H. J. Pirner

P-wave polarization of the ϱ-meson and the dilepton spectrum in dense matter

We study the p-wave polarization operator of the ϱ-meson due to ϱN interactions via the N∗(1720) and Δ(1905) resonances and compute the corresponding production rate for e+e− pairs at finite temperature and baryon density. At high baryon density we find a significant shift of the spectrum to lower invariant masses.

Vector meson leptoproduction and nonperturbative gluon fluctuations in QCD

We present a nonperturbative QCD calculation of diffractive vector meson production in virtual photon nucleon scattering at high energy. We use the nonperturbative model of the stochastic QCD vacuum which yields linear confinement and makes specific predictions for the dependence of high-energy scattering cross sections on the hadron size. Using light cone wave functions of the photon and vector mesons, we calculate electroproduction cross sections for {rho}, {omega}, {phi}, and J/{psi}. We emphasize the behavior of specific observables such as the ratio of longitudinal to transverse production cross section and the t dependence of the differential cross section. {copyright} {ital 1997} {ital The American Physical Society}

The equation of state of quarks and mesons in a renormalization group flow picture

Abstract Non-perturbative flow equations within an effective linear sigma model coupled to constituent quarks for two quark flavours are derived and solved. A heat kernel regularization is employed for a renormalization group improved effective potential. We determine the initial values of the coupling constants in the effective potential at zero temperature. Solving the evolution equations with the same initial values at finite temperature in the chiral limit, we find a second-order phase transition at Tc≈150 MeV. Due to the smooth decoupling of massive modes, we can directly link the low-temperature four-dimensional theory to the three-dimensional high-temperature theory. We calculate the equation of state in the chiral limit and for finite pion masses and determine universal critical exponents.Nonperturbative flow equations within an effective linear sigma model coupled to constituent quarks for two quark flavors are derived and solved. A heat kernel regularization is employed for a renormalization group improved effective potential. We determine the initial values of the coupling constants in the effective potential at zero temperature. Solving the evolution equations with the same initial values at finite temperature in the chiral limit, we find a second order phase transition at T_c \approx 150 MeV. Due to the smooth decoupling of massive modes, we can directly link the low-temperature four-dimensional theory to the three-dimensional high-temperature theory. We calculate the equation of state in the chiral limit and for finite pion masses and determine universal critical exponents.

Confining QCD strings, Casimir scaling, and a Euclidean approach to high-energy scattering

We compute the chromo-field distributions of static color-dipoles in the fundamental and adjoint representation of SU(Nc) in the loop-loop correlation model and find Casimir scaling in agreement with recent lattice results. Our model combines perturbative gluon exchange with the non-perturbative stochastic vacuum model which leads to confinement of the color-charges in the dipole via a string of color-fields. We compute the energy stored in the confining string and use low-energy theorems to show consistency with the static quark-antiquark potential. We generalize Meggiolaros analytic continuation from parton-parton to gauge-invariant dipole-dipole scattering and obtain a Euclidean approach to high-energy scattering that allows us in principle to calculate S-matrix elements directly in lattice simulations of QCD. We apply this approach and compute the S-matrix element for high-energy dipole-dipole scattering with the presented Euclidean loop-loop correlation model. The result confirms the analytic continuation of the gluon field strength correlator used in all earlier applications of the stochastic vacuum model to high-energy scattering.

K+N scattering in the constituent quark model

Abstract We calculate S-wave phase shifts of the KN system in the framework of a nonrelativistic quark model, using variational methods (generator coordinates). We show that the I = 0 channel is determined by the QCD residual interaction, which leads to a small repulsion, and that the I = 1 channel receives strong repulsive contributions already from quark exchange. We discuss the relevance and physical origin of these results. We furthermore construct an approximate local potential extracted from the nonlocal cluster interaction.

Atomic mass dependence of hadron production in deep inelastic scattering on nuclei

Abstract Hadron production in lepton–nucleus deep inelastic scattering is studied in an absorption model. In the proposed model, the early stage of hadronization in the nuclear medium is dominated by prehadron formation and absorption, controlled by flavor-dependent formation lengths and absorption cross sections. Computations for hadron multiplicity ratios are presented and compared with the HERMES experimental data for pions, kaons, protons and antiprotons. The mass-number dependence of hadron attenuation is shown to be sensitive to the underlying hadronization dynamics. Contrary to common expectations for absorption models, a leading term proportional to A 2 / 3 is found. Deviations from the leading behavior arise at large mass-numbers and large hadron fractional momenta.

Convergence of the expansion of the renormalization group flow equation

We compare and discuss the dependence of a polynomial truncation of the effective potential used to solve an exact renormalization group flow equation for a model with a fermionic interaction (linear sigma model) with a grid solution. The sensitivity of the results to the underlying cutoff function is discussed. We explore the validity of the expansion method for second- and first-order phase transitions. (c) 2000 The American Physical Society.

Linking the quark meson model with QCD at high temperature

We model the transition of a system of quarks and gluons at high energies to a system of quarks and mesons at low energies in a consistent renormalization group approach. Flow equations interpolate between the physics of the high-temperature degrees of freedom and the low-temperature dynamics at a scale of 1 GeV. We also discuss the dependence of the equation of state on baryon density and compare our results with recent lattice gauge simulations.

Volume dependence of the pion mass in the quark-meson-model

We consider the quark-meson-model in a finite three-dimensional volume using the Schwinger proper-time renormalization group. We derive and solve the flow equations for finite volume in local potential approximation. In order to break chiral symmetry in the finite volume, we introduce a small current quark mass. The corresponding effective meson potential breaks chiral O(4) symmetry explicitly, depending on {sigma} and {pi}{yields} fields separately. We calculate the volume dependence of the pion mass and of the pion decay constant with the renormalization group flow equations and compare with recent results from chiral perturbation theory in a finite volume.

Transverse Momentum Broadening in Semi-inclusive DIS on Nuclei

Abstract Using a three stage model of hadron formation we calculate the change of the transverse momentum distribution of hadrons produced in semi-inclusive deep inelastic scattering (SIDIS) on nuclei. In the first stage after its interaction with the virtual photon, the struck quark propagates quasi free in the nuclear environment undergoing multiple collisions with nucleons. During this stage it can acquire transverse momentum. In the second stage a prehadron is formed which has a very small elastic cross section with the nucleons. In the third stage the prehadron turns into a hadron. For HERMES energies, prehadron elastic scatterings contribute little to p ⊥ -broadening. The acquired extra Δ p ⊥ 2 of hadrons can therefore be deduced entirely from the first stage of quasi free quark propagation with quark–nucleon collisions. We use this model to describe π -production on Ne, Kr, Xe and compare with the most recent HERMES data.