QCD Plasma Phase Research Articles

Static quark-antiquark free energy and the running coupling at finite temperature

We analyze the free energy of a static quark-antiquark pair in quenched QCD at short and large distances. From this we deduce running couplings, ${g}^{2}$ ($r$, $T$), and determine the length scale that separates at high temperature the short distance perturbative regime from the large distance nonperturbative regime in the QCD plasma phase. Ambiguities in the definition of a coupling beyond the perturbative regime are discussed in their relation to phenomenological considerations on heavy quark bound states in the quark gluon plasma. Our analysis suggests that it is more appropriate to characterize the nonperturbative properties of the QCD plasma phase close to ${T}_{c}$ in terms remnants of the confinement part of the QCD force rather than a strong Coulombic force.

Quark number susceptibilities, strangeness, and dynamical confinement

We report first results on the strange quark number susceptibility, chi_s, over a large range of temperatures, mainly in the plasma phase of QCD. Chi_s jumps across the phase transition temperature, T_c, and grows rapidly with temperature above but close to T_c. For all quark masses and susceptibilities in the entire temperature range studied, we found significant departures from ideal-gas values. We also observed a strong correlation between these quantities and the susceptibility in the scalar/pseudo-scalar channel, supporting ideas of ``dynamical confinement'' in the high temperature phase of the QCD plasma.

Static correlation lengths in QCD at high temperatures and finite densities

We use a perturbatively derived effective field theory and three-dimensional lattice simulations to determine the longest static correlation lengths in the deconfined QCD plasma phase at high temperatures (T\gsim 2 Tc) and finite densities (\mu\lsim 4 T). For vanishing chemical potential, we refine a previous determination of the Debye screening length, and determine the dependence of different correlation lengths on the number of massless flavours as well as on the number of colours. For non-vanishing but small chemical potential, the existence of Debye screening allows us to carry out simulations corresponding to the full QCD with two (or three) massless dynamical flavours, in spite of a complex action. We investigate how the correlation lengths in the different quantum number channels change as the chemical potential is switched on.

The non-perturbative QCD Debye mass from a Wilson line operator

According to a proposal by Arnold and Yaffe, the non-perturbative g2T-contribution to the Debye mass in the deconfined QCD plasma phase can be determined from a single Wilson line operator in the three-dimensional pure SU(3) gauge theory. We extend a previous SU(2) measurement of this quantity to the physical SU(3) case. We find a numerical coefficient which is more accurate and smaller than that obtained previously with another method, but still very large compared with the naive expectation: the correction is larger than the leading term up to T∼107Tc, corresponding to g2∼0.4. At moderate temperatures T∼2Tc, a consistent picture emerges where the Debye mass is mD∼6T, the lightest gauge invariant screening mass in the system is ∼3T, and the purely magnetic operators couple dominantly to a scale ∼6T. Electric (∼gT) and magnetic (∼g2T) scales are therefore strongly overlapping close to the phase transition, and the colour-electric fields play an essential role in the dynamics.

Evidence for quantum chaos in the plasma phase of QCD

We investigate the eigenvalue spectrum of the staggered Dirac matrix in SU(3) gauge theory and in full QCD on a $6^3\times 4$ lattice. As a measure of the fluctuation properties of the eigenvalues, we study the nearest-neighbor spacing distribution $P(s)$ for various values of $\beta$ both in the confinement and in the deconfinement phase. In both phases except far into the deconfinement region, the lattice data agree with the Wigner surmise of random-matrix theory which is indicative of quantum chaos. We do not find signs of a transition to Poisson regularity at the deconfinement phase transition.

New results and long-standing questions in finite temperature QCD

We review recent results on QCD at finite temperature. In particular we will discuss the chiral phase transition in two flavour QCD and new results on the excitation spectrum in the plasma phase of QCD.

Quantum effects in charmonium formation and signatures of a quark-gluon plasma

We consider initial conditions of a heavy charmed quark-antiquark pair produced in a spacetime region whose size is much less than that of an isolated bound state. This situation is of Interest for studying the effects of a QCD plasma phase possibly produced in heavy-ion collisions at high energy. The time evolution of such a system is studied, assuming that the quarks initially propagate as free particles to simulate the screened potential in the plasma phase. Due to the small initial size of the quark wave packets, large relativistic momentum components play an important part in the evolved spatial distributions. This feature produces significant changes to the classical description of such systems in which sharp particle trajectories and formation times are employed. To highlight this difference, we use a simple one-dimensional model to follow the evolution of the system during the plasma phase alone. The results indicate that only when such a procedure is carried through the entire time history of the system using realistic physical parameters can one have confidence that experiments are revealing some signature of bound state formation which is characteristic of the existence of a plasma phase.

Hadronic correlation functions in the QCD plasma phase.

We present results for static hadronic correlation functions in the high-temperature phase of QCD with four flavors of dynamical quarks. We confirm chiral-symmetry restoration through the degeneracies of the spectrum of screening lengths. We also find that the screening lengths for the \ensuremath{\rho} and N channels at T\ensuremath{\gtrsim}1.2${\mathit{T}}_{\mathit{c}}$ are close to that in a free quark gas.

Hadronic correlation functions in the QCD plasma phase

We present results for static hadronic correlation functions in the high temperature phase of QCD, obtained by the MT c collaboration. The measured screening lengths are compared with values for a gas of free quarks. In the ϱ and N channels deviations from free quark propagation are found to be small at temperatures T ≥ 1.5 T c.

Lattice QCD, quark-gluon plasma and heavy-ion collisions

These lectures give a strongly biased introduction into the physics of the quark gluon plasma. We will discuss the present status of Monte Carlo simulations of lattice regularized QCD at finite temperature. In particular we look at results for the order of the finite temperature phase transition and the quantitative determination of the transition temperature as well as the equation of state and screening lengths in the QCD plasma phase. We also discuss briefly first experimental attempts to study dense nuclear matter in heavy ion experiments.

The QCD plasma phase

We discuss perturbative and non-perturbative properties of the QCD plasma phase. In particular we present recent Monte Carlo results for the equation of state, the heavy quark potential and screening lengths and give a comparison with expectations based on high temperature perturbation theory.

Electron-positron pair production as a probe of chiral symmetry in a hot QCD plasma

Chiral symmetry in the high-temperature plasma phase of QCD is consistent with at least three qualitatively different quark dispersion relations. The rate for q q → e + e − , with zero total momentum of the e +e − pair in the plasma rest frame is direct measure of the quark dispersion relation. For the three plausible dispersion relations the rates differ by orders of magnitude when the e +e − invariant mass M < 2 T.