In a $\alpha$ particle, do the 4 nucleons stay distinct in any meaningful manner, or is it more accurately considered to be a hadron composed of 12 valence quarks that are not subdivided into nucleons?

I presume the answer is similar for $^2$H $^3$H and $^3$He. For heavier nuclei, Pauli exclusion appears to force internal structure of some sort.

  • $\begingroup$ Given that nuclear reactions, whether involving H, D, T, $^{3}$He, $^{4}$He, or other nuclei can all be dealt with using the same tools (nuclear energy level diagrams for one), it is pretty clear they are 4 nucleons. $\endgroup$ – Jon Custer Jun 5 '17 at 13:09
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    $\begingroup$ I suspect the literal answer to your question may be "yes." That is, I suspect one could meaningfully apply either model, and (after working out all the nontrivial QCD interactions between the quarks) obtain essentially the same results. But then again, I'm not really a nuclear physicist, so this is just a semi-educated guess. $\endgroup$ – Ilmari Karonen Jun 5 '17 at 14:39
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    $\begingroup$ Pretty close to a duplicate of physics.stackexchange.com/q/310820 (exact except that you might argue that alphas are special) and related to physics.stackexchange.com/q/13581 and physics.stackexchange.com/q/171037 and possibly others. $\endgroup$ – dmckee --- ex-moderator kitten Jun 5 '17 at 16:12

Definitively 4 distinct nucleons. Combinations of more than 4 quarks have never been observed. The existence of tetraquarks is pretty much confirmed [1]: the so-called Z(4430) whose quark content is $c\bar{c}d\bar{u}$. The next-lightest candidate, the pentaquark, has been entertained but the conclusion is currently that it does not exist. So 12 quarks!

Interestingly, note that the tetraquark mentioned above is heavier than an $\alpha$ particule (4.4 vs 3.7 GeV/c$^2$). The putative pentaquark resonances have masses around 4.4 GeV/c$^2$ too. Thus even without all the evidences provided by a century of nuclear physics which point to the fact that $\alpha$ particle are made of nucleons, clearly a dodecaquark would be far too heavy…

[1] https://arxiv.org/abs/1404.1903

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    $\begingroup$ Researchers at the LHC think they have found evidence of pentaquarks[1]. But I agree it's a long reach to go from pentaquarks to dodecaquarks. [1]: arxiv.org/abs/1507.03414 $\endgroup$ – Mike Scott Jun 5 '17 at 13:10
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    $\begingroup$ Yes, I agree that a pentaquark is the most appealing model for this observed resonance. All other alternatives, among which a molecular bound state between a baryon and a meson, have issues. But that's a single measurement in a single channel: wait and see! $\endgroup$ – user154997 Jun 5 '17 at 13:25
  • $\begingroup$ Analogues of light nuclei with quarks other than u or d exist, they're called hypernuclei. I see no actual evidence in this answer as to whether light (hyper)nuclei are quark soup or have a nucleonic substructure. $\endgroup$ – Mitchell Porter Jun 5 '17 at 18:05
  • $\begingroup$ @Michel Porter Only quark s as far as I know, I.e. Hypéron(s) among nucléons. I know little about those, sorry. $\endgroup$ – user154997 Jun 5 '17 at 19:44

According to this link the binding of the quarks in the protons and neutrons is much stronger than the spill over color forces, the nuclear force, combining into the alpha particle.

The structure for the alpha particles can almost be considered crystalline like in state. The sections on the "Deuteron & Alpha Steps” illustrates how the progression of stable nuclei can be visualized as deuteron or alpha building. The table of stable elements shows a construct similar to what would be created by a long chain organic polymer or the deposition pattern of a crystalline sub-straight. The pattern of stable nuclides is indicative of filling crystalline subsets as well as building a larger crystalline set. This statement will become clearer as you proceed.

So no, nuclei are held together by the residual color forces between protons and neutrons, modeled with pion exchange.

  • $\begingroup$ Interestingly that description has the alpha composed of two PN pairs, giving it a more complex structure than just 4 nucleons! $\endgroup$ – user1998586 Jun 5 '17 at 9:57
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    $\begingroup$ unclear2nuclear is not a reliable source. It promotes a personal "ball-and-stick" theory according to which individual nucleons are rigid triangles of quarks, and neighboring nucleons are connected by bonds between individual quarks... Even if some part of this were true (it is qualitatively reminiscent of the skyrmion theory of nuclei), it is very unlikely that the overall picture is true. It has no connection to QCD, the authors calculate nothing, it's total guesswork. $\endgroup$ – Mitchell Porter Jun 5 '17 at 12:12
  • $\begingroup$ @MitchellPorter fair enough. Do you have a link where the pion exchanges are calcualted by QCD? $\endgroup$ – anna v Jun 5 '17 at 12:29
  • $\begingroup$ Not so far. It seems lattice QCD has been used to obtain couplings in a nuclear effective field theory, but I don't know if that counts... Lattice QCD has also directly calculated properties of a di-baryon bound state, and there are theoretical arguments for the value of the pion-nucleon coupling in large-N (colors>>3) QCD... I am still looking, but clearly there are still big gaps in what can be deduced or calculated from fundamental theory here. Heuristic models still rule nuclear physics. $\endgroup$ – Mitchell Porter Jun 5 '17 at 13:11

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