The energy loss d E / d x for a heavy lepton propagating through a high-temperature QED plasma is calculated to leading order in the QED coupling constant. The screening effects of the plasma are computed consistently using a resummation of perturbation theory in the small-momentum-transfer region. At large momentum transfer, recoil effects are properly taken into account. Our complete leading-order calculation differs significantly from previous calculations.

No abstract

No abstract

An experimental determination of particle charge in a bulk dc discharge plasma covering a wide range of neutral gas pressures, was recently reported [S. Ratynskaia, Phys. Rev. Lett. 93, 085001 (2004)]. The charges obtained were several times smaller than the predictions of collisionless orbital motion limited theory. This discrepancy was attributed to the effect of ion-neutral collisions. In the present paper a more detailed description of this experiment is provided and additional experimental results obtained with particles of different sizes are reported. The measurements are compared with molecular dynamics simulations of particle charging for conditions similar to those of the experiment, with other available experimental data on particle charge in the bulk of gas discharges, and with a simple analytical model accounting for ion-neutral collisions. All the considered evidence indicates that ion-neutral collisions represent a very important factor, which significantly affects (reduces) the particle charge under typical discharge conditions.

The appearance of quark phases in the dense interior of neutron stars provides one possibility to soften the equation of state (EOS) of neutron star matter at high densities. This softening leads to more compact equilibrium configurations of neutron stars compared to pure hadronic stars of the same mass. We investigate the question to which amount * Supported by DFG and GSI Darmstadt. † Heisenberg Fellow. the compactness of a neutron star can be attributed to the presence of a quark phase. For this purpose we employ several hadronic EOS in the framework of the relativistic mean-field (RMF) model and an extended MIT bag model to describe the quark phase. We find that -almost independent of the model parameters -the radius of a pure hadronic neutron star gets typically reduced by 20 − 30% if a pure quark phase in the center of the star does exist. For some EOS we furthermore find the possibility of a third family of compact stars which may exist besides the two known families of white dwarfs and neutron stars. We show how an experimental proof of the existence of a third family by mass and radius measurements may provide a unique signature for a phase transition inside neutron stars.

Direct photons have been proposed as a promising signature for the quark-gluon plasma (QGP) formation in relativistic heavy-ion collisions. Recently WA98 presented the first data on direct photons in P b+P b-collisions at SPS. At the same time RHIC started with its experimental program. The discovery of the QGP in these experiments relies on a comparison of data with theoretical predictions for QGP signals. In the case of direct photons new results for the production rates of thermal photons from the QGP and a hot hadron gas as well as for prompt photons from initial hard parton scatterings have been proposed recently. Based on these rates a variety of different hydrodynamic models, describing the space-time evolution of the fireball, have been adopted for calculating the direct photon spectra. The results have been compared to the WA98 data and predictions for RHIC and LHC have been made. So far the conclusions of the various models are controversial.The aim of the present review is to provide a comprehensive and up-to-date survey and status report on the experimental and theoretical aspects of direct photons in relativistic heavy-ion collisions. References 83Now we will discuss the various attempts for calculating the production rate of energetic photons (E ≫ T ) from an equilibrated QGP.Pre-HTL rate: Before the invention of the Hard-Thermal-Loop (HTL) improved perturbation theory (see below), the QGP photon rates have been calculated using the perturbative matrix elements for the processes of Fig. 1 together with Eq.(2) [15,16,20,22]. In Ref.[20], even bremsstrahlung has been considered in this way. The derivation of the differential production rate of energetic photons (E ≫ T ), 4 The strong coupling constant at finite temperature depends on the temperature (effective, temperature-dependent running coupling constant) [29]. However, for most applications in the following we will use a mean value of α s = 0.2 -0.5, which is typical for temperatures reachable in relativistic heavy-ion collisions.7 In Ref.[49] a numerical error led to an overestimation of the rate by a factor of 4 [51].

We investigate the properties of strange quark matter at zero temperature including medium effects. The quarks are considered as quasiparticles which acquire an effective mass generated by the interaction with the other quarks of the dense system. The effective quark masses are derived from the zero momentum limit of the dispersion relations following from an effective quark propagator obtained from resumming one-loop self energy diagrams in the hard dense loop approximation. This leads to a thermodynamic selfconsistent description of strange quark matter as an ideal Fermi gas of quasiparticles. Within this approach we find that medium effects reduce the overall binding energy with respect to $^{56}Fe$ of strange quark matter. For realistic values of the strong coupling constant strange quark matter is not absolutely stable. The application to pure strange quark matter stars shows that medium effects have, nevertheless, no impact on the mass-radius relation of the stars. However, a phase transition to hadronic matter at the surface of the stars becomes more likely.Comment: 16 pages, LaTeX, 14 eps-figures included. To be published in Nuclear Physics

Anisotropic systems of quarks and gluons, which at least for sufficiently short space-time intervals can be treated as homogeneous and static, are considered. The gluon polarization tensor of such a system is explicitly computed within the semiclassical kinetic and hard loop diagrammatic theories. The equivalence of the two approaches is demonstrated. The quark self-energy is computed as well, and finally, the dispersion relations of quarks and gluons in the anisotropic medium are discussed.