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I was reading QED by Feynman (a dated book) and he seemed to be suggesting we have discovered hundreds of particles. But I thought there were only the ones we saw in the standard model (ie leptons, quarks, gauge bosons, higgs boson)? Why aren't all those hundreds of particles included in the typical "periodic table" for elementary particles?

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  • $\begingroup$ In Feynman's time, physicists didn't fully realize that among the 100s of particles they thought existed, many of which were in fact the same. $\endgroup$ Dec 24 '14 at 17:17
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    $\begingroup$ The particles were composite particles. $\endgroup$
    – HDE 226868
    Dec 24 '14 at 17:17
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After the quark model was established and the standard model was accepted as the theoretical framework for elementary particles, it became obvious that the great number of resonances could be organized into group representations of the standard model's group structures.

These are still recorded and sought after according to the predictions of the standard model. The particle data group does a good job of keeping the records.

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Part of the problem lies with the definition of "elementary". Before the discovery of nuclear physics atoms were considered elementary. This chemical definition overlooked the (then unknown) fact that most naturally occurring elements are mixtures of different isotopes. After nuclear physics was established, the electron, proton and later neutron were considered elementary, although that didn't last long. Soon it became clear that protons and neutrons themselves are made of constituents. Now we consider quarks elementary. This, of course, is a very good definition up to the TeV energy range because the current experiments show no sign of there being an internal quark structure (and much less so an internal structure for electrons).

While some are sufficiently confident that we have peeled back the last layer of this onion, it's a good idea to keep in mind that our predecessors thought the same thing after they had discovered atoms etc.. On some level, of course, we have become more cautious with the use of the word, as string theory shows, which tries to reduce the current zoo of "elementary" particles to an even smaller one, in which everything can be described as vibrations of strings.

I think it's a pretty good bet that today's "elementary particles" will be modeled as excitations of something even more fundamental tomorrow.

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  • $\begingroup$ Although the difference is "stuff" smaller than the SM has not been proven to exist and I am asking questions about known to exist "stuff". Still point taken. One should always keep an open mind. $\endgroup$ Dec 24 '14 at 17:45
  • $\begingroup$ @StanShunpile: Good point. One can still consider atoms elementary for the purposes of chemistry and we will probably continue to consider quarks and electrons as elementary for sub-TeV physics purposes, so the definition is not just a matter of history, but also one of the hierarchy of models to describe reality in increasing level of detail. Anna basically gave you the correct answer to that... the grailskeepers of the standard model like the PDG are keeping track of the properties of these particles. $\endgroup$
    – CuriousOne
    Dec 24 '14 at 17:51

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