When the quark model wasn' t there yet, was there evidence that hadrons are composite structures? In hindsight of course yes. But at the time back then, were there signs? Maybe not noticed, but which could have made people thinking this could be the case, without having compositeness in mind a priori?

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    $\begingroup$ As noted in en.wikipedia.org/wiki/Quark_model, "Developing classification schemes for hadrons became a timely question after new experimental techniques uncovered so many of them that it became clear that they could not all be elementary" - yes there was evidence. Mesons were "easy" to figure out, Baryons took more work. $\endgroup$
    – Jon Custer
    Jul 12 at 22:25
  • $\begingroup$ Strange question. How would it get there if there were no signs pointing in its direction before it was? And after atoms, literally "indivisibles", turned out to be divisible it was hard to find physicists who did not have compositeness in mind a priori as one of the options for everything. $\endgroup$
    – Conifold
    Jul 13 at 3:20
  • $\begingroup$ @Conifold You look too naive. All processes could be explained by normal physics. As current processes can by the quark model. There is a deeper explanation of the quark model. Without evidence. Although the muon g2 experiment is a hint for quark and lepton sub structure. So this model is simply conceived as a way to organize diversity. So was the quark model. Only afterwards were experiments made to look fir substructure. I think, thats why the question is asked. $\endgroup$ Jul 13 at 4:31
  • $\begingroup$ @JonCuster Why cant they be elementary if there are many? Arent there many quarks? $\endgroup$ Jul 13 at 5:47
  • $\begingroup$ The problem wasn't just that they kept discovering more particles. Sure, in theory they were all independent, completely unrelated things. It was the fact that the experimental results weren't random - there clearly were systematic rules behind how colliding $A+B$ would yield $C+D$, but to get an $E$ you needed $B+C$. With enough particles and enough experiments one could start piecing together the logic and break the code. $\endgroup$
    – Jon Custer
    Jul 13 at 13:53

As @Jon Custer points out, WP covers the history of such models pretty well. Once the classification of hadrons was completed by Gell-Mann and Ne'eman, in 1961, the patterns involved, underlain by flavor SU(3), begged the question of compositeness. A black box rattles and squeals (spectra, couplings) in orderly, non-random ways: this is catnip for theorists. So, discussions by Sakata, and Peterman, succeeded in the end by Gell-Mann and, independently, Zweig's successful introduction of quarks in 1964.

The point is systematic patterns of atomic properties in the periodic table classification of Mendeleev (1870) were ultimately understood by Moseley in 1913 in terms of nuclear charges, building on Rutherford's van den Broeck, and Bohr's work, in a virtually constituent model of the nucleus, let us say. Since then, nuclear physicists cleaned the structure up in detail.

The leap from hadron classification to constituent structure took 3, instead of 45 years, and involved partially the same investigator, MGM, who landed on "mathematical quarks"$^\natural$ through SU(3) group theory (!). George Zweig, a continent apart from MGM, and conceptually so as well, was unique in that he was not driven by expediting SU(3) representation theory, but, instead, dynamics (long life of the φ meson), so he introduced "nuclear physicists' quarks" (constituent quarks; very harshly received at the time: an astounding saga).

For a long time, both types of quarks were highly speculative entities, to be considered or not by the mainstream of the field, certainly not by experimentalists; until, finally, Feynman's parton model (1969) explained scaling through them, and appreciated them as DIS essentially phenomenological objects.

The book by A Pais, Inward Bound, (1986; ISBN13:9780198519713) explains the saga quite wonderfully.

$\natural$ "A search for stable quarks of charge -1/3 or +2/3 and/or stable di-quarks of charge -2/3 or +1/3 or +4/3 at the highest energy accelerators would help to reassure us of the non-existence of real quarks." MGM, 1964

  • $\begingroup$ You should write a book yourself! Im not kidding. Your choice of words (A black box rattles and squeals (spectra, couplings) in orderly, non-random ways: this is catnip for theorists.) and way of telling would make it a new Greek mythology! Well, no myth of course, but nevertheless. $\endgroup$ Jul 13 at 11:08
  • $\begingroup$ I know its not the place to ask, but I ask nevertheless. Can you state (in a very small and thin nutshell) what the problem with the rishon model (Harari, already in 1981) is? Apart from that they are not observed but neither were quarks back then. $\endgroup$ Jul 13 at 11:27
  • $\begingroup$ Too late for myth making... Rishons rose and fell meteorically 40 years ago. Leptons, like the electron, are "clean" and well-studied, so they set a huge deviation-from-elementarity scale, and there are good limits of lepton number conservation... In addition, they appear far more contrived, technically, than the quark model, but this is always subjective... $\endgroup$ Jul 13 at 13:16
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    $\begingroup$ Yes, model-builders have never stopped speculating about compositeness, but it's just them. Indeed, the saga of QMPS is something that nobody had chronicled scientifically... I further advised Moyal's widow, now deceased, on his biography she wrote... $\endgroup$ Jul 13 at 13:38
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    $\begingroup$ :) I have looked at the PDF in a wink. The Lone Star Lemma. Sounds nice! I also looked at a video you made (with beard). Again, very clear! But Ill stop now... $\endgroup$ Jul 13 at 14:00

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