Can you distinguish between the two different situations in the picture - a single particle, and a complex of two particles? Is the difference obvious for a physicist?

How you would call that kind of complex particle?

Composite, Compound, Complex or something else? Which term for that would the best for understanding what it is for a physicist?

"Meson-baryon molecule" sounds understandable, but a bit not correct, as molecule is a system of nuclei and electrons.

enter image description here

  • $\begingroup$ en.m.wikipedia.org/wiki/Pentaquark $\endgroup$ May 23, 2019 at 9:03
  • $\begingroup$ No, no, without collision. Two separate particles, which creates complex (compound) but still are sepatate particles. Like deuteron - one proton and one neutron. What would be correct term for particle, consisting of proton and neutron? $\endgroup$
    – coodan
    May 23, 2019 at 9:08
  • $\begingroup$ A "meson-baryon molecule" in that particular case? $\endgroup$
    – coodan
    May 23, 2019 at 9:16
  • $\begingroup$ What does it mean for a particle to "consist only of quarks"? $\endgroup$ May 23, 2019 at 9:46
  • $\begingroup$ That means two types of particles: first one consist of only quarks (directly) and second one consist of other particles (which may in turn consist of quarks) $\endgroup$
    – coodan
    May 23, 2019 at 10:32

2 Answers 2


These are the elementary particles of the standard model of particle physics:


All other hadrons ,(like mesons and baryons and nuclei)and atoms and molecules are composite, in a complex manner.

Have you checked this site?


The standard model of a hadron, a proton in this case, does not have just the three valence quarks, but includes a sea of quark-antiquark pairs and gluons. These are in a virtual state, i.e. off mass shell, and any pairs with the appropriate flavor of a pion can be considered a virtual pion. The complexity overcomes the concept of compounds.

The nucleon nucleon interaction is closer to your "compound" view, as the nuclear force is modeled by a meson exchange , i.e. a quark antiquark pair makes the proton+neutron bind into a deuteron nucleus.

Although, in the light of QCD, meson theory is not perceived as fundamental anymore, the meson exchange concept continues to represent the best working model for a quantitative nucleon-nucleon potential

So penta quarks get a name by themselves, because they also have four valence quarks and one valence antiquark, in addition to the sea of quarks antiquarks and gluons in their definition.

From the wikipedia link on pentaquarks:

The binding mechanism for pentaquarks is not yet clear. They may consist of five quarks tightly bound together, but it is also possible that they are more loosely bound and consist of a three-quark baryon and a two-quark meson interacting relatively weakly with each other via pion exchange (the same force that binds atomic nuclei) in a "meson-baryon molecule

The "penta" name is enough at the moment and this is a research question. If it is found in measurements that a meson has to be modeled for the existence of a pentaquark, then maybe a new word will be proposed to describe them.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – tpg2114
    May 23, 2019 at 19:19

A particle consisting of 1 neutron and 1 proton is actually a nucleus. You could also consider the proton as Hydrogen nucleus. Concerning your proton / pion bound states, I am not even sure we should try to find a name for them since they are not observed. Anyway you certainly cannot call it a molecule since it has no electrons to bind things together.

  • $\begingroup$ Nucleus is the concrete name for particular case of system which consist of neutron and proton.But what should be term to characterize it, more generally? Molecule - agree with you completely is bad, as molecule is concrete name for system consisting of nuclei and electrons. Just in the Pantaquark article of Wiki the type like that was called "meson-baryon molecule". $\endgroup$
    – coodan
    May 23, 2019 at 10:38
  • $\begingroup$ Probably, term "Compound" would be more optimal for, say, both molecules and such type of systems? Please, see picture in wiki for pentaquark of type "meson-baryon molecule". en.m.wikipedia.org/wiki/Pentaquark $\endgroup$
    – coodan
    May 23, 2019 at 10:41
  • 2
    $\begingroup$ @coodan Nuclei that have particles other than protons and neutrons in them are typically called hypernuclei. But in general, this being such a newly-studied phenomenon, it's likely that there's no specific term for it yet, and either 1) a completely new term will have to be invented, or 2) an existing term will have to be repurposed to include these objects (for example, "baryonic molecule"). In my opinion, 2 is far more likely, especially since the system is already a lot like a molecule: it's a group of strongly-bound objects held together by weaker bonds. $\endgroup$ May 23, 2019 at 12:06
  • $\begingroup$ Exactly, probably_someone. Hypernuclei - great example of such system, thank you. And yes, you are right that a feature is that there is very strong connections inside the blocks (particles) and weaker connection of blocks (particles). Systems like that in chemistry have name "complex". Complex is compound like that - strongly bonded blocks with less strong connection. $\endgroup$
    – coodan
    May 23, 2019 at 12:26
  • $\begingroup$ Please, probably_someone, copy this your comment as answer, as you right and it is important. And I will explain what it can mean. $\endgroup$
    – coodan
    May 23, 2019 at 13:00

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