Signatures of the quark gluon plasma: a personal overview

We first discuss, on a no-QGP basis, the two prominent indications of (a) enhanced strangeness production and of (b) anomalous $J/\psi$-suppression: We elaborate in particular on a recent idea of antihyperon production solely by multi-mesonic reactions. As a possible source for an enhanced dissociation of $c \bar{c}$ pairs we summarize the findings within the 'early'-comover absorption scenario of prehadronic string excitations. As an exotic candidate we then finally adress the stochastic formation of so called disoriented chiral condensates: An experimentally feasible DCC, if it does exist, has to be a rare event following an unusual and nontrivial distribution on an event by event basis.Comment: 10 pages, 5 figures, uses ws-p8-50x6_00.cls style file, contribution for the proceedings of `Luederitz 2000: Fundamental and Applied Aspects of Modern Physics', November 2000, Luederitz, Namibi

Do chemically saturated antihyperon abundancies signal the quark gluon plasma?

We first review the production and the possible chemical equilibration of strange particles at CERN-SPS energies within a microscopic hadronic transport calculation. It is shown in particular that the strange quarks are produced initially via string excitations in the primary, secondary and ternary interactions. We then further elaborate on a recent idea of antihyperon production by multi-mesonic reactions like $n_1\pi + n_2 K \to \bar{Y}+p$ corresponding to the inverse of the strong binary baryon-antibaryon annihilation process. It is argued that by these reactions the (rare) antihyperons are driven towards local chemical equilibrium with pions, nucleons and kaons on a timescale of 1--3 fm/c in the still moderately baryon-dense initial hadronic environment after the termination of the prehadronic string phase. Accordingly this mechanism can provide a convenient explanation for the antihyperon yields at CERN-SPS energies without any need of a deconfined quark gluon plasma phase.Comment: invited contribution to the symposium on `Fundamental Issues in Elementary Matter' in honor of M. Danos, 25-29 september, Bad Honnef, Germany; 12 pages, 4 .eps figures, uses eppsf.sty, hip-artc.st

Theoretical Review of Charmonium Production with Different pTp_T in the Hot Medium

Charmonia with different transverse momentum $p_T$ usually comes from different mechanisms in the relativistic heavy ion collisions. This work tries to review the theoretical studies on quarkonium evolutions in the deconfined medium produced in p-Pb and Pb-Pb collisions. The charmonia with high $p_T$ are mainly from the initial hadronic collisions, and therefore sensitive to the initial energy density of the bulk medium. For those charmonia within $0.1<p_T<5$ GeV/c at the energies of Large Hadron Collisions (LHC), They are mainly produced by the recombination of charm and anti-charm quarks in the medium. In the extremely low $p_T\sim 1/R_A$ ($R_A$ is the nuclear radius), additional contribution from the coherent interactions between electromagnetic fields generated by one nucleus and the target nucleus plays a non-negligible role in the $J/\psi$ production even in semi-central Pb-Pb collisions.Comment: 6 pages, 13 figures, proceeding for CHARM 201

Interpretation and resolution of pinch singularities in non-equilibrium quantum field theory

Ill-defined pinch singularities arising in a perturbative expansion in out of equilibrium quantum field theory have a natural analogue to standard scattering theory. We explicitly demonstrate that the occurrence of such terms is directly related to Fermi's golden rule known from elementary scattering theory and is thus of no mystery. We further argue that within the process of thermalization of a plasma one has to resum such contributions to all orders as the process itself is of non-perturbative nature. In this way the resummed propagators obtain a finite width. Within the Markov approximation of kinetic theory the actual phase space distribution at a given time of the evolution enters explicitly.Comment: REVTeX, 11 pages, 3 eps-figures, minor changes, some remarks adde