M. Buballa

Technical University Darmstadt, Darmstadt, Hesse, Germany

Are you M. Buballa?

Claim your profile

Publications (84)255.31 Total impact

  • Source
    Michael Buballa · Stefano Carignano
    [Show abstract] [Hide abstract]
    ABSTRACT: An increasing number of model results suggests that chiral symmetry is broken inhomogeneously in a certain window at intermediate densities in the QCD phase diagram. This could have significant effects on the properties of compact stars, possibly leading to new astrophysical signatures. In this contribution we discuss this idea by reviewing recent results on inhomogeneous chiral symmetry breaking under an astrophysics-oriented perspective. After introducing two commonly studied spatial modulations of the chiral condensate, the chiral density wave and the real kink crystal, we focus on their properties and their effect on the equation of state of quark matter. We also describe how these crystalline phases are affected by different elements which are required for a realistic description of a compact star, such as charge neutrality, the presence of magnetic fields, vector interactions and the interplay with color-superconductivity. Finally, we discuss possible signatures of inhomogeneous chiral symmetry breaking in the core of compact stars, considering the cases of mass-radius relations and neutrino emissivity explicitly.
    Preview · Article · Aug 2015
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We investigate the effects of isospin asymmetry on inhomogeneous chiral symmetry breaking phases within the two-flavor NJL model. After introducing a plane-wave ansatz for each quark-flavor condensate, we find that, as long as their periodicities are enforced to be equal, a non-zero isospin chemical potential shrinks the size of the inhomogeneous phase. The asymmetry reached in charge neutral matter is nevertheless not excessively large, so that an inhomogeneous window is still present in the phase diagram. Lifting the constraint of equal periodicities alters the picture significantly, as the inhomogeneous phase survives in a much larger region of the phase diagram.
    Full-text · Article · Jun 2015
  • Source
    Federico Marquez · Aftab Ahmad · Michael Buballa · Alfredo Raya
    [Show abstract] [Hide abstract]
    ABSTRACT: We study the beahviour of the dual quark condensate $\Sigma_1$ in the Nambu-Jona-Lasinio (NJL) model and its nonlocal variant. In quantum chromodynamics $\Sigma_1$ can be realted to the breaking of the center symmetry and is therefore an (approximate) order parameter of confinement. The deconfinement transition is then signaled by a strong rise of $\Sigma_1$ as a function of temperature. However, a similar behaviour is also seen in the NJL model, which is known to have no confinement. Indeed, it was shown that in this model the rise of $\Sigma_1$ is triggered by the chiral phase transition. In order to shed more light on this issue, we calculate $\Sigma_1$ for several variants of the NJL model, some of which have been suggested to be confining. Switching between "confining" and "non-confining" models and parametrizations we find no qualitative difference in the behaviour of $\Sigma_1$, namely, it always rises in the region of the chiral phase transition. We conclude that without having established a relation to the center symmetry in a given model, $\Sigma_1$ should not blindly be regarded as an order parameter of confinement.
    Full-text · Article · Apr 2015 · Physics Letters B
  • Source
    D. Blaschke · A. Dubinin · M. Buballa
    [Show abstract] [Hide abstract]
    ABSTRACT: A quark-meson-diquark plasma is considered within the PNJL model for dynamical chiral symmetry breaking and restoration in quark matter. Based on a generalized Beth-Uhlenbeck approach to mesons and diquarks we present the thermodynamics of this system including the Mott dissociation of mesons and diquarks at finite temperature. A striking result is the suppression of the diquark abundance below the chiral restoration temperature by the coupling to the Polyakov loop, because of their color degree of freedom. This is understood in close analogy to the suppression of quark distributions by the same mechanism. Mesons as color singlets are unaffected by the Polyakov-loop suppression. At temperatures above the chiral restoration mesons and diquarks are both suppressed due to the Mott effect, whereby the positive resonance contribution to the pressure is largely compensated by the negative scattering contribution in accordance with the Levinson theorem.
    Full-text · Article · Dec 2014 · Physical Review D
  • Source
    Michael Buballa · Stefano Carignano
    [Show abstract] [Hide abstract]
    ABSTRACT: The chiral condensate, which is constant in vacuum, may become spatially modulated at moderately high densities and temperatures, where in the traditional picture of the QCD phase diagram a first-order chiral phase transition occurs. We review the current status of this idea, which originally dates back to Migdal's pion condensation, but recently received new momentum by studies on the nature of the chiral critical point and by the conjecture of a quarkyonic-matter phase. We discuss how these nonuniform phases emerge in generalized Ginzburg-Landau analyses as well as in specific calculations, both within effective models and in Dyson-Schwinger or large-$N_c$ approaches to QCD. Questions about the most favored shape of the modulations and in particular its dimension, and about the effects of nonzero isospin chemical potential, strange quarks, color superconductivity, and external magnetic fields, will be adressed as well.
    Full-text · Article · Jun 2014 · Progress in Particle and Nuclear Physics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Inhomogeneous chiral-symmetry breaking phases at non-vanishing chemical potential and temperature are studied within a two-flavor quark-meson model in the chiral limit. The analysis is performed beyond the standard mean-field approximation by taking into account the Dirac-sea contributions of the quarks. Compared with the case where the Dirac sea is neglected, we find that the inhomogeneous phase shrinks, but in general does not disappear. It is shown within a Ginzburg-Landau analysis that the Lifshitz point of the inhomogeneous phase coincides with the tricritical point if the ratio between sigma-meson and constituent quark mass in vacuum is chosen to be $m_\sigma/M = 2$, corresponding to the fixed mass ratio in the Nambu--Jona-Lasinio model. In the present model, however, this ratio can be varied, offering the possibility to separate the two points. This is confirmed by our numerical calculations, which demonstrate a strong sensitivity of the size of the inhomogeneous phase on $m_\sigma$. Finally, we uncover a general instability of the model with respect to large wave numbers of the chiral modulations, which calls for further improvements beyond the present approximation.
    Full-text · Article · Mar 2014 · Physical Review D
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The recent measurement of two solar mass pulsars has initiated an intense discussion on its impact on our understanding of the high-density matter in the cores of neutron stars. A task force meeting was held from October 7-10, 2013 at the Frankfurt Institute for Advanced Studies to address the presence of quark matter in these massive stars. During this meeting, the recent oservational astrophysical data and heavy-ion data was reviewed. The possibility of pure quark stars, hybrid stars and the nature of the QCD phase transition were discussed and their observational signals delineated.
    Full-text · Article · Feb 2014 · Journal of Physics G Nuclear and Particle Physics
  • Source
    Daniel Müller · Michael Buballa · Jochen Wambach
    [Show abstract] [Hide abstract]
    ABSTRACT: 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.
    Full-text · Article · Aug 2013 · Physics Letters B
  • Source
    D. Blaschke · D. Zablocki · M. Buballa · A. Dubinin · G. Roepke
    [Show abstract] [Hide abstract]
    ABSTRACT: An important first step in the program of hadronization of chiral quark models is the bosonization in meson and diquark channels. This procedure is presented at finite temperatures and chemical potentials for the SU(2) flavor case of the NJL model with special emphasis on the mixing between scalar meson and scalar diquark modes which occurs in the 2SC color superconducting phase. The thermodynamic potential is given the Beth-Uhlenbeck form which allows a detailed discussion of bound state dissociation in hot, dense matter (Mott effect) in terms of the in-medium scattering phase shift of two-particle correlations. It is shown for the case without meson-diquark mixing that the phase shift can be separated into a continuum and a resonance part. In the latter, the Mott transition manifests itself by a change of the phase shift by \pi\ in accordance with the Levinson theorem, when a bound state transforms to a resonance in the scattering continuum. The question of a selfconsistent quasiparticle picture is discussed as a preparation for the second step in the hadronization program: the introduction of baryons and "integrating out" the diquark fields.
    Preview · Article · May 2013 · Annals of Physics
  • Source
    Daniel Müller · Michael Buballa · Jochen Wambach
    [Show abstract] [Hide abstract]
    ABSTRACT: 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.
    Full-text · Article · Mar 2013 · European Physical Journal A
  • Source
    Michael Buballa · Stefano Carignano
    [Show abstract] [Hide abstract]
    ABSTRACT: The existence and the properties of self-bound quark matter in the NJL model at zero temperature are investigated in mean-field approximation, focusing on inhomogeneous structures with one-dimensional spatial modulations. It is found that the most stable homogeneous solutions which have previously been interpreted as schematic quark droplets are unstable against formation of a one-dimensional soliton-antisoliton lattice. The solitons repel each other, so that the minimal energy per quark is realized in the single-soliton limit. The properties of the solitons and their interactions are discussed in detail, and the effect of vector interactions is estimated. The results may be relevant for the dynamics of expanding quark matter.
    Preview · Article · Oct 2012 · Physical review D: Particles and fields
  • Stefano Carignano · Michael Buballa
    [Show abstract] [Hide abstract]
    ABSTRACT: We investigate the phase structure of the Nambu-Jona-Lasinio model at zero temperature, allowing for a two-dimensional spatial dependence of the chiral condensate. Applying the mean-field approximation, we consider various periodic structures with rectangular and hexagonal geometries, and minimize the corresponding free energy. We find that these two-dimensional chiral crystals are favored over homogeneous phases in a certain window in the region where the phase transition would take place when the analysis was restricted to homogeneous condensates. It turns out, however, that in this regime they are disfavored against a phase with a one-dimensional modulation of the chiral condensate. On the other hand, we find that square and hexagonal lattices eventually get favored at higher chemical potentials. Although stretching the limits of the model to some extent, this would support predictions from quarkyonic-matter studies.
    No preview · Article · Oct 2012 · Physical review D: Particles and fields
  • Source
    D. S. Zablocki · D. Blaschke · M. Buballa
    [Show abstract] [Hide abstract]
    ABSTRACT: We study the Gaussian fluctuations of the two-flavor, meson-diquark bosonized NJL model for two and three colors at the color superconducting phase transition. The analysis is based on analytical properties of the polarization matrix. Pionic fluctuations are shown to be stabilised in the 2SC phase compared to the two-color result where they are right on threshold.
    Preview · Article · Jul 2012 · Physics of Atomic Nuclei
  • Source
    Sanjin Benić · David Blaschke · Michael Buballa
    [Show abstract] [Hide abstract]
    ABSTRACT: We show that the CJT thermodynamic potential of dynamical quark models with a quark propagator represented by complex conjugate mass poles inevitably exhibits thermodynamic instabilities. We find that the minimal coupling of the quark sector to a Polyakov loop potential can strongly suppress but not completely remove such instabilities. This general effect is explicitly demonstrated in the framework of a covariant, chirally symmetric, effective quark model.
    Preview · Article · Jun 2012 · Physical review D: Particles and fields
  • Source
    M. Buballa · S. Carignano · D. Nickel
    [Show abstract] [Hide abstract]
    ABSTRACT: We investigate inhomogeneous chiral symmetry breaking phases in the phase diagram of the two-flavor Nambu-Jona-Lasinio model, concentrating on phases with one-dimensional modulations. It is found that the first-order transition line in the phase diagram of homogeneous phases gets completely covered by an inhomogeneous phase which is bordered by second-order transition lines. The inhomogeneous phase turns out to be remarkably stable when vector interactions are included.
    Preview · Article · Jun 2012 · Physics of Atomic Nuclei
  • [Show abstract] [Hide abstract]
    ABSTRACT: A nonlocal chiral quark model is consistently extended beyond mean field using a strict 1/N c expansion scheme. It is found that the 1/N c corrections lead to a lowering of the temperature of the chiral phase transition in comparison with the mean-field result. On the other hand, near the phase transition the 1/N c expansion breaks down and a nonperturbative scheme for the inclusion of mesonic correlations is needed in order to describe the phase transition region.
    No preview · Article · Jun 2012 · Physics of Atomic Nuclei
  • M. Buballa · K. Heckmann · J. Wambach
    [Show abstract] [Hide abstract]
    ABSTRACT: The properties of strongly interacting matter are probed in ultra-relativistic heavy-ion collisions. In the context of measurements of the elliptic flow at RHIC and the LHC the shear viscosity is of particular interest. In this presentation we discuss recent results for η/sη/s in hadronic matter at vanishing baryo-chemical potential within kinetic theory. Using the Nambu Jona-Lasinio model, special attention is paid to effects arising from the restoration of spontaneously broken chiral symmetry with increasing temperature.
    No preview · Article · Apr 2012 · Progress in Particle and Nuclear Physics
  • Source
    Stefano Carignano · Michael Buballa
    [Show abstract] [Hide abstract]
    ABSTRACT: We investigate the phase structure of the Nambu--Jona-Lasinio model at zero temperature, allowing for a two-dimensional spatial dependence of the chiral condensate. Applying the mean-field approximation, we consider various periodic structures with rectangular and hexagonal geometries, and minimize the corresponding free energy. We find that these two-dimensional chiral crystals are favored over homogeneous phases in a certain window in the region where the phase transition would take place when the analysis was restricted to homogeneous condensates. It turns out, however, that in this regime they are disfavored against a phase with a one-dimensional modulation of the chiral condensate. On the other hand, we find that square and hexagonal lattices eventually get favored at higher chemical potentials. Although stretching the limits of the model to some extent, this would support predictions from quarkyonic-matter studies.
    Preview · Article · Mar 2012
  • Source
    Klaus Heckmann · Michael Buballa · Jochen Wambach
    [Show abstract] [Hide abstract]
    ABSTRACT: 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.
    Full-text · Article · Feb 2012 · European Physical Journal A
  • Source
    J. Wambach · K. Heckmann · M. Buballa
    [Show abstract] [Hide abstract]
    ABSTRACT: The properties of strong-interaction matter are probed in ultra-relativistic heavy-ion collisions. In the context of measurements of the elliptic flow at RHIC and the LHC the shear viscosity is of particular interest. In this presentation we discuss recent results for eta/s in hadronic matter at vanishing baryo-chemical potential within kinetic theory. Using the Nambu-Jona-Lasinio model, special attention is paid to effects arising from the restoration of spontaneously broken chiral symmetry with increasing temperature.
    Full-text · Article · Nov 2011

Publication Stats

3k Citations
255.31 Total Impact Points

Institutions

  • 2004-2015
    • Technical University Darmstadt
      • Institute of Nuclear Physics
      Darmstadt, Hesse, Germany
  • 2002
    • GSI Helmholtzzentrum für Schwerionenforschung
      Darmstadt, Hesse, Germany
  • 1996
    • Stony Brook University
      Stony Brook, New York, United States
  • 1994-1996
    • State University of New York
      New York City, New York, United States
  • 1991
    • Forschungszentrum Jülich
      • Institut für Kernphysik (IKP)
      Düren, North Rhine-Westphalia, Germany