# Polarisation inside neutron

What happens to a neutron when it is placed in an electric field? Does polarisation take place inside the neutron (As it consists of charged quarks) in a similar way as that of neutral atom?

You are asking if the neutron can have an "induced neutron electric dipole moment" (quotes are for search terms): yes, it can. Whether is does is unknown.

Before addressing that, note that a permanent electric dipole moment of the neutron (nEDM) is an active area of research, both theoretically and experimentally. Since and electric dipole is parity odd and time reversal even, its alignment with the neutron spin violates both parity and time reversal symmetry.

A proposed experimental test of induced neutron dipole moment is presented in https://arxiv.org/abs/1104.1260. The effect is a non-linear QED effect requiring fields above the critical electric field strength:

$$E_c = \frac{m_e^2c^3}{\hbar e}=1.3 \times 10^{18}\,{\rm V/m}$$

which is not something you make in the lab. The authors suggest observing a 1 part in 1,000 (at best) asymmetry in polarized neutron scattering from heavy nuclei (where large electric fields are present).

@ÁrpádSzendrei: It is not clear how the neutron consituents are "shielded from the external EM field" and why there is "not enough time for them to be polarized". The sea itself is permanently in an external electric field, so it is affected somehow and may be polarized.

There is a some analogy with atomic "orbitals": in a certain state $$|n,l,m\rangle$$ the are "charge clouds" with generally fractional charges resulting from the nucleus-electron mutual motion, in full analogy with the neutron consituents:

These fractionnally charged clouds cannot be made "free" as such ("extracted" from an atom), also in full analogy with valence quarks.

Finally, apart from pure QCD "domination", there still is an electromagnetic vacuum contributions in the polarization effects of a neutron, as mentionned in the JEB's answer.

By the way, the atomic polarization happens due to inducing atomic excited states $$E_n>E_0$$ including the ionization (continuous) spectrum of $$E_n$$. I do not know about possible "excited but still bound states" of neutron, but it has a continuous spectrum due to a weak decay channel, which is "open" even in the absence of any external electric field. It may contribute too into the neutron polarization effect under question.

• This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review – John Rennie Mar 27 at 11:30
• Yes, it answers the question with pointing out the analogy with atomic case. Atom is also globally neutral, but is may be polarized. – Vladimir Kalitvianski Mar 27 at 12:45
• The current version of this answer (v1) seems more like a reply to another answer than an answer on its own. A clarifying edit might be in order, especially if the other answer were to disappear for some reason. – rob Mar 27 at 14:05
• In response to the physics of this answer: the dominance of the strong interaction in the QCD vacuum makes the comparison to the EM-only atomic case less than obvious. – rob Mar 27 at 14:07