No abstract

Matter described by quantum chromodynamics (QCD), the theory of strong interactions, may undergo phase transitions when its temperature and the chemical potentials are varied. QCD at finite temperature is studied in the laboratory by colliding heavy ions at varying beam energies. We present a test of QCD in the nonperturbative domain through a comparison of thermodynamic fluctuations predicted in lattice computations with the experimental data of baryon number distributions in high-energy heavy ion collisions. This study provides evidence for thermalization in these collisions and allows us to find the crossover temperature between normal nuclear matter and a deconfined phase called the quark gluon plasma. This value allows us to set a scale for the phase diagram of QCD.

When the free energy density of QCD is expanded in a Taylor series in the chemical potential, µ, the coefficients are the non-linear quark number susceptibilities. We show that these depend on the prescription for putting chemical potential on the lattice, making all extrapolations in chemical potential prescription dependent at finite lattice spacing. To put bounds on the prescription dependence, we investigate the magnitude of the non-linear susceptibilities over a range of temperature, T , in QCD with two degenerate flavours of light dynamical quarks at lattice spacing 1/4T . The prescription dependence is removed in quenched QCD through a continuum extrapolation, and the dependence of the pressure, P , on µ is obtained.

The electrical conductivity in the hot phase of the QCD plasma is extracted from a quenched lattice measurement of the Euclidean time vector correlator for 1.5 < T/T_c < 3. The spectral density in the vicinity of the origin is examined using a method specially adapted to this region, and a peak at small energies is seen. The continuum limit of the electrical conductivity, and the closely related soft photon emissivity of the QCD plasma, are then extracted from a fit to the Fourier transform of the temporal vector correlator.Comment: 4 pages, 3 figures; estimate of shear viscosity & comparison with AdS/CFT adde

We present lattice QCD results along the freezeout curve of heavy-ion collisions. The variance, skew and kurtosis of the event distribution of baryon number are studied through Padé resummations. We predict smooth behaviour of three ratios of these quantities at current RHIC and future LHC energies. Deviations from this at lower energies signal the presence of a nearby critical point.