Critical Point

There are good reasons to think that at zero temperature, as one varies the chemical potential, there is a first-order quantum phase transition between nuclear and quark matter. There may in fact be several transitions of various kinds, including meson condensation and alternative pairings in color superconductivity. I expect that most of these are essentially low-temperature phenomena, leaving only a single first-order transition above $\sim 50$ MeV. In any case, for purposes of discussion let me assume this. Within this context one discovers a somewhat unconventional but I think sharp and insightful perspective on the definition of “quark-gluon plasma”. At sufficiently high (but not too high) chemical potential, as one increases the temperature, or at sufficiently low (but not too low) temperature, as one increases the chemical potential, there is a sharp phase transition. We are justified in calling this a hadron-to-quark transition, since the respective phases on either side evolve without further discontinuities into matter accurately described as nearly free hadrons or nearly free quarks.

As the temperature rises the distinction between hadronic and quark matter becomes less significant; the discontinuities associated with the first-order transition grow smaller and eventually vanish altogether. The precise point in the 