Neutron Electric Dipole Moment
Sham Chun Hong
Spin-Dressing Effect
Sham Chun Hong
Abstract
Part 1: Basic knowledge Part 2: The measurement of neutron electric
dipole moment (nEDM) Part 3: My work Default audience: Any undergraduates in CU
HK
Part I
Basic knowledge
Basic knowledge
What is electric dipole moment?
Under an electric field, the EDM subjects to a torque given by
EpN
Basic knowledge
What is magnetic dipole moment?
A magnetic dipole moment feels the torque
BN
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.
S
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.
BN
Part II
Measurement of nEDM
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.
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?
Measurement of neutron spin precession Spin-dependent reaction rate between He3
and neutron. Measure the scintillation light from the
reaction. Difficulty: spin relaxation
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.
Solution to the difficulty
Do “magic” to equalize the effective gyromagnetic ratio of He3 and neutron.
My work is a to demonstrate this “magic”.
Part III
My work
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.
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dressBJ
The experimental work
Spin-dressing effect was demonstrated in particle beams. We demonstrate the effect in a cell.
How to demonstrate?
Optical pumping
Purpose: obtain a preferential direction for spin to point
Holding field
Holding field is provided by a tower of Helmholtz coils, which will be shown in next slide.
More about the setup
The measurement cycle
Optical pumping, stop pumping, pi/2 burst, apply dress field, observe precession signal from He3, optical pumping again,…
How do we determine the precession frequency? Technique of beat
oscillation
Result
Only a preliminary result. Limited by the stability of dress field.
Last day’s result
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.
Acknowledgements
Physics department of CUHK Supervisor: Jen-Chieh Peng Coworkers: Jacob Yoder, Pinghan Chu and S
teve Williamson
THE END
Q&A Session!
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