Hagedorn Temperature - Transition To Quark-Gluon Plasma

Transition To Quark-Gluon Plasma

In parallel to the SBM development the newly discovered quarks became progressively more trusted building blocks of hadrons. Hagedorn saw a compressed gas of hadrons as another hadron. In the quark picture this is, however, a drop of quark matter. In quark matter at high temperature gluons would also be present and as temperature is increased, asymptotic freedom assures that all constituents are relatively weakly interacting. There seemed to be nothing that would stop a dense assembly of hadrons from dissolving into a plasma of quarks and gluons. This new state of matter could be heated to a very high temperature and no limit was in sight. What is the meaning of Hagedorn temperature in this enlarged context?

In the SBM as conceived before quarks, hadrons were point particles. Now a subtle modification is required when considering quarks as building blocks. Hadrons made of quarks need a finite proper volume growing with hadron mass. Hagedorn and Rafelski discovered that at the Hagedorn temperature finite size hadrons dissolve into a quark-gluon liquid. Both a phase transition and a smoother transformation is possible, depending on the precise nature of the mass spectrum. The most physically attractive alternative was a first order phase transition. In this case the latent heat is delivered to the hadron phase at a constant Hagedorn temperature. One then reaches a new phase wherein the hadron constituents, the quarks and the gluons, are no longer confined. The system temperature can now rise again.