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

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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.

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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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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!