arXiv subjects
Publications and source records attributed to M. H. Thoma.
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.
A calculation of the thermal quark propagator is presented taking the gluon condensate above the critical temperature into account. The quark dispersion relation and the dilepton production following from this propagator are derived.
We present an extension of the HTL resummation technique to non-equilibrium situations, starting from the real time formalism in the Keldysh representation. As an example we calculate the HTL photon self energy, from which we derive the resummed photon propagator. This propagator is applied to the computation of the interaction rate of a hard electron in a non-equilibrium QED plasma. In particular we show the absence of of pinch singularities in these quantities. Finally we show that the Ward identities relating the HTL electron self energy to the HTL electron-photon vertex hold as in equilibrium.
We investigate the influence of medium effects on the structure of hybrid stars, i.e. neutron stars possessing a quark matter core. We found that medium effects reduce the extent of a pure quark matter phase in the interior of a hybrid star significantly in favor of a mixed phase of quark and hadronic matter. Over a wide range of the strong coupling constant - which parameterizes the influence of medium effects - quark matter is able to exist at least in a mixed phase in the interior of neutron stars.
One of the most intriguing consequence of the extreme conditions inside neutron stars is the possibility of the natural existence of a deconfined strange quark matter phase in the high density interior of the star. The equation of state (EOS) of strange quark matter (SQM) was recently improved in the framework of the MIT bag model by including medium effects. It was found that medium effects increase the energy per baryon of SQM and therefore lower the stability of this phase. In this work we investigate the influence of medium effects on the structure of hybrid stars within this model. We found that the medium effects reduce the extent of a pure SQM phase in the interior of an hybrid star significantly in favor of a mixed phase of quark and hadronic matter.
We estimate the radiative energy-loss of heavy quarks, produced from the initial fusion of partons, while propagating in a quark-gluon plasma which may be formed in the wake of relativistic heavy ion collisions. We find that the radiative energy-loss for heavy quarks is larger than the collisional energy-loss for all energies. We point out the consequences on possible signals of the quark-gluon plasma.
The complete one-loop self energies (real and imaginary parts) for photons, gluons, electrons and quarks at finite temperature are calculated numerically and compared to the results of the hard thermal loop (HTL) approximation used for the resummation technique of Braaten and Pisarski. In this way some light is shed on the validity of the weak coupling limit assumption ($g \ll 1$) or equivalently the high temperature assumption, on which the HTL approximation is based. Furthermore, the gauge dependence of the fermion self energy beyond the HTL approximation is considered. Finally the dispersion relations following from the real part of the self energies are compared to the HTL results.
The hierarchy of equations of motion for equal-time Green functions in temporal gauge SU(N) Yang-Mills theory is truncated using an expansion in terms of connected Green functions. A second hierarchy of constraint equations arises from Gauss law and can be truncated in a similar way. Within this approximation scheme we investigate SU(2) Yang-Mills theory on a torus in 2+1 spacetime dimensions in a finite basis of plane wave states and focus on infrared and ultraviolet properties of the approach. We study the consequences of restoring the hierarchy of Gauss law constraints and of different momentum cutoffs for the 2- and the 3-point functions. In all truncation schemes considered up to the 4-point level the connected Green function approach is found to be UV divergent and either violating gauge invariance and/or energy conservation. The problems associated with adiabatically generating a perturbed ground state are discussed as well.
The resummation technique of Braaten and Pisarski is used for a complete calculation of the $α^2 α_s$-contribution to the production rate of dileptons with energy E >> T and invariant mass M <~ T from a quark-gluon plasma. In particular the cancellation of the infrared singularity by medium effects is discussed in detail.
A dynamical model of confinement based on a microscopic transport description of the Friedberg-Lee model is extended to explicit color degrees of freedom. The string tension is reproduced by an adiabatic string formation from the nucleon ground state. As a particular application, we address the question of how a charmonium state might be dissociated by the strong color electric fields when moving through a color electric flux tube and speculate on the importance of such an effect with respect to the issue of $J/ψ$--suppression observed in ultrarelativistic heavy ion collisions. Furthermore, we show the dynamical breakup of flux tubes via $q-\bar{q}$-particle production and the disintegration into mesons. There we encounter some problems within the Vlasov-type realization of describing the motion of the quarks which can be resolved by a molecular dynamical approach.
We present two novel dynamical features of flux tubes in the Friedberg-Lee model. First the fusion of two (anti-)parallel flux tubes, where we extract a string-string interaction potential which has a qualitative similarity to the nucleon-nucleon potential in the Friedberg-Lee model obtained by Koepf et al. Furthermore we show the dynamical breakup of flux tubes via $q\bar{q}-$particle production and the disintegration into mesons. We find, as a shortcoming of the present realization of the model, that the full dynamical transport approach presented in a previous publication fails to provide the disintegration mechanism in the semiclassical limit. Therefore, in addition, we present here a molecular dynamical approach for the motion of the quarks and show, as a first application, the space-time development of the quarks and their mean-fields for Lund-type string fragmentation processes.
The properties of strange quark matter at zero temperature are investigated 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. Within this approach we find that these medium effects reduce the binding energy of strange quark matter with respect to $^{56}Fe$.
We investigate the convergence properties of the cluster expansion of equal-time Green functions in scalar theories with quartic self-coupling in (0+1), (1+1), and (2+1) space-time dimensions. The computations are carried out within the equal-time correlation dynamics approach. We find that the cluster expansion shows good convergence as long as the system is in a single phase configuration and that it breaks down in a two phase configuration, as one would naively expect. In the case of dynamical calculations with a time dependent Hamiltonian we find two timescales determining the adiabaticity of the propagation; these are the time required for adiabaticity in the single phase region and the time required for tunneling into the non-localized lowest energy state in the two phase region.
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.
The damping rate of a Higgs field at zero momentum is calculated using the Braaten-Pisarski method and compared to the Lyapunov exponent of the classical SU(2) Yang-Mills Higgs system.
A dynamical model of confinement based on a transport theoretical description of the Friedberg-Lee model is extended to explicit color degrees of freedom. The string tension is reproduced by an adiabatic string formation from the nucleon ground state. Color isovector oscillation modes of a $q\bar{q}$-system are investigated for a wide range of relative $q\bar{q}$-momenta and the dynamical impact of color confinement on the quark motion is shown.
We compute the photon production rate of a quark-gluon plasma (QGP) at finite quark chemical potential $μ$ using the Braaten-Pisarski method, thus continuing the work of Kapusta, Lichard, and Seibert who did the calculation for $μ=0$.
Using the cluster expansions for n-point Green functions we derive a closed set of dynamical equations of motion for connected equal-time Green functions by neglecting all connected functions higher than $4^{th}$ order for the $λΦ^4$-theory in $1+1$ dimensions. We apply the equations to the investigation of spontaneous ground state symmetry breaking, i.e. to the evaluation of the effective potential at temperature $T=0$. Within our momentum space discretization we obtain a second order phase transition (in agreement with the Simon-Griffith theorem) and a critical coupling of $λ_{crit}/4m^2=2.446$ as compared to a first order phase transition and $λ_{crit}/4m^2=2.568$ from the Gaussian effective potential approach.