Renormalization and resummation in the O(N) model

In the O(N) model for the large N expansion one needs resummation which makes the renormalization of the model difficult. In the paper it is discussed, how can one perform a consistent perturbation theory at zero as well as at finite temperature with the help of momentum dependent renormalization schemes.Comment: 4 pages, presented at International Conference on Strong and Electroweak matter (SEWM 2008), Amsterdam, The Netherlands, 26-29 Aug 200

Transport coefficients in non-quasiparticle systems

Transport coefficeints, in particular the shear viscosity to entropy density ratio is studied in systems where the small-width quasiparticle assumption is not valid. It is found that $\eta/s$ has no unversal lower bound, the minimal value depends on the system and the temperature, and can be even zero. We construct models where the $1/4\pi$ conjectured bound is violated.Comment: 5 pages, uses svjour; talk presented in HCBM 2010, International Workshop on Hot & Cold Baryonic Matter 15-20 August 2010, Budapest - Hungar

Hadron melting and QCD thermodynamics

We study in this paper mechanisms of hadron melting based on the spectral representation of hadronic quantum channels, and examine the hadron width dependence of the pressure. The findings are applied to a statistical hadron model of QCD thermodynamics, where hadron masses are distributed by the Hagedorn model and a uniform mechanism for producing hadron widths is assumed. According to this model the hadron - quark gluon plasma transition occurs at $T\approx 200$-250 MeV, the numerically observable $T_c=156$ MeV crossover temperature is relevant for the onset of the hadron melting process.Comment: 10 pages, 12 figures, revtex

Resummation and damping in the O(N) model

In this talk I summarize the one loop and higher loop calculations of the effective equations of motion of the O(N) symmetric scalar model in the linear response approximation. At one loop one finds essential difference in long time behavior for the fields below and above a dynamically generated length scale. A partial resummation assuming quasi-particle propagation seems to cancel the relevance of this scale.Comment: Talk given at SEWM2000. 5 page