arXiv subjects

Dirk Roder

Publications and source records attributed to Dirk Roder.

2 recordsLinked to original sources

arXiv2006

Selfconsistent calculations of sigma-meson properties at finite temperature

I study the properties of the scalar sigma-meson [also referred to as f_0(600)] at nonzero temperature in the O(N)-model in the framework of the Cornwall-Jackiw-Tomboulis formalism. In the standard Hartree (or large-N) approximation one only takes into account the double-bubble diagrams in the effective potential. I improve this approximation by additionally taking into account the sunset diagrams, which lead to 4-momentum dependent real and imaginary parts of the Dyson-Schwinger equations. By solving these and the equation for the chiral condensate selfconsistently, one obtains the decay width and the spectral density of the sigma-meson. I compare the results in the case with explicit chiral symmetry breaking with the chiral limit. I found that the 4-momentum dependent real part of the selfenergy does not lead to major qualitative changes in the spectral density.

hep-ph↗

arXiv2006

Selfconsistent calculations of mesonic properties at nonzero temperature

After a brief introduction on QCD and effective models in the first chapter, I analyze the dependence of the QCD transition temperature on the quark (or pion) mass in the second chapter. I found that a linear sigma model, which links the transition to chiral symmetry restoration, predicts a much stronger dependence of T_c on m_pi than seen in present lattice data for m_pi >~ 0.4 GeV. On the other hand, an effective Lagrangian for the Polyakov loop requires only small explicit symmetry breaking to describe T_c(m_pi) in the above mass range. In the third and fourth chapter, I study the linear sigma model with O(N) symmetry at nonzero temperature in the framework of the Cornwall-Jackiw-Tomboulis formalism. Extending the set of two-particle irreducible diagrams by adding sunset diagrams to the usual Hartree-Fock (or Hartree) contributions, I derive a new approximation scheme which extends the standard Hartree-Fock (or Hartree) approximation by the inclusion of nonzero decay widths.

hep-ph↗