So in a similar way to electrons moving in atoms, causing induced dipole-dipole interactions, can neutrons momentarily attract or repel?


1 Answer 1


It seems that within the standard model of particle physics

A permanent electric dipole moment of a fundamental particle violates both parity (P) and time reversal symmetry (T). These violations can be understood by examining the neutron's magnetic dipole moment and hypothetical electric dipole moment. Under time reversal, the magnetic dipole moment changes its direction, whereas the electric dipole moment stays unchanged. Under parity, the electric dipole moment changes its direction but not the magnetic dipole moment. As the resulting system under P and T is not symmetric with respect to the initial system, these symmetries are violated in the case of the existence of an EDM. Having also CPT symmetry, the combined symmetry CP is violated as well.

Thus experiments trying for new physics beyond the standard model have tried to measure it for the neutron:

The neutron electric dipole moment (nEDM) is a measure for the distribution of positive and negative charge inside the neutron. A finite electric dipole moment can only exist if the centers of the negative and positive charge distribution inside the particle do not coincide. So far, no neutron EDM has been found. The current best upper limit amounts to |dn| $< 3.0×10^−26 e⋅cm.$

As CP violation has been observed in particle data, to a very small amount, the value due to CP violation for the electric dipole moment of the neutron is expected to be around $ 10^−31 e⋅cm$, way smaller than the present limit.

can neutrons momentarily attract or repel?

Do not forget that the neutrons are in the realm of strong interactions , and will attract other neutrons or protons, in orders of magnitude stronger reaction. The technique for the electric dipole measurement is described here.

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    $\begingroup$ but OP is asking about non-permanent dipole interactions, like London dispersion force $\endgroup$
    – DavePhD
    Apr 6, 2018 at 16:10
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    $\begingroup$ @DavePhD the standard model does not allow either permanent or instantaneous violation. Beyond the standard model , as the CP violation number shows, the effect should be equally very small, afaik. It is a moot point since quarks are a soup of strongly interacting gluons and quark antiquark pairs within the neutron , cannot be compared with orbitals in molecules and atoms. Will need lattice QCD for calculations. $\endgroup$
    – anna v
    Apr 6, 2018 at 17:32
  • $\begingroup$ Stationary states can not have electric dipole moments, but time dependent states can. robotics.cs.tamu.edu/dshell/cs689/papers/… $\endgroup$
    – DavePhD
    Apr 6, 2018 at 23:43
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    $\begingroup$ Since a neutron is not a fundamental particle in the Standard Model, is the first quote on point? $\endgroup$ Apr 6, 2018 at 23:44
  • $\begingroup$ @HenningMakholm , it should say "quanrum mechanical entitites" I guess. CP violation in hadrons , it is not simple : ncnr.nist.gov/summerschool/ss09/pdf/Filippone_FP09.pdf $\endgroup$
    – anna v
    Apr 7, 2018 at 3:39

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