Hadron Physics To Do

One of my long-standing to dos for a blog post, which keeps getting put off because it is a pretty big project, is to survey the current state of the literature regarding hadron and/or hadron molecule resonances that aren't simple pseudo-scalar valance quark-antiquark, and simple three valance quark baryons, with u, d, s, c, and b valance quarks.

These include scalar mesons, axial-vector mesons, tetraquarks, pentaquarks, hexaquarks (if any), quarkonia, toponium, glueballs, mixed/blended meson resonances, glueball-quark hybrids, hadron molecules, excited hadron resonances, and other XYZ resonances. 

There are also "leptonic atoms" which substitute positively charged leptons for protons in an atomic nucleus and are bound by quantum electrodynamics (i.e. by electromagnetism) rather than by the strong force, that probably belong in the same discussion (and generally have a mass of less than 4 GeV).

As a prelude, the big bottom line is that there is not a global solution, really, even to any large group of unclassified resonances. Each resonance has to be figured out on its own. It is sometimes quite an epic effort to discriminate between plausible explanations of their structure. 

But there is also no BSM physics. QCD can explain it, but you have to be open to more involved hadron and hadron molecule structures than the vanilla mesons and baryons display. Thus, we are slowly and painfully, but inexorably, reaching a point where essentially all resonances have a Standard Model explanation.

Also, except for toponium, this highly sophisticated analysis and classification of hadron resonances, while it requires lots of data points, doesn't require the extreme high energies of the 13-14 TeV LHC (Large Hadron Collider). 

Generally speaking, all hadron resonances are somewhere between 135 MeV (the lightest pion) and about 30 GeV (a hypothetical six b quark hexaquark), and the lower middle part of this range is very crowded with all sorts of resonances. This is comfortably below the energy scale of even a W or Z or Higgs boson, and is also below the energy scale of a top quark-antitop quark pair. 

Maybe a post just spelling out the possibilities would be a good prelude to a post putting forth the leading theories about which resonances are most likely matches to which possibilities.