Joseph I. Kapusta

Quantum Chromodynamics at High Temperature

Abstract The thermodynamic potential of quantum chromodynamics is calculated in orders 1, α c and α c 3 2 . The renormalization group is used to improve the expansion by allowing αc to be a function of the temperature and chemical potentials. The results are valid for an arbitrary number of quark flavors and masses. Numerical calculations are made for the special case of up, down and strange quarks. Particular attention is paid to the breakdown of the perturbation expansion, and possible connection to the thermodynamics of Hagedorn is discussed. The MIT bag model, which has a fixed coupling constant, does not appear to yield good convergence at high temperatures.

Strongly interacting low-viscosity matter created in relativistic nuclear collisions.

Substantial collective flow is observed in collisions between large nuclei at BNL RHIC (Relativistic Heavy Ion Collider) as evidenced by single-particle transverse momentum distributions and by azimuthal correlations among the produced particles. The data are well reproduced by perfect fluid dynamics. A calculation of the dimensionless ratio of shear viscosity eta to entropy density s by Kovtun, Son, and Starinets within anti-de Sitter space/conformal field theory yields eta/s=variant Plancks over 2pi/4pikB, which has been conjectured to be a lower bound for any physical system. Motivated by these results, we show that the transition from hadrons to quarks and gluons has behavior similar to helium, nitrogen, and water at and near their phase transitions in the ratio eta/s. We suggest that experimental measurements can pinpoint the location of this transition or rapid crossover in QCD.

Vector dominance model at finite temperature

Abstract The neutral ϱ-meson propagator is computed at finite temperature using standard π-ρ dynamics. The vector dominance model states that the hadronic electromagnetic current operator is proportional to a linear combination of the neutral vector meson field operators. Neglecting the φ-ω complex, this should give a good account of the rate of lepton pair emission from a hot pion gas. The mass and width of both transverse and longitudinal ϱ-mesons increase with temperature, but temperature effects are still relatively small up to T = 150 MeV. If substantial shifts in these quantities were to be seen experimentally, it would seem to require other new physics like chiral symmetry restoration or deconfinement. The results obtained here may be viewed as generalizing the Gounaris-Sakurai formula to finite temperature.

Entropy and Cluster Production in Nuclear Collisions

When nuclei collide at beam energies from several tens of MeV to several GeV per nucleon considerable disorder is generated. Nuclear fragments ranging from nucleons all the way up in mass to the target and projectile nuclei themselves have been observed experimentally. Theoretical models for the dynamics of the formation and emission of these clusters of nucleons are reviewed. Most of the models, but not all, are statistical in origin, following from the assumption that the phase space available for cluster formation and emission is the dominant factor. The entropy generated during the collision may be studied in diverse dynamical models, such as intranuclear cascade and nuclear fluid dynamics. The entropy of the system may be estimated from the measured abundances of nuclear clusters, thus providing information on the properties of hot and dense nuclear matter. Critical analysis of both conventional and exotic interpretations of the data are given.

Behaviour of Gluons at high temperature

The screening of colour electric and magnetic fields and plasma oscillations in a high-temperature gluon plasma are investigated using linear response theory and self-consistent nonperturbative solutions to the Schwinger-Dyson equation. Static electric fields are screened, with m/sup 2//sub e//sub l/ = 1/3g/sup 2/NT/sup 2/-(3/..pi..)1/6g/sup 2/N)/sup 3/2/T/sup 2/. This result is proven to be gauge invariant in two ways: by computing ..pi../sub 00/ in temporal axial, Coulomb and covariant gauges, and by computing the physical free energy of a heavy quark pair in the plasma in temporal axial gauge. To order g/sup 3/ static magnetic fields are not screened.

Chiral symmetry breaking in the soft-wall AdS/QCD model

We show how to incorporate chiral-symmetry breaking in the soft-wall version of the anti-de Sitter/QCD model by using a modified dilaton profile and a quartic term in the bulk scalar potential. This allows one to separate the dependence on spontaneous and explicit chiral-symmetry breaking. Moreover, our 5D model automatically incorporates linear trajectories and non chiral-symmetry restoration for highly excited radial states. We compare our resulting mass spectra in the scalar, vector, and axial-vector sectors with the respective QCD resonances and find reasonable agreement using the known values for the pion mass and decay constant.

Relativistic theory of hydrodynamic fluctuations with applications to heavy-ion collisions

We develop the relativistic theory of hydrodynamic fluctuations for application to high-energy heavy-ion collisions. In particular, we investigate their effect on the expanding boost-invariant (Bjorken) solution of the hydrodynamic equations.We discover that correlations over a long rapidity range are induced by the propagation of the sound modes. Due to the expansion, the dispersion law for these modes is nonlinear and attenuated even in the limit of zero viscosity. As a result, there is a nondissipative wake behind the sound front which is generated by any instantaneous pointlike fluctuation. We evaluate the two-particle correlators using the initial conditions and hydrodynamic parameters relevant for heavy-ion collisions at RHIC and LHC. In principle these correlators can be used to obtain information about the viscosities because the magnitudes of the fluctuations are directly proportional to them.

Strangeness, glue and quark matter content of neutrons stars

Abstract We show that uncertainties in the strength of interactions of hyperons among themselves and with nucleons lead to a large uncertainty in the maximum allowed neutron star mass, even if the properties of nuclear and neutron matter are known with infinite precision around normal nuclear matter density and below. The presence of hyperons in the neutron star will generate a φ-meson condensate, however, and this reduces the sensitivity to the strengths of the couplings. The possibility that nucleons have a high strangeness content is explored, but it turns out to have negligible influence on neutron star structure. We consider a novel mechanism for nuclear attraction, a density-dependent glueball condensate. Finally, we determine which of these nuclear equations of state lead to a stable quark matter core in the star, via a first- or second-order phase transition.

Properties of ρ and ω mesons at finite temperature and density as inferred from experiment

The mass shift, width broadening, and spectral density for \ensuremath{\rho} and \ensuremath{\omega} mesons in a heat bath of nucleons and pions are calculated using a general formula which relates the self-energy to the real and imaginary parts of the forward scattering amplitude. We use experimental data to saturate the scattering amplitude at low energies with resonances and include a background Pomeron term, while at high energies a Regge parametrization is used. The real part obtained directly is compared with the result of a dispersion integral over the imaginary part. The peaks of the spectral densities are little shifted from their vacuum positions, but the widths are considerably increased due to collisional broadening. Where possible we compare with the UrQMD model and find quite good agreement. At normal nuclear matter density and a temperature of 150 MeV the spectral density of the \ensuremath{\rho} meson has a width of 345 MeV, while that for the \ensuremath{\omega} is in the range 90\char21{}150 MeV.

Rates for dilepton production at RHIC and LHC between the J Ψ and υ are big

Abstract We study the problem of dilepton production in heavy ion collisions at RHIC and LHC energies. We find that, due to the expected enhanced multiplicities and larger transverse momenta of hadrons arising from minijet production, the dilepton production rate dramatically increases. We consider two extreme limits: a thermalized, hydrodynamically expanding quark-gluon plasma and a free-streaming, non-interacting gas of quarks and gluons. In both cases we find that the dilepton rate arising from electromagnetic annihilations of the quarks is significantly larger than that of the Drell-Yan process for dilepton masses between the J Ψ and the υ.