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Q & A: color multipoles

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Most recent answer: 06/06/2010
Q:
It has been explained to me that the "color force" which holds neutrons and protons together is strong enough that all free matter must be "color neutral". If we consider this in terms of the familiar electric-magnetic forces, this would be like saying something can't have a net electric charge. But even neutral objects can have electric dipoles, or magnetic dipoles, etc. So my question is: We know the neutron, despite being electrically neutral, has a permanent magnetic dipole moment. Does it also have a permanent "color-magnetic" dipole moment? What about quadrupole, octopole moment, etc? --- The related question is then: if the color force is so strong that all these moments have to be zero for protons, neutrons, etc. How then can protons and neutrons interact via the color force if they are completely color neutral to ever possible level? What holds the nucleus together? --- Thank you.
- John Marsh (age 20)
Springfield, IL, USA
A:
Your point is right on the money. The chromodynamic force only prevents free color monopoles. That's because the energy of separating a color and anti-color grows linearly with the distance between them, for large distances. At short distances this interaction is relatively weak, an effect called "asymptotic freedom".  That allows all sorts of higher chromo multipoles (just not monopoles) to exist, so that nuclear interactions at distances of a typical nucleon size are quite strong.  The chromodynamic forces are indeed the main ones holding the nucleus together.

Mike W.

(published on 06/06/2010)

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