ar X iv : h ep - p h / 99 10 35 3 v 1 1 5 O ct 1 99 9 1 Dynamical screening in hot systems away from ( chemical ) equilibrium ∗ )

Within the Closed Time Path Formalism of Thermal Field Theory we calculate the hard photon emission rate as well as the collisional energy-loss rate for a quark-gluon plasma away from chemical equilibrium. Mass singularities are shown to be dynamically screened within HTL-resummed perturbation theory also away from equilibrium. Additional (pinch) singularities are absent and well defined results are obtained. At present, major efforts are being made to experimentally probe a new deconfinement state of strongly interacting matter (Quark-Gluon Plasma, QGP), and much related research is being done in order to establish a sufficient theoretical understanding of the systems under investigation here. As a central result of research done to date, the hard-thermal-loop (HTL) resummation scheme 1)-3) allows to take collective effects into account within an improved per-turbative approach. For systems in thermal equilibrium, based on HTL-effective perturbation theory, predictions for potentially observable emission rates can be obtained, with medium effects providing the required (dynamical) screening of singularities. More recently, need for a better theoretical understanding of non-equilibrium effects has become apparent, since systems actually under investigation in a heavy-ion collision are likely to stay away from equilibrium for important periods. In this work, we attempt to study the role of collective effects in systems away from equilibrium and to extend the HTL-resummation prescription appropriately. We will study hard thermal photon production 4)-10) as well as the collisional energy loss 11)-13) , which both are known to be sensitive to soft scale physics. The following is based on 14), 15) , where further details can be found. §2. The approximation scheme-chemical non-equilibrium The scenario of chemical non-equilibrium 16)-19) has been argued to be a valid simplification of the actual, complicated pre-equilibrium dynamics in a heavy-ion collision. It is inspired from an analysis of the processes leading to equilibration eventually, from which the effect of elastic collisions is more pronounced than the effect of inelastic processes. Consequently, while local thermal equilibrium might be quickly established, chemical equilibrium among the different particle species and their respective number-densities will be delayed. In particular the number density of quarks is expected to be low while gluons will reach their equilibrium density more quickly, as has been formulated in the hot-glue scenario 16). In this work we follow the analysis, e.g., of 17), parameterizing chemical non-equilibrium in terms of fugacity factors λ multiplying the distribution functionsñ(X, p) = λ q (X)n F for quarks and n(X, …