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Subject: Re: Why don't neutrons have temporary charge?

Date: Mon Apr 26 11:03:28 1999
Posted by Samuel Silverstein
Position: faculty, physics, Stockholm University

Alan,

Experimentally, no positive measurement has yet been made of the neutron's electric dipole moment (EDM), and the current Standard Model of particle physics predicts a value of the neutron EDM which is a million to a hundred million times smaller than the sensitivity of current experiments. The reasons for this are based on the conservation of certain symmetries in nature.

What keeps the neutron from posessing an electric dipole moment is that it does have a magnetic moment. In electromagnetic theory, it is impossible to have both without violating something known as parity conservation . In simple terms, parity conservation means that the mirror images of objects and processes have to behave in the same way as their non-mirrored counterparts. But electric dipoles and magnetic moments behave differently under parity transformations.

To illustrate this, suppose the neutron did have both an EDM and a magnetic moment (MM). The EDM is a vector quantity, while the MM is an axial vector quantity, comparable to the angular momentum vector of a spinning top. If I start with the EDM and the MM pointing in the same direction and then look at the system in the mirror (perform a parity transformation), I get the following:

       EDM     |     EDM
    -------->  |  <--------
        MM     |     MM
    -------->  |  -------->
               |
            Parity
After the parity transformation, the EDM and the MM are no longer pointing the same way! Since the electromagnetic force obeys parity, a simultaneous non-zero EDM and MM is therefore forbidden.

However, this is not the complete story. Although the electromagnetic force forbids parity violation, the weak nuclear force does not. So, the neutron is expected to have a non-zero EDM, but it will be very small (remember, we call it the weak nuclear force!). In the July 1988 Particle Physics Booklet (issued by the Particle Data Group in Berkeley, California) the EDM of the neutron has been experimentally shown to be less than 0.97E-25 e cm (where e is the charge of an electron), while the current Standard Model of particle physics predicts a neutron EDM between around 1E-31 to 1E-33 e cm.

There is a great deal of interest in measuring the neutron EDM. Theoretical extensions of the Standard Model invariably give rise to much larger EDMs than the Standard Model does, which are within the reach of new experiments. If a large value was measured, it would be strong evidence for new, exotic physics processes! So the search continues...

I am sure I have raised more questions here than I have answered. If anything here didn't make sense to you, or if you have further questions, feel free to contact me at silver@physto.se .

Cheers,

Sam Silverstein


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