arXiv subjects

G. Roepke

Publications and source records attributed to G. Roepke.

59 records · Page 4Linked to original sources

arXiv1996

1/N_c- expansion of the quark condensate at finite temperature

Previously the quark and meson properties in a many quark system at finite temperature have been studied within effective QCD approaches in the Hartree approximation. In the present paper we consider the influence of the mesonic correlations on the quark self-energy and on the quark propagator within a systematic $1/N_c$- expansion. Using a general separable ansatz for the nonlocal interaction, we derive a selfconsistent equation for the $1/N_c$ correction to the quark propagator. For a separable model with cut-off formfactor, we obtain a decrease of the condensate of the order of 20\% at zero temperature. A lowering the critical temperature for the onset of the chiral restoration transition due to the inclusion of mesonic correlations is obtained what seems to be closer to the results from lattice calculations.

nucl-th↗

arXiv1995

Coherent and squeezed condensates in massless $λφ^4$ theory

Generalizing the Bogoliubov model of a weakly non-ideal Bose gas to massless $λφ^4$ theory we show that spontaneous breaking of symmetry occurs due to condensation in a coherent vacuum % which is energetically favoured compared to the perturbative one. and leads to a vacuum energy density which is lower than that obtained by Coleman and Weinberg using the one-loop effective potential method. We discuss the alternative of a squeezed condensate and find that for the massless $λφ^4$ theory spontaneous symmetry breaking to a squeezed vacuum does not occur.

hep-th↗

arXiv1995

Bogoliubov condensation of gluons and spontaneous gauge symmetry breaking in QCD

The problem of the gluonic quasiparticle excitations in QCD is considered under the aspect of the condensation of gluon pairs in the ''squeezed'' vacuum. The present approach is a field theoretical generalization of the Bogoliubov model which successfully reproduced the Landau spectrum in the microscopic theory of superfluidity. We construct a gauge invariant QCD Hamiltonian by formally solving the Gauss equation such that the physical variables are separated by a non-Abelian projection operator, instead of fixing a gauge. By using Dirac quantization we show that the Bogoliubov condensation of gluon pairs destroys this projection operator, and the spontaneous appearance of a gluon mass is accompanied by a longitudinal component for the gluon field in correspondence with the relativistic covariance. Gauge symmetry is broken spontaneously since the gauge invariance of the Hamiltonian is not shared by the vacuum. The squeezed vacuum in the present model is characterized by one free parameter related to the contraction of a pair of zero momentum gluon fields which is fixed from the difference of the $η'$ and the $η$ - meson masses ($ U(1)$-problem) and results in a value for the gluon condensate which is in good agreement with the value obtained by Shifman, Vainshtein and Zakharov.

hep-ph↗

arXiv1995

The Nucleon Spectral Function at Finite Temperature and the Onset of Superfluidity in Nuclear Matter

Nucleon selfenergies and spectral functions are calculated at the saturation density of symmetric nuclear matter at finite temperatures. In particular, the behaviour of these quantities at temperatures above and close to the critical temperature for the superfluid phase transition in nuclear matter is discussed. It is shown how the singularity in the thermodynamic T-matrix at the critical temperature for superfluidity (Thouless criterion) reflects in the selfenergy and correspondingly in the spectral function. The real part of the on-shell selfenergy (optical potential) shows an anomalous behaviour for momenta near the Fermi momentum and temperatures close to the critical temperature related to the pairing singularity in the imaginary part. For comparison the selfenergy derived from the K-matrix of Brueckner theory is also calculated. It is found, that there is no pairing singularity in the imaginary part of the selfenergy in this case, which is due to the neglect of hole-hole scattering in the K-matrix. From the selfenergy the spectral function and the occupation numbers for finite temperatures are calculated.

nucl-th↗

arXiv1994

Critical Enhancement of the In-medium Nucleon-Nucleon Cross Section at low Temperatures

The in-medium nucleon-nucleon cross section is calculated starting from the thermodynamic T-matrix at finite temperatures. The corresponding Bethe-Salpeter-equation is solved using a separable representation of the Paris nucleon-nucleon-potential. The energy-dependent in-medium N-N cross section at a given density shows a strong temperature dependence. Especially at low temperatures and low total momenta, the in-medium cross section is strongly modified by in-medium effects. In particular, with decreasing temperature an enhancement near the Fermi energy is observed. This enhancement can be discussed as a precursor of the superfluid phase transition in nuclear matter.

nucl-th↗