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Jochen Wambach

Publications and source records attributed to Jochen Wambach.

At least 37 records · Page 2Linked to original sources
arXiv2014

Spectral functions from the functional renormalization group at finite temperature and density

We present a viable method to obtain real-time quantities such as spectral functions or transport coefficients at finite temperature and density within a non-perturbative Functional Renormalization Group approach. Our method is based on a thermodynamically consistent truncation of the flow equations for 2-point functions with analytically continued frequency components in the originally Euclidean external momenta. We demonstrate its feasibility by calculating the mesonic spectral functions in the quark-meson model at different temperatures and quark chemical potentials, in particular around the critical endpoint in the phase diagram of the model.

hep-ph↗
arXiv2014

Spectral Functions from the Functional Renormalization Group

In this article we wish to present a new method to obtain spectral functions at finite temperature and density from the Functional Renormalization Group (FRG). The FRG offers a powerful non-perturbative tool to deal with phase transitions in strong-interaction matter under extreme conditions and their fluctuation properties. Based on a thermodynamically consistent truncation we derive flow equations for pertinent two-point functions in Minkowski space-time. We demonstrate the feasibility of the method by calculating mesonic spectral functions in hot and dense hadronic matter using the quark-meson model as a simple example.

hep-ph↗
arXiv2014

Real-Time Correlation Functions in the O(N) Model from the Functional Renormalization Group

In the framework of the functional renormalization group (FRG) we present a simple truncation scheme for the computation of real-time mesonic n-point functions, consistent with the derivative expansion of the effective action. Via analytic continuation on the level of the flow equations we perform calculations of mesonic spectral functions in the scalar O(N) model, which we use as an exploratory example. By investigating the renormalization-scale dependence of the 2-point functions we shed light on the nature of the sigma meson, whose spectral properties are predominantly of dynamical origin.

hep-ph↗
arXiv2013

Finite-Temperature Spectral Functions from the Functional Renormalization Group

We present a method to obtain spectral functions at finite temperature from the Functional Renormalization Group. Our method is based on a thermodynamically consistent truncation of the flow equations for 2-point functions with analytically continued frequency components in the originally Euclidean external momenta. For the uniqueness of this continuation at finite temperature we furthermore implement the physical Baym-Mermin boundary conditions. Results are presented for mesonic spectral functions obtained from a two-flavor quark-meson model.

hep-lat↗
arXiv2013

Dyson-Schwinger study of chiral density waves in QCD

The formation of inhomogeneous chiral condensates in QCD matter at nonzero density and temperature is investigated for the first time with Dyson-Schwinger equations. We consider two massless quark flavors in a so-called chiral density wave, where scalar and pseudoscalar quark condensates vary sinusoidally along one spatial dimension. We find that the inhomogeneous region covers the major part of the spinodal region of the first-order phase transition which is present when the analysis is restricted to homogeneous phases. The triple point where the inhomogeneous phase meets the homogeneous phases with broken and restored chiral symmetry, respectively, coincides, within numerical accuracy, with the critical point of the homogeneous calculation. At zero temperature, the inhomogeneous phase seems to extend to arbitrarily high chemical potentials, as long as pairing effects are not taken into account.

hep-ph↗
arXiv2013

Dyson-Schwinger approach to color superconductivity at finite temperature and density

We investigate the phases of dense QCD matter at finite temperature with Dyson-Schwinger equations for the quark propagator for N_f=2+1 flavors. For the gluon propagator we take a fit to quenched lattice data and add quark-loop effects perturbatively in a hard-thermal-loop--hard-dense-loop approximation. We consider 2SC and CFL-like pairing with chiral up and down quarks and massive strange quarks and present results for the condensates and the phase diagram. We find a dominant CFL phase at chemical potentials larger than 500-600 MeV. At lower values of the chemical potential we find a 2SC phase, which also exists in a small band at higher temperatures for larger chemical potentials. With values of 20-30 MeV, the critical temperatures to the normal phase turn out to be quite small.

hep-ph↗
arXiv2013

Neutron Star Matter

In this presentation I discuss two aspects of the neutron-matter equation of state. One relates to the symmetry energy of nuclear matter and empirical constraints on its slope parameter at saturation density. The second deals with spatially inhomogeneous chiral phases of deconfined quark matter in the inner core of a neutron star.

nucl-th↗
arXiv2012

Fluctuations in the quark-meson model for QCD with isospin chemical potential

We study the two-flavor quark-meson (QM) model with the functional renormalization group (FRG) to describe the effects of collective mesonic fluctuations on the phase diagram of QCD at finite baryon and isospin chemical potentials, $μ_B$ and $μ_I$. With only isospin chemical potential there is a precise equivalence between the competing dynamics of chiral versus pion condensation and that of collective mesonic and baryonic fluctuations in the quark-meson-diquark model for two-color QCD at finite baryon chemical potential. Here, finite $μ_B=3μ$ introduces an additional dimension to the phase diagram as compared to two-color QCD, however. At zero temperature, the ($μ_I,μ$)-plane of this phase diagram is strongly constrained by the "Silver Blaze problem." In particular, the onset of pion condensation must occur at $μ_I= m_π/2$, independent of $μ$ as long as $μ+ μ_I$ stays below the constituent quark mass of the QM model or the liquid-gas transition line of nuclear matter in QCD. In order to maintain this relation beyond mean field it is crucial to compute the pion mass from its timelike correlator with the FRG in a consistent way.

hep-ph↗
arXiv2012

Chiral restoration effects on the shear viscosity of a pion gas

We investigate the shear viscosity of a pion gas in relativistic kinetic theory, using the Nambu-Jona-Lasinio model to construct the pion mass and the pi-pi interaction at finite temperature. Whereas at low temperatures the scattering properties and, hence, the viscosity are in agreement with lowest-order chiral perturbation theory, we find strong medium modifications in the crossover region. Here the system is strongly coupled and the scattering lengths diverge, similarly as for ultra-cold Fermi gases at a Feshbach resonance. As a consequence, the ratio eta/s is found to be strongly reduced as compared to calculations without medium-modified masses and scattering amplitudes. However, the quantitative results are very sensitive to the details of the applied approximations.

hep-ph↗
arXiv2011

Towards finite density QCD with Taylor expansions

We analyze general convergence properties of the Taylor expansion of observables to finite chemical potential in the framework of an effective 2+1 flavor Polyakov-quark-meson model. To compute the required higher order coefficients a novel technique based on algorithmic differentiation has been developed. Results for thermodynamic observables as well as the phase structure obtained through the series expansion up to 24th order are compared to the full model solution at finite chemical potential. The available higher order coefficients also allow for resummations, e.g. Pade series, which improve the convergence behavior. In view of our results we discuss the prospects for locating the QCD phase boundary and a possible critical endpoint with the Taylor expansion method.

hep-lat↗
arXiv2011

Towards finite density QCD with Taylor expansions

Convergence properties of Taylor expansions of observables, which are also used in lattice QCD calculations at non-zero chemical potential, are analyzed in an effective N_f = 2+1 flavor Polyakov-quark-meson model. A recently developed algorithmic technique allows the calculation of higher-order Taylor expansion coefficients in functional approaches. This novel technique is for the first time applied to an effective N_f = 2+1 flavor Polyakov-quark-meson model and the findings are compared with the full model solution at finite densities. The results are used to discuss prospects for locating the QCD phase boundary and a possible critical endpoint in the phase diagram.

hep-ph↗
arXiv2011

Recent Theoretical Developments in the QCD Phase Diagram

In this talk I discuss three recent developments in the theoretical understanding of the phase diagram of the strong interaction. The first topic deals with the comparison of model calculations of the quark-hadron transition at vanishing quark chemical potential with state-of-the-art lattice QCD results. In the second relates to the size of a possible 'quarkyonic phase'. The third deals with the occurence of inhomogeneous chiral phases.

hep-ph↗
arXiv2010

The Quark Propagator in the NJL Model in a self-consistent 1/Nc Expansion

The quark propagator is calculated in the Nambu-Jona-Lasinio (NJL) model in a self-consistent 1/Nc-expansion at next-to-leading order. The calculations are carried out iteratively in Euclidean space. The chiral quark condensate and its dependence on temperature and chemical potential is calculated directly and compared with the mean-field results. In the chiral limit, we find a second-order phase transition at finite temperature and zero chemical potential, in agreement with universality arguments. At zero temperature and finite chemical potential, the phase transition is first order. In comparison with the mean-field results, the critical temperature and chemical potential are slightly reduced. We determine spectral functions from the Euclidean propagators by employing the Maximum-Entropy-Method (MEM). Thereby quark and meson masses are estimated and decay channels identified. For testing this method, we also apply it to evaluate perturbative spectral functions, which can be calculated directly in Minkowski space. In most cases we find that MEM is able to reproduce the rough features of the spectral functions, but not the details.

hep-ph↗
arXiv2010

Thermodynamics of (2+1)-flavor QCD: Confronting Models with Lattice Studies

The Polyakov-quark-meson (PQM) model, which combines chiral as well as deconfinement aspects of strongly interacting matter is introduced for three light quark flavors. An analysis of the chiral and deconfinement phase transition of the model and its thermodynamics at finite temperatures is given. Three different forms of the effective Polyakov loop potential are considered. The findings of the (2+1)-flavor model investigations are confronted to corresponding recent QCD lattice simulations of the RBC-Bielefeld, HotQCD and Wuppertal-Budapest collaborations. The influence of the heavier quark masses, which are used in the lattice calculations, is taken into account. In the transition region the bulk thermodynamics of the PQM model agrees well with the lattice data.

hep-ph↗
arXiv2010

On the appearance of hyperons in neutron stars

By employing a recently constructed hyperon-nucleon potential the equation of state of β-equilibrated and charge neutral nucleonic matter is calculated. The hyperon-nucleon potential is a low-momentum potential which is obtained within a renormalization group framework. Based on the Hartree-Fock approximation at zero temperature the densities at which hyperons appear in neutron stars are estimated. For several different bare hyperon-nucleon potentials and a wide range of nuclear matter parameters it is found that hyperons in neutron stars are always present. These findings have profound consequences for the mass and radius of neutron stars.

nucl-th↗
arXiv2009

Cosmic Matter under Extreme Conditions: CSQCD II Summary

After the first meeting in Copenhagen in 2001 QSQCD II is the second workshop in this series dealing with cosmic matter at very high density and its astrophysical implications. The aim is to bring together reseachers in the physics of compact stars, both theoretical and observational. Consequently a broad range of topics was presented, reviewing extremely energetic cosmological events and their relation to the high-density equation of state of strong-interaction matter. This summary elucidates recent progress in the field, as presented by the participants, and comments on pertinent questions for future developments.

astro-ph.SR↗
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↗