Phases of QCD: perspectives, challenges and opportunities
Seminar Room 1, Newton Institute
We will give a critical discussion of the phase diagram of QCD in the temperature -- chemical potential plane. We will explain the mechanism of chiral symmetry breaking at nonzero chemical potential and emphasize the importance of the the phase of the fermion determinant. Among others it is shown that the Banks-Casher formula, which relates the chiral condensate and the Dirac spectrum, is not valid for QCD at nonzero chemical potential. Instead, because of the phase of the fermion determinant, intricate cancellations lead to a nonzero chiral condensate. The distribution of this phase is calculated by means of chiral perturbation theory, and a region of the phase diagram where Monte Carlo simulations can be performed is identified.