Markus H. Thoma

Hard Loop Approach to Anisotropic Systems

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.

Hard thermal photon production in relativistic heavy ion collisions

The recent status of hard thermal photon production in relativistic heavy ion collisions is reviewed and the current rates are presented with emphasis on corrected bremsstrahlung processes in the quark-gluon plasma (QGP) and quark-hadron duality. Employing Bjorken hydrodynamics with an EOS supporting the phase transition from QGP to hot hadron gas (HHG), thermal photon spectra are computed. For SPS 158 GeV Pb+Pb collisions, comparison with other theoretical results and the WA98 direct photon data indicates significant contributions due to prompt photons. Extrapolating the presented approach to RHIC and LHC experiments, predictions of the thermal photon spectrum show a QGP outshining the HHG in the high-pT-region.

Finite temperature meson correlation functions in HTL approximation

Abstract We calculate temporal correlators and their spectral functions with meson quantum numbers in the deconfined phase of QCD using the hard thermal loop (HTL) approximation for the quark propagator. Although this approach does not result in a complete next-to-leading order perturbative calculation it takes into account important medium effects such as thermal quark masses and Landau damping in the quark–gluon plasma. We show that both effects lead to competing modifications of the free mesonic correlation functions. We find that correlators in scalar channels are only moderately influenced by the HTL medium effects, while the HTL-vertex corrections lead to divergent vector correlators.

Bremsstrahlung from an equilibrating quark-gluon plasma

The photon production rate from a chemically equilibrating quark-gluon plasma likely to be produced at RHIC (BNL) and LHC (CERN) energies is estimated taking into account bremsstrahlung. The plasma is assumed to be in local thermal equilibrium, but with a phase space distribution that deviates from the Fermi or Bose distribution by space-time dependent factors (fugacities). The photon spectrum is obtained by integrating the photon rate over the space-time history of the plasma, adopting a boost invariant cylindrically symmetric transverse expansion of the system with different nuclear profile functions. Initial conditions obtained from a self-screened parton cascade calculation and, for comparison, from the HIJING model are used. Compared to an equilibrated plasma at the same initial energy density, taken from the self-screened parton cascade, a moderate suppression of the photon yield by a factor of 1 to 5 depending on the collision energy and the photon momentum is observed. The individual contributions to the photon production, however, are completely different in the both scenarios.

Leontovich relations in thermal field theory

Abstract. The application of generalized Kramers-Kronig relations, the so-called Leontovich relations, to thermal field theory is discussed. Medium effects contained in the full, thermal propagators can easily be taken into account by this method. As examples the collisional energy loss of a charged particle in a relativistic plasma and the radiation of energetic photons from a quark-gluon plasma are considered. Within the leading logarithmic approximation the results based on the hard thermal loop resummation technique are reproduced easily. However, the method presented here is more general and provides exact expressions, which allow in principle non-perturbative calculations.

Photon-photon interaction in a photon gas

Using the effective Lagrangian for the low-energy photon-photon interaction, the lowest-order photon self-energy at finite temperature and in non-equilibrium is calculated within the real-time formalism. The Debye mass, the dispersion relation, the dielectric tensor, and the velocity of light following from the photon self-energy are discussed. As an application, we consider the interaction of photons with the cosmic microwave background radiation.

Gluon Condensate and Non-Perturbative Quark-Photon Vertex ∗

We evaluate the quark-photon vertex non-perturbatively taking into account the gluon condensate at finite temperature. This vertex is related to the previously derived effective quark propagator by a QED like Ward-Takahashi identity. The importance of the effective vertex for the dilepton production rate from a quark-gluon plasma is stressed.We evaluate the quark-photon vertex non-perturbatively taking into account the gluon condensate at finite temperature. This vertex is related to the previously derived effective quark propagator by a QED like Ward-Takahashi identity. The importance of the effective vertex for the dilepton production rate from a quark-gluon plasma is stressed. PACS numbers: 12.38.Lg Typeset using REVTEX ∗Supported by BMBF, GSI Darmstadt, DFG, and Humboldt foundation †Humboldt fellow and on leave of absence from Saha Institute of Nuclear Physics, 1/AF Bidhan Nagar, Calcutta 700 064, India

Van Hove singularities in the quark-gluon plasma

Abstract General arguments as well as different approximations for the in-medium quark propagator in a quark-gluon plasma lead to quark dispersion relations that exhibit a minimum in one branch (plasmino). This minimum causes Van Hove singularities in the dilepton production rate and mesonic correlators, which might have observable consequences.

Quantum Field Theoretic Description of Matter in the Universe

Quantum field theory at finite temperature and density can be used for describing the physics of relativistic plasmas. Such systems are frequently encountered in astrophysical situations, such as the early universe, supernova explosions, and the interior of neutron stars. After a brief introduction to thermal field theory the usefulness of this approach in astrophysics will be exemplified in three different cases. First the interaction of neutrinos within a supernova plasma will be discussed. Then the possible presence of quark matter in a neutron star core and finally the interaction of light with the Cosmic Microwave Background will be considered.

Hadron correlators in the deconfined phase

Temporal meson correlators and their spectral functions are calculated in the deconfined phase using the hard thermal loop resummation technique. The spectral functions exhibit strong medium effects coming from the hard thermal loop approximation for the quark propagator. The correlators, on the other hand, do not differ significantly from free correlators, for which bare quark propagators are used. This is in contrast to lattice calculations showing a clear deviation from the free correlations functions.