K. K. Szabo

Forschungszentrum Jülich, Jülich, North Rhine-Westphalia, Germany

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Publications (98)318.75 Total impact

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    ABSTRACT: Axions are one of the most attractive dark matter candidates. The evolution of their number density in the early universe can be determined by calculating the topological susceptibility $\chi(T)$ of QCD as a function of the temperature. Lattice QCD provides an ab initio technique to carry out such a calculation. A full result needs two ingredients: physical quark masses and a controlled continuum extrapolation from non-vanishing to zero lattice spacings. We determine $\chi(T)$ in the quenched framework (infinitely large quark masses) and extrapolate its values to the continuum limit. The results are compared with the prediction of the dilute instanton gas approximation (DIGA). A nice agreement is found for the temperature dependence, whereas the overall normalization of the DIGA result still differs from the non-perturbative continuum extrapolated lattice results by a factor of order ten. We discuss the consequences of our findings for the prediction of the amount of axion dark matter.
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    ABSTRACT: We present the crossover line between the quark gluon plasma and the hadron gas phases for small real chemical potentials. First we determine the effect of imaginary values of the chemical potential on the transition temperature using lattice QCD simulations. Then we use various formulas to perform an analytic continuation to real values of the baryo-chemical potential. Our data set maintains strangeness neutrality to match the conditions of heavy ion physics. The systematic errors are under control up to $\mu_B\approx 300$ MeV. For the curvature of the transition line we find that there is an approximate agreement between values from three different observables: the chiral susceptibility, chiral condensate and strange quark susceptibility. The continuum extrapolation is based on $N_t=$ 10, 12 and 16 lattices. By combining the analysis for these three observables we find, for the curvature, the value $\kappa = 0.0149 \pm 0.0021$.
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    ABSTRACT: We calculate second- and fourth-order cumulants of conserved charges in a temperature range stretching from the QCD transition region towards the realm of (resummed) perturbation theory. We perform lattice simulations with staggered quarks; the continuum extrapolation is based on $N_t=10\dots24$ in the crossover-region and $N_t=8\dots16$ at higher temperatures. We find that the Hadron Resonance Gas model predictions describe the lattice data rather well in the confined phase. At high temperatures (above $\sim$250 MeV) we find agreement with the three-loop Hard Thermal Loop results.
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    ABSTRACT: Die Feinabstimmung physikalischer Größen ist entscheidend für die Struktur des Universums, die Entstehung von Materie und letztlich auch des Lebens. Eine dieser Größen ist die Massendifferenz von Proton und Neutron. Unser Team theoretischer Physiker konnte nun mit Hilfe der Gitter-Quantenchromodynamik zeigen, dass sich diese Massendifferenz auf die Kombination von Massen und Ladungen der Quarks zurückführen lässt.
    Physik in unserer Zeit 07/2015; 46(4). DOI:10.1002/piuz.201590064
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    ABSTRACT: Electromagnetic effects are increasingly being accounted for in lattice quantum chromodynamics computations. Because of their long-range nature, they lead to large finite-size effects over which it is important to gain analytical control. Nonrelativistic effective field theories provide an efficient tool to describe these effects. Here we argue that some care has to be taken when applying these methods to quantum electrodynamics in a finite volume.
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    ABSTRACT: We present preliminary results of a 2+1-flavor study of finite-volume effects in the lattice QCD computation of the leading-order hadronic contribution to the muon anomalous magnetic moment. We also present methods for obtaining directly the invariant hadronic polarization function, $\Pi(Q^2)$, and the Adler function at all discrete lattice values of $Q^2$, including $Q^2=0$. Results are obtained with HEX-smeared clover fermions.
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    ABSTRACT: We study the correlators of Polyakov loops, and the corresponding gauge invariant free energy of a static quark-antiquark pair in 2+1 flavor QCD at finite temperature. Our simulations were carried out on $N_t$ = 6, 8, 10, 12, 16 lattices using Symanzik improved gauge action and a stout improved staggered action with physical quark masses. The free energies calculated from the Polyakov loop correlators are extrapolated to the continuum limit. For the free energies we use a two step renormalization procedure that only uses data at finite temperature. We also measure correlators with definite Euclidean time reversal and charge conjugation symmetry to extract two different screening masses, one in the magnetic, and one in the electric sector, to distinguish two different correlation lengths in the full Polyakov loop correlator.
    Journal of High Energy Physics 01/2015; 2015(4). DOI:10.1007/JHEP04(2015)138 · 6.22 Impact Factor
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    ABSTRACT: We perform a detailed, fully-correlated study of the chiral behavior of the pion mass and decay constant, based on 2+1 flavor lattice QCD simulations. These calculations are implemented using tree-level, O(a)-improved Wilson fermions, at four values of the lattice spacing down to 0.054 fm and all the way down to below the physical value of the pion mass. They allow a sharp comparison with the predictions of SU(2) chiral perturbation theory (\chi PT) and a determination of some of its low energy constants. In particular, we systematically explore the range of applicability of NLO SU(2) \chi PT in two different expansions: the first in quark mass (x-expansion), and the second in pion mass (\xi-expansion). We find that these expansions begin showing signs of failure around M_\pi=300 MeV for the typical percent-level precision of our N_f=2+1 lattice results. We further determine the LO low energy constants (LECs), F=88.0 \pm 1.3\pm 0.3 and B^\msbar(2 GeV)=2.58 \pm 0.07 \pm 0.02 GeV, and the related quark condensate, \Sigma^\msbar(2 GeV)=(271\pm 4\pm 1 MeV)^3, as well as the NLO ones, l_3=2.5 \pm 0.5 \pm 0.4 and l_4=3.8 \pm 0.4 \pm 0.2, with fully controlled uncertainties. We also explore the NNLO expansions and the values of NNLO LECs. In addition, we show that the lattice results favor the presence of chiral logarithms. We further demonstrate how the absence of lattice results with pion masses below 200 MeV can lead to misleading results and conclusions. Our calculations allow a fully controlled, ab initio determination of the pion decay constant with a total 1% error, which is in excellent agreement with experiment.
    Physical Review D 12/2014; DOI:10.1103/PhysRevD.90.114504 · 4.86 Impact Factor
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    ABSTRACT: We report on a continuum extrapolated result (arXiv:1309.5258) for the equation of state (EoS) of QCD with $N_f=2+1$ dynamical quark flavors and discuss preliminary results obtained with an additional dynamical charm quark ($N_f=2+1+1$). For all our final results, the systematics are controlled, quark masses are set to their physical values, and the continuum limit is taken using at least three lattice spacings corresponding to temporal extents up to $N_t=16$.
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    ABSTRACT: Finite temperature charmonium spectral functions in the pseudoscalar(PS) and vector(V) channels are studied in lattice QCD with 2+1 flavours of dynamical Wilson quarks, on fine isotropic lattices (with a lattice spacing of 0.057fm), with a non-physical pion mass of 545MeV. The highest temperature studied is approximately 1.4Tc. Up to this temperature no significant variation of the spectral function is seen in the PS channel. The V channel shows some temperature dependence, which seems to be consistent with a temperature dependent low frequency peak related to heavy quark transport, plus a temperature independent term at omega > 0. These results are in accord with previous calculations using the quenched approximation.
  • S Borsanyi · Z Fodor · S D Katz · S Krieg · C Ratti · K K Szabo
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    ABSTRACT: We present our latest results for fluctuations of electric charge and baryon number, simulated on the lattice in a system of 2+1 dynamical quark flavors at the physical quark masses and continuum extrapolated. In order to extract the chemical freeze-out temperature and chemical potential, we compare our results to the moments of multiplicity distribution of the corresponding conserved charges, measured in heavy ion collision experiments by the STAR collaboration. Consistency between the freeze-out parameters obtained through different conserved charges is discussed.
    Journal of Physics Conference Series 09/2014; 535(1):012030. DOI:10.1088/1742-6596/535/1/012030
  • C. Ratti · S. Borsanyi · Z. Fodor · S.D. Katz · S. Krieg · K.K. Szabo
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    ABSTRACT: The latest results on fluctuations of electric charge and baryon number, simulated on the lattice by the Wuppertal–Budapest collaboration, are compared to the moments of multiplicity distribution of the corresponding conserved charges, measured in heavy ion collision experiments by the STAR collaboration. The purpose of this study is to extract the chemical freeze-out parameters (temperature and chemical potential) as a function of the collision energy, from first principles. Consistency between the freeze-out parameters obtained through the two different conserved charges used in the analysis is discussed.
    Nuclear Physics A 08/2014; 931. DOI:10.1016/j.nuclphysa.2014.08.014 · 2.50 Impact Factor
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    ABSTRACT: The existence and stability of atoms rely on the fact that neutrons are more massive than protons. The measured mass difference is only 0.14% of the average of the two masses. A slightly smaller or larger value would have led to a dramatically different universe. Here, we show that this difference results from the competition between electromagnetic and mass isospin breaking effects. We performed lattice quantum-chromodynamics and quantum-electrodynamics computations with four nondegenerate Wilson fermion flavors and computed the neutron-proton mass-splitting with an accuracy of 300 kilo-electron volts, which is greater than 0 by 5 standard deviations. We also determine the splittings in the Σ, Ξ, D, and Ξcc isospin multiplets, exceeding in some cases the precision of experimental measurements. Copyright © 2015, American Association for the Advancement of Science.
    Science 06/2014; 347(6229). DOI:10.1126/science.1257050 · 31.48 Impact Factor
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    S. Borsanyi · Z. Fodor · S. D. Katz · S. Krieg · C. Ratti · K. K. Szabo
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    ABSTRACT: Recent results for moments of multiplicity distributions of net-protons and net-electric charge from the STAR collaboration are compared to lattice QCD results for higher order fluctuations of baryon number and electric charge by the Wuppertal-Budapest collaboration, with the purpose of extracting the freeze-out temperature and chemical potential. All lattice simulations are performed at the physical mass for light and strange quarks; all results are continuum extrapolated. We show that it is possible to extract an upper value for the freeze-out temperature, as well as precise baryo-chemical potential values corresponding to the four highest collision energies of the experimental beam energy scan. Consistency between the freeze-out parameters obtained from baryon number and electric charge fluctuations is found. The freeze-out chemical potentials are now in agreement with the statistical hadronization model.
    Physical Review Letters 03/2014; 113(5). DOI:10.1103/PhysRevLett.113.052301 · 7.51 Impact Factor
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    ABSTRACT: Finite temperature charmonium spectral functions in the pseudoscalar and vector channels are studied in lattice QCD with 2+1 flavours of dynamical Wilson quarks, on fine isotropic lattices (with a lattice spacing of 0.057 fm), with a non-physical pion mass of $m_{\pi} \approx$ 545 MeV. The highest temperature studied is approximately $1.4 T_c$. Up to this temperature no significant variation of the spectral function is seen in the pseudoscalar channel. The vector channel shows some temperature dependence, which seems to be consistent with a temperature dependent low frequency peak related to heavy quark transport, plus a temperature independent term at \omega>0. These results are in accord with previous calculations using the quenched approximation.
    Journal of High Energy Physics 01/2014; 2014(4). DOI:10.1007/JHEP04(2014)132 · 6.22 Impact Factor
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    Kalman Szabo
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    ABSTRACT: A status of lattice QCD thermodynamics, as of 2013, is summarized. Only bulk thermodynamics is considered. There is a separate section on magnetic fields.
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    ABSTRACT: We report on a continuum extrapolated result [arXiv:1309.5258] for the equation of state (EoS) of QCD with $N_f=2+1$ dynamical quark flavors. In this study, all systematics are controlled, quark masses are set to their physical values, and the continuum limit is taken using at least three lattice spacings corresponding to temporal extents up to $N_t=16$. A Symanzik improved gauge and stout-link improved staggered fermion action is used. Our results are available online [ancillary file to arXiv:1309.5258].
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    S. Borsanyi · Z. Fodor · S. D. Katz · S. Krieg · C. Ratti · K. K. Szabo
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    ABSTRACT: We present continuum extrapolated lattice results for the higher order fluctuations of conserved charges in high temperature Quantum Chromodynamics. Through the matching of the grand canonical ensemble on the lattice to the net charge and net baryon distribution realized in heavy ion experiments the temperature and the chemical potential may be estimated at the time of chemical freeze-out
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    ABSTRACT: We present preliminary lattice results for the leading-order hadronic contribution to the muon anomalous magnetic moment, calculated with HEX-smeared clover fermions. In our calculation we include 2+1-flavor ensembles with pions at the physical mass.
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    ABSTRACT: We present a full result for the 2+1 flavor QCD equation of state. All the systematics are controlled, the quark masses are set to their physical values, and the continuum extrapolation is carried out. This extends our previous studies [JHEP 0601:089 (2006); 1011:077 (2010)] to even finer lattices and now includes ensembles with Nt = 6,8,10,12 up to Nt = 16. We use a Symanzik improved gauge and a stout-link improved staggered fermion action. Our findings confirm our earlier results. In order to facilitate the direct use of our equation of state we make our tabulated results available for download.
    Physics Letters B 09/2013; 730. DOI:10.1016/j.physletb.2014.01.007 · 6.02 Impact Factor

Publication Stats

3k Citations
318.75 Total Impact Points

Institutions

  • 2015
    • Forschungszentrum Jülich
      • Jülich Supercomputing Centre (JSC)
      Jülich, North Rhine-Westphalia, Germany
  • 2004–2015
    • Bergische Universität Wuppertal
      • Physical and Theoretical Chemistry
      Wuppertal, North Rhine-Westphalia, Germany
  • 2013
    • Universität Regensburg
      Ratisbon, Bavaria, Germany
  • 2002–2004
    • Eötvös Loránd University
      • Department of Theoretical Physics
      Budapeŝto, Budapest, Hungary