arXiv subjects

Bernd-Jochen Schaefer

Publications and source records attributed to Bernd-Jochen Schaefer.

47 records · Page 3Linked to original sources

arXiv2009

QCD Thermodynamics: Confronting the Polyakov-Quark-Meson Model with Lattice QCD

NJL-type effective models represent a low-energy realization of QCD and incorporate pertinent aspects such as chiral symmetry and its spontaneous breaking, the center symmetry in the heavy-quark limit as well as the axial anomaly. One such model, the Polyakov-quark-meson model for three light quark flavors, is introduced in order to study the phase structure of strongly-interacting matter. With recent high-statistics lattice QCD simulations of the finite-temperature equation of state, a detailed comparison with model results becomes accessible. Such comparisons allow to estimate volume and truncation effects of quantities, obtained on the lattice and provide possible lattice extrapolation procedures to finite chemical potential which are important to locate a critical endpoint in the QCD phase diagram.

hep-ph↗

arXiv2009

QCD thermodynamics with effective models

In this talk we extend the Polyakov-quark-meson model to N_f=2+1 quark flavors and study its bulk thermodynamics at finite temperatures in mean-field approximation. Three different Polyakov-loop potentials are considered. Our findings are confronted to recent QCD lattice simulations of the RBC-Bielefeld and HotQCD collaborations. Furthermore, the finite chemical potential expansion of the quark-number susceptibility in a Taylor series around vanishing chemical potential is analyzed. By means of a novel algorithmic differentiation technique, we have calculated Taylor coefficients up to 24th order in the model for the first time. This allows the systematic study of convergence properties of the Taylor series.

hep-ph↗

arXiv2008

On the QCD phase structure from effective models

Some recent theoretical developments of the QCD phase diagram are summarized. Chiral symmetry restoration and the confinement/deconfinement transition at nonzero temperature and quark densities are analyzed in the framework of an effective linear sigma model with three light quark flavors. The sensitivity of the chiral transition as well as the existence of a critical end point in the phase diagram on the value of the sigma mass is explored. The influence of the axial anomaly on the chiral critical surface is addressed. Finally, the modifications by the inclusion of the Polyakov loop on the phase structure are investigated.

hep-ph↗

arXiv2008

The three-flavor chiral phase structure in hot and dense QCD matter

Chiral symmetry restoration at nonzero temperature and quark densities are investigated in the framework of a linear sigma model with N_f=3 light quark flavors. After the derivation of the grand potential in mean-field approximation, the nonstrange and strange condensates, the in-medium masses of the scalar and pseudoscalar nonets are analyzed in hot and dense medium. The influence of the axial anomaly on the nonet masses and the isoscalar mixings on the pseudoscalar η-η' and scalar σ(600)-f_0(1370) complex are examined. The sensitivity of the chiral phase transition as well as the existence and location of a critical end point in the phase diagram on the value of the sigma mass is explored. The chiral critical surface with and without the influence of the axial U(1)_A anomaly is elaborated as a function of the pion and kaon masses for several values of the sigma mass.

hep-ph↗

arXiv2008

Hyperon-nucleon single-particle potentials with low-momentum interactions

Single-particle potentials in Hartree-Fock approximation for different hyperon-nucleon (YN) channels are calculated in the framework of the effective low-momentum YN interaction V_lowk. In contrast to the nucleon-nucleon interaction, the available experimental data for the YN interaction are scarce. As a consequence no unique YN low-momentum potential V_lowk can be predicted from the various bare potentials. The resulting momentum- and density-dependent single-particle potentials for several different bare OBE models and for chiral effective field theory are compared to each other.

nucl-th↗

arXiv2007

Critical structure of the QCD medium

Fluctuations in the vicinity of a phase transition are important but neglected in mean-field theory. In order to assess the influence of such fluctuations on the critical endpoint and the size of the critical region in the QCD phase diagram, a mean-field calculation of a two-flavor quark-meson model is compared with a renormalization group approach. However, due to the lack of confinement in this effective model the equation of state near the chiral phase transition is still unrealistic. A first improvement of this model can be achieved by coupling quark degrees of freedom to the Polyakov loop, consequently incorporating certain aspects of confinement. The influence of these modifications on the resulting phase diagram is discussed.

hep-ph↗

arXiv2007

The Phase Structure of the Polyakov--Quark-Meson Model

The relation between the deconfinement and chiral phase transition is explored in the framework of an Polyakov-loop-extended two-flavor quark-meson (PQM) model. In this model the Polyakov loop dynamics is represented by a background temporal gauge field which also couples to the quarks. As a novelty an explicit quark chemical potential and N_f-dependence in the Polyakov loop potential is proposed by using renormalization group arguments. The behavior of the Polyakov loop as well as the chiral condensate as function of temperature and quark chemical potential is obtained by minimizing the grand canonical thermodynamic potential of the system. The effect of the Polyakov loop dynamics on the chiral phase diagram and on several thermodynamic bulk quantities is presented.

hep-ph↗

arXiv2006

Renormalization Group Approach towards the QCD Phase Diagram

The idea of the functional renormalization group and one-loop improved renormalization group flows are reviewed. The associated flow equations and nonperturbative approximations schemes for its solutions are discussed. These techniques are then applied to the strong interaction in the framework of an effective quark meson model which is introduced in great detail. The renormalization group analysis of the two flavor quark meson model is extended to finite temperature and quark chemical potential which allows for an analysis of the chiral phase diagram beyond the mean field approximation.

hep-ph↗

arXiv2006

Susceptibilities near the QCD (tri)critical point

Based on the proper-time renormalization group approach, the scalar and the quark number susceptibilities in the vicinity of possible critical end points of the hadronic phase diagram are investigated in the two-flavor quark-meson model. After discussing the quark-mass dependence of the location of such points, the critical behavior of the in-medium meson masses and quark number density are calculated. The universality classes of the end points are determined by calculating the critical exponents of the susceptibilities. In order to numerically estimate the influence of fluctuations we compare all quantities with results from a mean-field approximation. It is concluded that the region in the phase diagram where the susceptibilities are enhanced is more compressed around the critical end point if fluctuations are included.

hep-ph↗

arXiv2005

The Phase Diagram of the Quark-Meson Model

Within the proper-time renormalization group approach, the chiral phase diagram of a two-flavor quark-meson model is studied. In the chiral limit, the location of the tricritical point which is linked to a Gaussian fixed point, is determined. For quark chemical potentials smaller than the tricritical one the second-order phase transition belongs to the O(4) universality class. For temperatures below the tricritical one we find initially a weak first-order phase transition which is commonly seen in model studies and also in recent lattice simulations. In addition, below temperatures of T <~ 17 MeV we find two phase transitions. The chiral restoration transition is initially also of first-order but turns into a second-order transition again. This leads to the possibility that there may be a "second tricritical" point in the QCD phase diagram in the chiral limit.

nucl-th↗

arXiv1994

How sharp is the chiral crossover phenomenon for realistic meson masses?

The mass dependence of the chiral phase transition is studied in the linear $SU(3)\times SU(3)$ sigma-model to leading order in a $1/N_f$-expansion, $N_f$ denoting the number of flavours. For realistic meson masses we find a smooth crossover between $T\sim181.5$ to 192.6~[MeV]. The crossover looks more rapid in the light quark condensate than in thermodynamic quantities like the energy and entropy densities. The change in the light quark condensate in this temperature interval is $\sim$~50\% of the zero-temperature condensate value, while the entropy density increases by ($5.5\pm0.8)\cdot10^{-3}$~[GeV$^3$]. Since the numerical error is particularly large in this region, we cannot rule out a finite latent heat smaller than 0.2~[GeV/fm$^3$]. The chiral transition is washed out for an average pseudoscalar meson octet mass of 203~[MeV]. This gives an upper bound on the first-order transition region in the meson mass parameter space. The corresponding ratio of critical to realistic light current quark masses $m^{crit}_{u,d}/m_{u,d}$ is estimated as $0.26\pm0.08$. This result is by an order of magnitude larger than the corresponding mean-field value. Therefore the

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