B. Kämpfer

An Effective model of the quark - gluon plasma with thermal parton masses

Recent “numerical experiments” provide thermodynamical quantities of pure SU(3) gauge theory in lattice approximation.1 These data, corrected at least partially for finite lattice effects, are thermodynamically selfconsistent,2 i.e., energy density and pressure are related via e = T∂p/∂T - p, and exhibit a striking difference from the popular bag model parametrization p = e/3 - 4B/3 (which would imply for the energy density and the pressure a temperature dependence e, p ∝ T 4).

The Equation of state of deconfined matter at finite chemical potential in a quasiparticle description

A quasiparticle description of the thermodynamics of deconfined matter, reproducing both the perturbative limit and nonperturbative lattice QCD data at finite temperature, is generalized to finite chemical potential. By a flow equation resulting from Maxwells relation, the equation of state is extended from zero to non-zero quark densities. The impact of the massive strange flavor is considered and implications for cold, charge-neutral deconfined matter in

Origin of the low-mass electron pair excess in light nucleus-nucleus collisions

\beta

Di-electron bremsstrahlung in intermediate-energy pn and Dp collisions

-equilibrium in compact stars are given.

Evaluation of QCD sum rules for light vector mesons at finite density and temperature

Abstract We report measurements of electron pair production in elementary p + p and d + p reactions at 1.25 GeV / u with the HADES spectrometer. For the first time, the electron pairs were reconstructed for n + p reactions by detecting the proton spectator from the deuteron breakup. We find that the yield of electron pairs with invariant mass M e + e − > 0.15 GeV / c 2 is about an order of magnitude larger in n + p reactions as compared to p + p . A comparison to model calculations demonstrates that the production mechanism is not sufficiently described yet. The electron pair spectra measured in C + C reactions are compatible with a superposition of elementary n + p and p + p collisions, leaving little room for additional electron pair sources in such light collision systems.

QCD sum rules for D and B mesons in nuclear matter

Abstract Invariant mass spectra of di-electrons stemming from bremsstrahlung processes are calculated in a covariant diagrammatical approach for the exclusive reaction D p → p sp n p e + e − with detection of a forward spectator proton, p sp . We employ an effective nucleon–meson theory for parameterizing the subreaction n p → n p e + e − and, within the Bethe–Salpeter formalism, derive a factorization of the cross section in the form d σ D p → p sp n p e + e − / d M = d σ n p → n p e + e − / d M × kinematical factor related solely to the deuteron (M is the e + e − invariant mass). The effective nucleon–meson interactions, including the exchange mesons π, σ, ω and ρ as well as excitation and radiative decay of Δ ( 1232 ) , have been adjusted to the process p p → p p e + e − at energies below the vector meson production threshold. At higher energies, contributions from ω and ρ meson excitations are analyzed in both, NN and Dp collisions. A relation to two-step models is discussed. Subthreshold di-electron production in Dp collisions at low spectator momenta is investigated as well. Calculations have been performed for kinematical conditions envisaged for forthcoming experiments at HADES.

Family of equations of state based on lattice QCD: Impact on flow in ultrarelativistic heavy-ion collisions

Abstract:QCD sum rules are evaluated at finite nucleon densities and temperatures to determine the change of mass parameters for the lightest vector mesons ρ, ω and φ in a strongly interacting medium. For conditions relevant for the starting experiments at HADES we find that the in-medium mass shifts of the ρ- and ω-mesons are governed, within the Borel QCD sum rule approach, by the density and temperature dependence of the four-quark condensate. In particular, the variation of the strength of the density dependence of the four-quark condensate reflects directly the decreasing mass of the ρ-meson and can lead to a change of the sign of the ω-meson mass shift as a function of the density. In contrast, the in-medium mass of the φ-meson is directly related to the chiral strange quark condensate which seems correspondingly accessible.

The QCD equation of state near Tc within a quasi-particle model

QCD sum rules for D and B mesons embedded in cold nuclear matter are evaluated. We quantify the mass splitting of D-D and B-B mesons as a function of the nuclear matter density; extrapolated to saturation density it is in the order of 60 and 130 MeV, respectively, driven essentially by the condensates , , and . The poorly known condensate may leave room for an even larger D-D mass splitting. The genuine chiral condensate , amplified by heavy-quark masses, enters the Borel transformed sum rules for the mass splitting beyond linear density dependence. The inclusion of strange quark condensates reveals a numerically smaller and opposite effect for the D{sub s}-D{sub s} mass splitting.

Is there a unique thermal source of dileptons in Pb(158 A·GeV) + Au, Pb reactions?

We construct a family of equations of state within a quasiparticle model by relating pressure, energy density, baryon density, and susceptibilities adjusted to first-principles lattice QCD calculations. The relation between pressure and energy density from lattice QCD is surprisingly insensitive to details of the simulations. Effects from different lattice actions, quark masses, and lattice spacings used in the simulations show up mostly in the quark-hadron phase transition region, which we bridge over by a set of interpolations to a hadron resonance gas equation of state. Within our optimized quasiparticle model we then examine the equation of state along isentropic expansion trajectories at small net baryon densities, as relevant for experiments and hydrodynamic simulations at RHIC and LHC energies. We illustrate its impact on azimuthal flow anisotropies and on the transverse momentum spectra of various hadron species.

In-medium spectral change of ω mesons as a probe of QCD four-quark condensate

Abstract We present a description of the equation of state of strongly interacting matter within a quasi-particle model. The model is adjusted to lattice QCD data near the deconfinement temperature T c . We compare in detail the excess pressure at non-vanishing chemical potential and its Taylor expansion coefficients with two-flavor lattice QCD calculations and outline prospects of the extrapolation to large baryon density.