As neutrons are neutral an electric field should not attract or repel them. However, as they are composed of a positive and a negative part, called one 'up' and two 'down' quarks, shouldn't those quarks somehow slightly react to that field making the neutron at least a mini electric dipole?
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1$\begingroup$ The property you're looking for is the polarizability of the neutron. See e.g. physics.stackexchange.com/questions/354044/… $\endgroup$– probably_someoneCommented Aug 17, 2020 at 7:37
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$\begingroup$ @probably_someone I should suppose that the same effect for electrons has not been confimed? $\endgroup$– Krešimir BradvicaCommented Aug 17, 2020 at 7:46
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$\begingroup$ @KrešimirBradvica I suppose that it is possible in principle for the electron itself to be polarized (i.e. acquire a higher EDM when an external electric field is applied), but there doesn't seem to be any literature describing it, at least at first blush. However, note that the electron is charged (unlike the neutron), so the primary response to an external field is for the electron's center of mass to accelerate. $\endgroup$– Emilio PisantyCommented Aug 17, 2020 at 9:03
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$\begingroup$ @EmilioPisanty But the proton was also thought of as entirely positive but now it is known it has also one negative quark.... $\endgroup$– Krešimir BradvicaCommented Aug 17, 2020 at 9:06
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$\begingroup$ @KrešimirBradvica Yes, because the evidence required it. There is no evidence for any composite structure of the electron. If you don't like that, tough luck. $\endgroup$– Emilio PisantyCommented Aug 17, 2020 at 9:07
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Neutrons can indeed have a nonzero (permanent) electric dipole moment (same as electrons).
- In the Standard Model of particle physics, this is predicted to have the value of $|d_n| \sim 10^{-31}\:e\rm \:cm$.
- It is possible to test experimentally for this value to extremely high precision, but this is still short of detecting the SM value. Current experiments are limited to precisions of the order $\Delta d_n \sim 10^{-26}\:e\rm \:cm$, i.e., about five orders of magnitude larger than the expected value in the SM.
(The direction of the neutron's intrinsic EDM is strictly parallel to its spin.)
Improving these measurements down to a tighter precision is a high-priority area and the focus of active research efforts, because many extensions of the Standard Model predict higher EDM values than the SM for both the neutron and the electron; thus, getting a nonzero value of either quantity would give us a good idea of how the SM needs to be extended.
In addition to that, neutrons can indeed get a higher dipole moment if you place them in an electric field, i.e., they are polarizable. This is explained in more depth in a thread linked to in the comments. This is quite small, but it is nonzero, and it has been measured.
That said, you should be wary of this picture:
as they are composed of a positive and a negative part, called one 'up' and two 'down' quarks, shouldn't those quarks somehow slightly react to that field making the neutron at least a mini electric dipole?
The picture of one-up-and-two-down-quarks is a simplified picture of the neutron, and it has limited validity; moreover, to the extent that it does hold, it is subject to quantum mechanics, which tells us that the quarks never have well-defined positions inside the proton, much like the way electrons inside an atom occupy orbitals instead of well-defined positions. (Even worse, you can't even use the language of orbitals the way you can for electrons, because QCD is a highly-coupled, highly-correlated theory.)
Some of the classical intuition in terms of displacing charge distributions still holds for the neutron, but it needs to be taken with a grain of salt -- it is not quantitatively accurate, and the neutron is a highly nonclassical object if you look at it from up close.
answered Aug 17, 2020 at 8:20
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$\begingroup$ Why then electrons are firmly considered not composed particles vith volume as protons are. What should be discovered to change that. Electrons also have intrinsic precession, Larmour precession, their magnetic moment is two times than the maximal possible for an unit charge... and also there is Compton scattering with high energy photons which are very small in let call it 'cross section' so it would be very difficult for that kind of photons to strike a pointlike electron...... For me this is very confusing. $\endgroup$ Commented Aug 17, 2020 at 9:02
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$\begingroup$ @KrešimirBradvica All of those are perfectly reasonable results within quantum mechanics -- if they seem unintuitive that's just because your intuition is based on classical mechanics. The evidence required for a composite structure for the electron would have to be monumental, and far outside the range of anything currently under consideration. $\endgroup$ Commented Aug 17, 2020 at 9:06
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$\begingroup$ @KrešimirBradvica "their [electron] magnetic moment is two times than the maximal possible for an unit charge" This statement assumes that the charge distribution is a uniform sphere. For other distributions a factor of two is achievable. $\endgroup$– my2ctsCommented Aug 17, 2020 at 10:41