# 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 '19 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 '19 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 '19 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 '19 at 14:07
• @rob: I added some explanations in my answer. – Vladimir Kalitvianski Mar 27 '19 at 14:22

It is a misconception that neutrons are made of three quarks. Neutrons in reality are made up of a sea of quarks and gluons. These quarks and gluons pop in and out of existence, with their anti-particle. Now if you net out these particles, then you get a net of three quarks, we call them valence quarks.

It is interesting that the EM charge of a quark is one third of an electron (or multiples of it). But still, the net EM charge of the neutron is 0.

It is a misconception that the EM field does not have an effect on the neutron. The neutron does have a magnetic dipole moment, and is deflected in the magnetic field. So magnetic fields do have an effect on the neutron.

You are asking whether the valence quarks inside the neutron will be polarized or not. Now as I wrote, the valence quarks are just a net of the ever changing sea of quarks and gluons. At a given moment in time, there are three valence quarks, but that changes every time you check, since quarks pop in and out of existence. So no, the quarks are not polarized in a certain direction. There are two reasons for this:

1. quarks are always confined inside the sea of quarks and gluons, and they are somewhat shielded from the external EM field (though, magnetic fields do deflect the whole neutron)
2. is just not enough time for them to be polarized, as the quarks pop in and out of existence almost instanteniously

You are imagining the inside of a neutron as like if it was as stable as an atom, having stable electron energy levels around it. No, in reality, the inside of a neutron is just an ever changing sea of quarks and gluons and though i the atom there is a stable electron energy level where electrons can be polarized by an EM field, and electrons can stay there in a pretty stable polarized state in a certain direction, the inside of a neutron just isn't this kind of a place, and quarks can't take that kind of a polarization.