QCD Critical Surfaces at Real and Imaginary μ

Quantumchromodynamics is the fundamental theory of the strong interactions. At finite temperatures and baryon densities, it predicts a transition from the known hadronic phase to a plasma phase, where quarks and gluons are no longer confined. For small and large quark masses, these are first order chiral and deconfinement transitions, respectively, whereas for intermediate masses they are only analytic crossovers, separated by critical surfaces. In a long term project, we calculate the critical surface bounding the region featuring chiral phase transitions in the quark mass and chemical potential parameter space of QCD with three flavours of quarks by means of lattice simulations. Our calculations are valid for small to moderate quark chemical potentials, μ≲T. Previous calculations were done on coarse N t =4 lattices, corresponding to a lattice spacing a∼0.3 fm. Here we present results for three degenerate flavours at zero and finite density on N t =6 lattices, corresponding to a lattice spacing of a∼0.2 fm. Furthermore, we compute the phase structure at imaginary chemical potential μ/T=iπ/3, finding tricritical lines which bound the continuation of the chiral as well as the deconfinement transition surfaces to imaginary chemical potentials and explain their curvature.