Hadron mass spectrum, QCD, and the quark-gluon plasma

Partly settled. Some of it is understood, some is still argued about.

Ginzburg's problem #14 grouped the hadron mass spectrum with quark-gluon plasma. Since 1999, RHIC announced strong evidence for a strongly-coupled 'perfect fluid' quark-gluon plasma in April 2005, and heavy-ion collisions at the LHC (from 2010, via ALICE) confirmed and extended this at much higher temperature. Separately, first-principles lattice-QCD calculations reproduced the light hadron mass spectrum to a few percent from QCD alone (Durr et al., Science 2008). A full analytic proof of quark confinement, however — why quarks are never observed free — remains an unsolved Clay Millennium Prize problem (the Yang-Mills mass gap).

For a few microseconds after the Big Bang, and for a few million-billionths of a second inside a heavy-ion collider, matter 'melts' into a soup of free quarks and gluons.

References

• What problems of physics and astrophysics seem now to be especially important and interesting (thirty years later, already on the verge of the XXI century)?, V L Ginzburg, Physics-Uspekhi 42 (4) 353-373, 1999
• Ab Initio Determination of Light Hadron Masses, S. Durr et al., Science 322, 1224, 2008
• Yang-Mills Existence and Mass Gap, Clay Mathematics Institute, Millennium Prize Problems, 2000