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Markus H. Thoma

Publications and source records attributed to Markus H. Thoma.

72 records · Page 4Linked to original sources
arXiv1999

Dilepton Production from Rho-Mesons in a Quark-Gluon Plasma

Assuming that rho-mesons exist in a quark-gluon plasma close to the QCD phase transition, we calculate the dilepton production rate from quark-antiquark annihilation via a rho-meson state using Vector Meson Dominance. The result is compared to the rates from direct quark-antiquark annihilation and from pion annihilation. Furthermore we discuss the suppression of low mass dileptons if the quarks assume an effective mass in the quark-gluon plasma.

nucl-th↗
arXiv1999

Gluon Condensate and Quark Propagation in the QGP

A calculation of the thermal quark propagator is presented taking the gluon condensate above the critical temperature into account. The quark dispersion relation following from this propagator is derived.

hep-ph↗
arXiv1998

Equilibrium and Non-Equilibrium Hard Thermal Loop Resummation in the Real Time Formalism

We investigate the use of the hard thermal loop (HTL) resummation technique in non-equilibrium field theory. We use the Keldysh representation of the real time formalism (RTF). We derive the HTL photon self energy and the resummed photon propagator. We show that no pinch singularities appear in the non-equilibrium HTL effective propagator. We discuss a possible regularization mechanism for these singularities at higher orders. As an example of the application of the HTL resummation method within the RTF we discuss the damping rate of a hard electron.

hep-ph↗
arXiv1998

Ward Identities in Non-equilibrium QED

We verify the QED Ward identity for the two- and three -point functions at non-equilibrium in the HTL limit. We use the Keldysh formalism of real time finite temperature field theory. We obtain an identity of the same form as the Ward identity for a set of one loop self-energy and one loop three-point vertex diagrams which are constructed from HTL effective propagators and vertices.

hep-th↗
arXiv1998

QCD Perturbation Theory at Finite Temperature/Density and its Application

In order to describe properties of an equilibrated quark-gluon plasma, QCD at finite temperature (and density) has to be considered. Besides lattice calculations, which can be applied only to static quantities at zero density, perturbative QCD has been used. At finite temperature, however, serious problems such as infrared divergent and gauge dependent results have been encountered. These difficulties can be (partially) avoided if one starts from effective Green functions constructed by resumming a certain class of diagrams (hard thermal loops). Within the last few years this improved perturbation theory (Braaten-Pisarski method) turned out to be a powerful tool for computing interesting quantities of the quark-gluon plasma phase. In the present talk a basic introduction to the Braaten-Pisarski method is provided and its applications and limits are reviewed. In particular, damping rates, the energy loss of energetic partons, thermalization times, the viscosity of the quark-gluon plasma, and the production of photons and dileptons are discussed.

hep-ph↗
arXiv1996

Damping Rate of a Scalar Particle in Hot Scalar QED

In contrast to the damping of partons in a quark-gluon plasma, the damping of a scalar particle in a hot scalar QED plasma can be calculated to leading order for the whole momentum range using the Braaten-Pisarski method. In this way the evolution of the logarithmic infrared singularity caused by the exchange of a transverse photon from soft to hard momenta can be studied.

hep-ph↗
arXiv1995

Photon Emission from a Parton Gas at Chemical Non-Equilibrium

We compute the hard photon production rate of a chemically non-equilibrated quark-gluon plasma. We assume that the plasma is already thermally equilibrated, i.~e. describable by a temperature, but with a phase-space distribution that deviates from the Fermi/Bose distribution by a time dependent factor (fugacity). The photon spectrum is obtained by integrating the photon rate over the space-time evolution of the quark-gluon plasma. Some consequences for ultrarelativistic heavy ion collisions are discussed.

hep-ph↗
arXiv1995

COMBINATORIAL COMPUTATION OF CLEBSCH-GORDAN COEFFICIENTS

The addition of angular momenta can be reduced to elementary coupling processes of spin-$\frac{1}{2}$-particles. In this way, a method is developed which allows for a non-recursive, simultaneous computation of all Clebsch-Gordan coefficients concerning the addition of two angular momenta. The relevant equations can be interpreted easily, analogously to simple probabilistic considerations. They provide an improved understanding of the addition of angular momenta as well as a practicable evaluation of Clebsch-Gordan coefficients in an easier way than within the well-known methods.

quant-ph↗
arXiv1995

APPLICATIONS OF HIGH-TEMPERATURE FIELD THEORY TO HEAVY-ION COLLISIONS

A recent development in finite temperature field theory, the so-called Braaten-Pisarski method, and its application to properties of a quark-gluon plasma, possibly formed in relativistic heavy ion collisions, are reviewed. In particular parton damping rates, the energy loss of energetic partons, thermalization times, viscosity, and production and damping rates of hard photons are discussed.

hep-ph↗
arXiv1995

Cluster Expansion Approach to the Effective Potential in $Φ^4_{2+1}$-Theory

We apply a truncated set of dynamical equations of motion for connected equal-time Green functions up to the 4-point level to the investigation of spontaneous ground state symmetry breaking in $Φ^4_{2+1}$ quantum field theory. Within our momentum space discretization we obtain a second order phase transition as soon as the connected 3-point function is included. However, an additional inclusion of the connected 4-point function still shows a significant influence on the shape of the effective potential and the critical coupling.

hep-th↗
arXiv1994

Braaten-Pisarski Method at Finite Chemical Potential

The effective perturbation theory developed by Braaten and Pisarski for gauge theories at finite temperature is extended to finite chemical potential. As a first application the collisional energy loss of a heavy quark propagating through a quark-gluon plasma with non-vanishing quark chemical potential is considered. Assuming $μ/T\simeq 1$, motivated by numerical simulations of heavy ion collisions at RHIC energies, we find that the effect of the quark chemical potential is rather small, unless the energy density instead of the temperature is fixed.

hep-ph↗
arXiv1994

Damping Rate of a Yukawa Fermion at Finite Temperature

The damping of a massless fermion coupled to a massless scalar particle at finite temperature is considered using the Braaten-Pisarski resummation technique. First the hard thermal loop diagrams of this theory are extracted and effective Green's functions are constructed. Using these effective Green's functions the damping rate of a soft Yukawa fermion is calculated. This rate provides the most simple example for the damping of a soft particle. To leading order it is proportional to $g^2T$, whereas the one of a hard fermion is of higher order.

hep-ph↗
arXiv1993

Parton Interaction Rates and Limits on Perturbative QCD

The relevance of parton interaction rates for observables of a quark- gluon plasma and for testing field theoretical methods is exhibited. Using the Braaten-Pisarski method the interaction rate as well as the transport rate beyond the leading logarithm approximation are estimated. Limts on the validity of perturbative QCD for relativistic heavy ion collisions are discussed.

hep-ph↗
arXiv1993

Fission Decay Rates Determined from a Quantal Transport Equation

The decay of a metastable system is described by extending Kramers' method to the quantal regime. For temperatures above twice the crossover value we recover the result known from applying Euclidean path integrals to solvable models. Our derivation is not restricted to a linearly coupled heat bath of oscillators, and thus applicable to nuclear systems.

nucl-th↗
arXiv1993

Parton Interaction Rates in the Quark-Gluon Plasma

The transport interaction rates of elastic scattering processes of thermal partons in the quark-gluon plasma are calculated beyond the leading logarithm approximation using the effective perturbation theory for QCD at finite temperatures developed by Braaten and Pisarski. The results for the ordinary and transport interaction rates obtained from the effective perturbation theory are compared to perturbative approximations based on an infrared cut-off by the Debye screening mass. The relevance of those interaction rates for a quark-gluon plasma possibly formed in ultrarelativistic heavy ion collisions are discussed.

hep-ph↗