A helpful analogy between the covalent bond and particle spectroscopy

It is proposed that meson resonances are linear combinations of and meson--meson (MM); baryon resonances are combinations of qq and meson--baryon (MB). Mixing between these combinations arises via decays of confined states to meson--meson or meson--baryon. There is a precise analogy with the covalent bond in molecular physics; it helps to visualize what is happening physically. One eigenstate is lowered by the mixing; the other moves up and normally increases in width. Cusps arise at thresholds. At sharp thresholds due to S-wave two-particle decays, these cusps play a conspicuous role in many sets of data. The overall pattern of light mesons is consistent with nearly linear Regge trajectories, hence qq components. There is no obvious reason why this pattern should arise from dynamically generated states without qq content.