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H. van Hees

Publications and source records attributed to H. van Hees.

45 records · Page 3Linked to original sources
arXiv2005

Thermalization of Heavy Quarks in the Quark-Gluon Plasma

Charm- and bottom-quark rescattering in a Quark-Gluon Plasma (QGP) is investigated with the objective of assessing the approach towards thermalization. Employing a Fokker-Planck equation to approximate the collision integral of the Boltzmann equation we augment earlier studies based on perturbative parton cross sections by introducing resonant heavy-light quark interactions. The latter are motivated by recent QCD lattice calculations which indicate the presence of "hadronic" states in the QGP. We model these states by colorless (pseudo-) scalar and (axial-) vector D- and B-mesons within a heavy-quark effective theory framework. We find that the presence of these states at moderate QGP temperatures substantially accelerates the kinetic equilibration of c-quarks as compared to using perturbative interactions. We also comment on consequences for $D$-meson observables in ultra-relativistic heavy-ion collisions.

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arXiv2005

The Delta(1232) at RHIC

We investigate properties of the Delta(1232) and nucleon spectral functions at finite temperature and baryon density within a hadronic model. The medium modifications of the Delta consist of a renormalization of its pion-nucleon cloud and resonant pi-Delta scattering. Underlying coupling constants and form factors are determined by the elastic π-N scattering phase shift in the isobar channel, as well as empirical partial decay widths of excited baryon resonances. For cold nuclear matter the model provides reasonable agreement with photoabsorption data on nuclei in the Delta-resonance region. In hot hadronic matter typical for late stages of central Au-Au collisions at RHIC we find the Delta-spectral function to be broadened by $\sim$65 MeV together with a slight upward mass shift of 5-10 MeV, in qualitative agreement with preliminary data from the STAR collaboration.

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arXiv2003

The renormalizability for massive Abelian gauge field theories re-visited

We give a simplified proof for the perturbative renormalizability of theories with massive vector particles. For renormalizability it is sufficient that the vector particle is treated as an gauge field, corresponding to an Abelian gauge group. Contrary to the non-Abelian case one does not need the Higgs mechanism to create the appropriate mass terms. The proof uses ``Stueckelberg's trick'' and the Ward-Takahashi identities from local Abelian gauge invariance. The simplification is due to the fact that, again contrary to the non-Abelian case, no BRST analysis is needed.

hep-th↗
arXiv2002

Renormalization of self-consistent $Φ$-derivable approximations

Within finite temperature field theory, we show that truncated non-perturbative self-consistent Dyson resummation schemes can be renormalized with local vacuum counterterms. For this the theory has to be renormalizable in the usual sense and the self-consistent scheme must follow Baym's Phi-derivable concept. Our BPHZ-renormalization scheme leads to renormalized self-consistent equations of motion. At the same time the corresponding 2PI-generating functional and the thermodynamic potential can be renormalized with the same counterterms used for the equations of motion. This guarantees the standard Phi-derivable properties like thermodynamic consistency and exact conservation laws also for the renormalized approximation schemes. We give also a short overview over symmetry properties of the various functions defined within the 2PI scheme for the case that the underlying classical field theory has a global linearly realized symmetry.

hep-ph↗
arXiv2002

Renormalization in Self-Consistent Approximation schemes at Finite Temperature III: Global Symmetries

We investigate the symmetry properties for Baym's $Φ$-derivable schemes. We show that in general the solutions of the dynamical equations of motion, derived from approximations of the $Φ$-functional, do not fulfill the Ward-Takahashi identities of the symmetry of the underlying classical action, although the conservation laws for the expectation values of the corresponding Noether currents are fulfilled exactly for the approximation. Further we prove that one can define an effective action functional in terms of the self-consistent propagators which is invariant under the operation of the same symmetry group representation as the classical action. The requirements for this theorem to hold true are the same as for perturbative approximations: The symmetry has to be realized linearly on the fields and it must be free of anomalies, i.e., there should exist a symmetry conserving regularization scheme. In addition, if the theory is renormalizable in Dyson's narrow sense, it can be renormalized with counter terms which do not violate the symmetry.

hep-ph↗
arXiv2002

Renormalization of Self-consistent Approximation schemes Finite Temperature II: Applications to the Sunset Diagram

The theoretical concepts for the renormalization of self-consistent Dyson resummations, deviced in the first paper of this series, are applied to first example cases for the $ϕ^4$-theory. Besides the tadpole (Hartree) approximation as a novel part the numerical solutions are presented which includes the sunset self-energy diagram into the self-consistent scheme based on the $Φ$-derivable approximation or 2PI effective action concept.

hep-ph↗
arXiv2000

Finite pion width effects on the rho--meson

We study the influence of the finite damping width of pions on the in-medium properties of the rho-meson in an interacting meson gas model at finite temperature. Using vector dominance also implications on the resulting dilepton spectra from the decay of the rho-meson are presented. A set of coupled Dyson equations with self energies up to the sunset diagram level is solved self consistently. Following a Phi-derivable scheme the self energies are dynamically determined by the self consistent propagators. Some problems concerning the self consistent treatment of vector or gauge bosons on the propagator level, in particular, if coupled to currents arising from particles with a sizable damping width, are discussed.

hep-ph↗
arXiv2000

Soft Modes, Resonances and Quantum Transport

Effects of the propagation of particles, which have a finite life-time and an according width in their mass spectrum, are discussed in the context of transport description. First, the importance of coherence effects (Landau-Pomeranchuk-Migdal effect) on production and absorption of field quanta in non-equilibrium dense matter is considered. It is shown that classical diffusion and Langevin results correspond to re-summation of certain field-theory diagrams formulated in terms of full non-equilibrium Green's functions. Then the general properties of broad resonances in dense and hot systems are discussed in the framework of a self-consistent and conserving Phi-derivable method of Baym at the examples of the rho-meson in hadronic matter and the pion in dilute nuclear matter. Further we address the problem of a transport description that properly accounts for the damping width of the particles. The Phi-derivable method generalized to the real-time contour provides a self-consistent and conserving kinetic scheme. We derive a generalized expression for the non-equilibrium kinetic entropy flow, which includes corrections from fluctuations and mass-width effects. In special cases an H-theorem is proved. Memory effects in collision terms give contributions to the kinetic entropy flow that in the Fermi-liquid case recover the famous bosonic type T^3 ln T correction to the specific heat of liquid Helium-3. At the example of the pion-condensate phase transition in dense nuclear matter we demonstrate important part played by the width effects within the quantum transport.

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