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The title speaks for itself really: I noted everywhere I learned about fundamental interactions that there are particles that interact ONLY via the Weak Force (and gravity, if it counts)(e.g. neutrinos), there are some that only participate in the EM component (and gravity, if...) (e.g. photons), there are some that have no charge, but interact both with strong and weak carriers (e.g. Higgs), etc. However, it seems that no one discusses why it seems to be the case that there is not a single known particle that only interacts via the strong force.

Why do you think this is?

(NB: there also seems to be no particle that is only affected by gravity, but that is perhaps not as surprising, given that it is a very weak interaction and it would be incredibly difficult to make a detector for such a particle, unless it showed up in huge masses... perhaps dark matter?? :))

EDIT: 2023/08/16 Let me generalize the question a bit in the form of a table:

Particle Strong Weak EM
gluon X
Z, neutrinos X
photon X
?? X X
W, charged leptons X X
?? X X
quarks X X X

Does this make more sense? Thank you for your answers.

EDIT: 2023/08/17

I would also note there is a huge difference between gluons and photons, so the table may still be a little misleading: gluons actually carry color charge, photons do not carry electric charge, so there is elementary gluon-gluon interaction, and there is only higher order (in fermion loops) interaction between photons. Similarly, W and Z do not have flavor, so also have the same note as photons. Thus the question might still include any knowledge of particles that are not affected by WI but have charge.

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  • $\begingroup$ Just a comment on the last paragraph: that we don't know if a particle is affected only by gravity is not because gravity is weak, but rather the opposite: that it does not seem to be affected by other forces (like dark matter). Gravity, despite being weak, was the first force to be described by humans. Mass and energy just add up instead of cancelling each others' effects, giving rise to ubiquitous gravitational phenomena. $\endgroup$
    – Avantgarde
    Commented Aug 9, 2023 at 16:05
  • $\begingroup$ I'm sorry, perhaps I wasn't clear on that. IF a particle existed that only interacted gravitationally, it would either have to be extremely massive for us to detect its effect individually, or it would -- as a particle -- simply go undetected on account of not interacting with anything, and showing up as a missing mass. To date -- as I understand -- there aren't any major gaps for missing mass. $\endgroup$
    – Istvan Cziegler
    Commented Aug 16, 2023 at 10:12
  • $\begingroup$ The graviton, assuming it exists, only feels gravity. $\endgroup$
    – PM 2Ring
    Commented Aug 16, 2023 at 10:44
  • $\begingroup$ Agreed. I thought of that too. But no sign of it so far -- again: to the best of my knowledge. $\endgroup$
    – Istvan Cziegler
    Commented Aug 16, 2023 at 10:47
  • $\begingroup$ Single gravitons would be very hard to detect. It's hard enough to detect even powerful gravitational waves, as I said here: physics.stackexchange.com/a/414094/123208 & physics.stackexchange.com/a/589121/123208 $\endgroup$
    – PM 2Ring
    Commented Aug 16, 2023 at 10:59

2 Answers 2

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You forgot the gluons (the bosons mediating the strong interaction). They interact only via the strong force, i.e. with quarks and other gluons.

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  • $\begingroup$ True, perhaps my original question should be restated without reference to photons. It is a bit odd still, don't you think, that leptons can weakly interact with or without EM components, quarks weakly AND strongly, but there isn't an elementary fermion that is only strongly interacting. $\endgroup$
    – Istvan Cziegler
    Commented Aug 16, 2023 at 10:01
  • $\begingroup$ In fact, come to think of it, it is unclear to me whether leptons having +-1 or 0 (x electron charge) electric charge and a fixed "weak charge", and quarks having +-1/3 or +-2/3 e, and again, unit "weak charge" has anything to do with the participation or lack of in strong interaction. Coincidence seems like a cop-out. $\endgroup$
    – Istvan Cziegler
    Commented Aug 16, 2023 at 10:17
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In grand unification theories, there is a single unified force that interacts with all the fermions. In such a theory, the forces that we know come from specific components of the unified force field, while certain other components of that unified force field become superheavy and effectively invisible. Thus, in SU(5) unified theory, we see gluons that change the color of quarks, and weak bosons that change the flavors of quarks or leptons, but e.g. we don't see the X and Y bosons that can turn a quark into a lepton.

In such a theory, the immediate answer to your question is "because the unified symmetry was broken according to a particular pattern". Going deeper might involve excursions into group theory, string theory, and the anthropic principle. However, all that would be highly speculative, since we do not even know which GUT model is correct, or indeed if any of them are correct. The same goes for any other speculative framework which proposes to explain why the known elementary particles are what they are.

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  • $\begingroup$ I do not wish to be abrasive, but this still basically restates the question in a hidden way. In other words: is your opinion then that we just don't know? That there is no theory that present an explanation (let alone a validated one)? $\endgroup$
    – Istvan Cziegler
    Commented Aug 17, 2023 at 11:41
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    $\begingroup$ From the standpoint of ordinary particle physics, it's just a contingent feature of the world. If you could prove that matter fields have to be bifundamental representations, that might do it (as it would imply that each fermion is coupled to two gauge fields), but then why would that be true? If there is a reason, it reaches beyond what we currently know. $\endgroup$
    – Mitchell Porter
    Commented Aug 17, 2023 at 12:17
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    $\begingroup$ Maybe the best candidate for a simple explanation would be that all those particles are "open strings" stretched between two different "brane stacks". The force particles are then the bosonic strings interior to each stack, and the open strings always feel two forces because they interact with two different stacks... This is not the only kind of model that exists within string theory (e.g. in heterotic models all particles correspond to closed strings), but it is a significant class of models. $\endgroup$
    – Mitchell Porter
    Commented Aug 22, 2023 at 7:56

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