Hadron Molecules v. True Composite States

Any time that you have four to six quarks in some sort of bound state, the question that is presented is whether it is a true tetra-, penta-, or hexaquark, or whether it is a hadron molecule with the same valence quarks.

In a true tetra-, penta-, or hexaquark, the valence quarks are bound direct to each other by gluons. In a hadron molecule, mesons or baryons are bound to each other either by something analogous to the nuclear binding force (carried by mesons in atomic nuclei, mostly, but not entirely, pions) or electromagnetically (as in an ordinary molecule made up of atoms bound electromagnetically).

A new preprint looks at four pentaquark states with a valence charm quark and finds that all of them are hadron molecules rather than true pentaquarks.