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Frithjof Karsch

Publications and source records attributed to Frithjof Karsch.

At least 73 records · Page 4Linked to original sources
arXiv2011

Universal properties of 3d O(4) symmetric models: The scaling function of the free energy density and its derivatives

We present direct representations of the scaling functions of the 3d O(4) model which are relevant for comparisons to other models, in particular QCD. This is done in terms of expansions in the scaling variable z=t/h^{1/βδ}. The expansions around z=0 and the corresponding asymptotic ones for z --> +/- infty, overlap such that no interpolation is needed. We explicitly present the expansion coefficients which have been determined numerically from data of a previous high statistics simulation of the O(4) model on a three-dimensional lattice of linear extension L=120. This allows to derive smooth representations of the first three derivatives of the scaling function of the free energy density, which determine universal properties of up to sixth order cumulants of net charge fluctuations in QCD.

hep-lat↗
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

Has Tc been measured by heavy ion experiments?

We discuss the role of cumulants of net baryon number fluctuations in the analysis of critical behavior in QCD and the study of freeze-out conditions in heavy ion experiments. Through the comparison of the current set of measurements of higher order cumulants of net baryon number fluctuations with lattice QCD calculations and results from hadron resonance gas model we can learn to what extent freeze-out as, determined by such cumulants, occurs close to the QCD transition temperature and thus can probe critical behavior at small values of the baryon chemical potential. Understanding how the relation between freeze-out conditions and the QCD crossover transition is reflected in properties of the experimentally determined cumulants is an important prerequisite to search for the QCD critical point. We point out that even if perfect continuum extrapolated lattice QCD results would be available, it would be inappropriate to use these observables to extract the value of the QCD transition temperature at vanishing baryon chemical potential from experimental data. We furthermore provide indications that a recently performed comparison of lattice QCD results on cumulants with data from heavy ion experiments suffer from systematic as well as statistical uncertainties in the lattice QCD calculations. This makes such comparison of lattice QCD calculations with experimental data at present not useful.

hep-ph↗
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

Continuum extrapolation of finite temperature meson correlation functions in quenched lattice QCD

We explore the continuum limit $a\rightarrow 0$ of meson correlation functions at finite temperature. In detail we analyze finite volume and lattice cut-off effects in view of possible consequences for continuum physics. We perform calculations on quenched gauge configurations using the clover improved Wilson fermion action. We present and discuss simulations on isotropic $N_σ^3\times 16$ lattices with $N_σ=32,48,64,128$ and $128^3 \times N_τ$ lattices with $N_τ=16,24,32,48$ corresponding to lattice spacings in the range of $0.01 fm \lsim a \lsim\ 0.031 fm$ at $T\simeq1.45T_c$. Continuum limit extrapolations of vector meson and pseudo scalar correlators are performed and their large distance expansion in terms of thermal moments is introduced. We discuss consequences of this analysis for the calculation of the electrical conductivity of the QGP at this temperature.

hep-lat↗
arXiv2010

Probing freeze-out conditions in heavy ion collisions with moments of charge fluctuations

We calculate the first four moments of baryon number, electric charge and strangeness fluctuations within the hadron resonance gas model. Different moments and their ratios as well as skewness and kurtosis are evaluated on the phenomenologically determined freeze-out curve in the temperature, baryon chemical potential plane. The model results and its predictions as well as relations between different moments are compared with the first data on net proton fluctuations in Au-Au collisions obtained at RHIC by the STAR Collaboration. We find good agreement between the model calculations and experimental results. We also point out that higher order moments should be more sensitive to critical behavior and will also distinguish hadron resonance gas model calculations from results obtained from lattice QCD.

hep-ph↗
arXiv2010

O(N) universality and the chiral phase transition in QCD

We discuss universal scaling properties of (2+1)-flavor QCD in the vicinity of the chiral phase transition at vanishing as well as non-vanishing light quark chemical potential (mu_l). We provide evidence for O(N) scaling of the chiral order parameter in (2+1)-flavor QCD and show that the scaling analysis of its derivative with respect to the light quark chemical potential provides a unique approach to the determination of the curvature of the chiral phase transition line in the vicinity of mu_l/T=0.

hep-lat↗
arXiv2009

Spectral Properties of Quarks at Finite Temperature in Lattice QCD

We analyze the quark spectral function above and below the critical temperature for deconfinement and at finite momentum in quenched lattice QCD. It is found that the temporal quark correlation function in the deconfined phase near the critical temperature is well reproduced by a two-pole ansatz for the spectral function. The bare quark mass and momentum dependences of the spectral function are analyzed with this ansatz. In the chiral limit we find that even near the critical temperature the quark spectral function has two collective modes corresponding to the normal and plasmino excitations in the high temperature (T) limit. The pole mass of these modes at zero momentum, which should be identified to be the thermal mass of the quark, is approximately proportional to T in a rather wide range of T in the deconfined phase.

hep-lat↗
arXiv2009

Quark propagator at finite temperature and finite momentum in quenched lattice QCD

We present an analysis of the quark spectral function above and below the critical temperature for deconfinement performed at zero and non-zero momentum in quenched lattice QCD using clover improved Wilson fermions in Landau gauge. It is found that the temporal quark correlation function in the deconfined phase near the critical temperature is well reproduced by a two-pole ansatz for the spectral function. This indicates that excitation modes of the quark field have small decay rates. The bare quark mass and momentum dependence of the spectral function is analyzed with this ansatz. In the chiral limit we find that the quark spectral function has two collective modes corresponding to the normal and plasmino excitations in the high temperature limit. Over a rather wide temperature range in the deconfined phase the pole mass of these modes at zero momentum, which corresponds to the thermal mass of the quark, is approximately proportional to temperature. With increasing bare quark masses the plasmino mode gradually disappears and the spectral function is dominated by a single pole. We also discuss quasi-particle properties of heavy quarks in the deconfined phase. In the confined phase, it is found that the pole ansatz for the spectral function fails completely.

hep-lat↗
arXiv2008

Fluctuations of Goldstone modes and the chiral transition in QCD

We provide evidence for the influence of thermal fluctuations of Goldstone modes on the chiral condensate at finite temperature. We show that at fixed temperature, T<Tc, in the vicinity of the chiral transition temperature this leads to a characteristic dependence of the chiral condensate on the square root of the light quark mass (m_l), which is expected for 3-dimensional models with broken O(N) symmetry. As a consequence the chiral susceptibility shows a strong quark mass dependence for all temperatures below Tc and diverges like 1/sqrt(m_l) in the chiral limit.

hep-lat↗
arXiv2008

Calculating Quark Number Susceptibilities with Domain-Wall Fermions

We present results from calculations of different quark number and hadronic susceptibilities on 2+1-flavor dynamical domain wall ensembles. We find that the iso-spin and electric charge susceptibilities are especially well suited to determine the transition temperature, as these quantities show only small statistical errors. Moreover, the transition values of the coupling obtained from iso-spin and electrical charge susceptibilities are in good agreement with the one obtained from the chiral condensate.

hep-lat↗
arXiv2008

Equation of state and more from lattice regularized QCD

We present results from a calculation of the QCD equation of state with two light (up, down) and one heavier (strange) quark mass performed on lattices with three different values of the lattice cut-off. We show that also on the finest lattice analyzed by us observables sensitive to deconfinement and chiral symmetry restoration, respectively, vary most rapidly in the same temperature regime.

hep-lat↗
arXiv2007

Recent lattice results on finite temperature and density QCD, part I

We discuss recent progress made studies of bulk thermodynamics of strongly interacting matter through lattice simulations of QCD with an almost physical light and strange quark mass spectrum. We present results on the QCD equation of state at vanishing and non-vanishing quark chemical potential and show first results on baryon number and strangeness fluctuations, which might be measured in event-by-event fluctuations in low energy runs at RHIC as well as at FAIR.

hep-lat↗
arXiv2007

Recent lattice results on finite temerature and density QCD, part II

We discuss recent progress in studies of QCD thermodynamics with almost physical light quark masses and a physical value of the strange quark mass. We summarize results on the transition temperature in QCD and analyze the relation between deconfinement and chiral symmetry restoration.

hep-lat↗
arXiv2007

Spectral properties of quarks above T_c in quenched lattice QCD

We analyze the quark spectral function above the critical temperature for deconfinement in quenched lattice QCD using clover improved Wilson fermions in Landau gauge. We show that the temporal quark correlator is well reproduced by a two-pole approximation for the spectral function and analyze the bare quark mass dependence of both poles as well as their residues. In the chiral limit we find that the quark spectral function has two collective modes which correspond to the normal and plasmino excitations. At large values of the bare quark mass the spectral function is dominated by a single pole.

hep-lat↗
arXiv2007

Spectral Properties of Quarks in the Quark-Gluon Plasma

We analyze the spectral properties of the quark propagator above the critical temperature for the deconfinement phase transition in quenched lattice QCD using clover improved Wilson fermions. The bare quark mass dependence of the quark spectral function is analyzed by varying the hopping parameter κin Landau gauge. We assume a two-pole structure for the quark spectral function, which is numerically found to work quite well for any value of κ. It is shown that in the chiral limit the quark spectral function has two collective modes that correspond to the normal and plasmino excitations, while it is dominated by a single-pole structure when the bare quark mass becomes large.

hep-lat↗
arXiv2007

Transition temperature in QCD with physical light and strange quark masses

We present results from a calculation of the transition temperature in QCD with two light and one heavier (strange) quark mass on lattices with temporal extent N_t =4 and 6. Calculations with improved staggered fermions have been performed with a strange quark mass fixed close to its physical value and for various light to strange quark mass ratios that correspond to light pseudo-scalar masses in the range (150-500) MeV. From a combined extrapolation to the chiral (m_l -> 0) and continuum (aT -> 0) limits we obtain for the transition temperature at the physical point T_c = 192(7)(4) MeV. We also present first results from an ongoing calculation of the QCD equation of state with almost realistic light and strange quark masses.

hep-ph↗