. My work is a to demonstrate this “magic”.

Part III My work

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

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

Optical pumping n Purpose: obtain a preferential direction for spin to point

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

More about the setup

The measurement cycle n Optical pumping, stop pumping, pi/2 burst, apply dress field, observe precession signal from He 3, optical pumping again, …

How do we determine the precession frequency? n Technique of beat oscillation

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

Last day’s result

Conclusion n Spin-dressing effect as a trick to improve the sensitivity of n. EDM 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.

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

THE END Q&A Session!