Recent SU͑3͒ gauge field lattice data for the equation of state are interpreted by a quasiparticle model with effective thermal gluon masses. The model is motivated by lowest-order perturbative QCD and describes very well the data. The proposed quasiparticle approach can be applied to study color excitations in the nonperturbative regime. As an example we estimate the temperature dependence of the Debye screening mass and find that it declines sharply when approaching the confinement temperature from above, while the thermal mass continuously rises. ͓S0556-2821͑96͒02815-9͔

The new matrix element generator AMEGIC++ is introduced, dedicated to describe multi-particle production in high energy particle collisions. It automatically generates helicity amplitudes for the processes under consideration and constructs suitable, efficient integration channels for the multi-channel phase space integration. The corresponding expressions for the amplitudes and the integrators are stored in library files to be linked to the main program. Title of the program : AMEGIC++ Program obtainable from : authors upon request Licensing provisions : noneOperating systems under which the program has been tested : UNIX, LINUX, VMS Programming language : C++ Separate documentation available : noNature of the physical problem:The theoretical description of multi particle production, even at the tree-level, suffers from two problems :1. The rapidly increasing number of amplitudes forbids the traditional method of summing and squaring individual Feynman amplitudes by means of the completeness relations for spinors and polarization vectors. Instead, the helicity method is employed, translating the amplitudes into complex numbers. Still, the helicity amplitudes for a large number of diagrams have to be constructed which itself is a formidable task.2. The complex structure of the high-dimensional phase space imperatively requires using Monte Carlo methods for its evaluation. Here, efficiency is of paramount importance, and one has to employ non-flat phase space measures which must be optimized for the process under consideration and its specific singularity structure in phase space. Method of solution:Automatic generation of helicity amplitudes related to Feynman diagrams describing the process at the tree-level. Translating of the amplitudes into character strings and storing in libraries for increased efficiency during evaluation, i.e. phase space integration. Integration by means of multichannel methods with specific channels which are constructed from the Feynman diagrams.

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We perform ab initio QED calculations of energy levels for the n = 1 and n = 2 states of He-like ions with the nuclear charge in the range Z = 12-100. The complete set of two-electron QED corrections is evaluated to all orders in the parameter αZ. Uncalculated contributions to energy levels come through orders α 3 (αZ) 2 , α 2 (αZ) 7 , and higher. The calculation presented is the first treatment for excited states of He-like ions complete through order α 2 (αZ) 4 . A significant improvement in accuracy of theoretical predictions is achieved, especially in the high-Z region.

A new approach to finite basis sets for the Dirac equation is developed. It solves the problem of spurious states and, as a result, improves the convergence properties of basis set calculations. The efficiency of the method is demonstrated for finite basis sets constructed from B splines by calculating the one-loop self-energy correction for a hydrogenlike ion.