The radius of a proton is described as a "charge radius", about 0.84 fm. The neutron is about the same size, 0.8 fm, but has no measureable charge. Is this a contradiction? Are the two radii measured in the same way?
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1$\begingroup$ See this possible duplicate, which links to Miller (2007). The neutron apparently has a negative core and a negative halo, balanced by a positive “mantle” in between. Beware that a more useful “radius” for a nucleon is given by the interplay between the attractive pion-mediated strong interaction versus the repulsive vector-meson-mediated strong interactions, which conspire to give ordinary nuclear matter roughly constant density, and make it reasonable to discuss “hard core repulsion” between nucleons. $\endgroup$– rob ♦Commented Sep 4, 2022 at 20:02
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2 Answers
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The neutron is a quantum mechanical entity, and as well as the proton it is a complex bound state of valence quarks and a sea of virtual quark antiquark and gluons, similar to the proton picture.
Quarks and antiquarks are charged and so a scattering of electrons on neutrons should show up a radius due to the charged constituents making up the neutron. This is a difficult experiment as neutrons decay and cannot be stationary targets as the proton. Methods have been devised to extract the radius, as explained in the publication below:
From Atac et al. (2021) abstract :
Despite the neutron zero-net electric charge, the asymmetric distribution of the positively- (up) and negatively-charged (down) quarks, a result of the complex quark-gluon dynamics, lead to a negative value for its squared charge radius, ${r_n}^2$.
The measurement from this work (red circle), with the error bar corresponding to the total uncertainty at the 1σ or 68% confidence level, and from references3,4,5,6 (black box) included in the PDG analysis for the radius. The orange-band indicates the PDG averaged value. The new weighted average of the world data is also shown (blue diamond) when the new measurement reported in this work is included in the calculation.
They give old and new measurements of the neutron charged radius
see link for details of calculations.
Charge radii are an open research project both theoretically and experimentally.
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$\begingroup$ “Should show up a radius due to charge constituents making up the neutron” Does that mean the neutron emits photons? $\endgroup$ Commented Sep 4, 2022 at 20:59
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$\begingroup$ @BillAlsept In a particular experiment of electrons scattering on neutrons photons may be "emitted" . No charge "emits" photons, in quantum mechanics there are "interactions " . $\endgroup$– anna vCommented Sep 5, 2022 at 2:59
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$\begingroup$ Is there an energy associated with the quark-gluon sea, perhaps in joules per cubic meter? If so, what is it? $\endgroup$ Commented Oct 13, 2022 at 17:25
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$\begingroup$ I failed to state it, but my curiosity was piqued when the measured radii of the neutron and proton are nearly identical. Perhaps the neutron is the smaller. My blind guess would have been the reverse. $\endgroup$ Commented Oct 13, 2022 at 17:28
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$\begingroup$ @IncredibleII I would guess that the neutron would have a smaller charge radius as it is neutral altogether, whereas the charge of the proton will affect larger scattering distances $\endgroup$– anna vCommented Oct 14, 2022 at 14:32
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Netrons have no net charge the same way a neutral hydrogen atom has no net charge. I don't know what the electric charge profile of a neutron looks like, but it's going to be rather complicated. See the parton distribution for hadrons - it describes how much each particle type contributes at each length scale.
At a guess, though, the neutron will have more positive charge at the center and more negative toward its outside. This is because it's more electromagnetically stable when you have one negative valence particle (the up quark) and two negative ones (the down quarks). I would expect it to be similar for protons - more positive charge toward the outside, less toward the center (possibly even going negative at the very center). This is all wild speculation, though, so take it with a fist sized lump of salt.
answered Sep 4, 2022 at 19:27
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$\begingroup$ Can you make a neutron radiate photons? $\endgroup$ Commented Sep 4, 2022 at 19:33
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2$\begingroup$ @BillAlsept That's a new question, but yes. It has a magnetic moment from its spin, and that will be the primary way for it to couple to the electromagnetic field. To get charge acceleration type coupling you'd need extremely high energy gamma rays. $\endgroup$ Commented Sep 4, 2022 at 19:35
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$\begingroup$ Thanks, Just one more question. This question got me thinking about it. If you fire neutrons at a slit experiment will there be ANY photon emissions from EITHER the accelerating neutrons OR the electrons on the surface of the slit edges? $\endgroup$ Commented Sep 4, 2022 at 20:00
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2$\begingroup$ See my links under the question. The positive core model was suggested by Fermi, with the toy explanation that the neutron should spend part of its time as a proton and a virtual $\pi^-$ meson. But the data suggest the positive-core model is wrong. $\endgroup$– rob ♦Commented Sep 4, 2022 at 20:04
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1$\begingroup$ @BillAlsept In a neutron interferometer, direction changes come from coherent strong interactions with the interferometer; electromagnetism is not involved. In my thesis experiment, we had a weak (10 G) polarizing field and manipulated neutron spins using resonant single-photon interactions with a 30 kHz oscillating field — that’s audio frequency, not radio frequency. The neutron magnetic moment is really small. $\endgroup$– rob ♦Commented Sep 4, 2022 at 20:24