J. Wambach

Rho meson propagation and dilepton enhancement in hot hadronic matter

A realistic model for the free rho meson with coupling to two-pion states is employed to calculate the rho propagator in a hot and dense hadron gas. The medium modifications are based on hadronic rescattering processes: intermediate two-pion states are renormalized through interactions with surrounding nucleons and deltas, and rho meson scattering is considered off nucleons, deltas, pions and kaons. Constraints from gauge invariance as well as the full off-shell dynamics of the interactions are accounted for. Within the vector dominance model we apply the resulting in-medium rho spectral function to compute e+e− production rates from π+π− annihilation. The calculation of corresponding e+e− spectra as recently measured in central collisions of heavy-ions at CERN/SpS energies gives reasonable agreement with the experimental data.

Quasiparticle interactions in neutron matter for applications in neutron stars

Abstract A microscopic model for the quasiparticle interaction in neutron matter is presented. Both particle-particle (pp) and particle-hole (ph) correlations are included. The pp correlations are treated in a semi-empirical way, while ph correlations are incorporated by solving coupled two-body equations for the particle-hole interaction and the scattering amplitude on the Fermi sphere. The resulting integral equations self-consistently sum the ph reducible diagrams. Antisymmetry is kept at all stages and hence the forward-scattering sum rules for the scattering amplitude are obeyed. Results for Landau parameters and transport coefficients in a density regime representing the crust of a neutron star are presented. We also estimate the 1 S 0 gap parameter for neutron superfluidity and comment briefly on neutron-star implications.

Effective interactions and superfluid energy gaps for low density neutron matter

Abstract We use a polarization potential approach to calculate properties of neutron matter at sub-nuclear densities with particular attention to the superfluid pairing interaction. Our model divides the quasi-particle scattering amplitude at the Fermi surface into two distinct parts: a direct particle-hole irreducible piece and an induced particle-hole reducible contribution. The direct part is constructed from realistic two-nucleon potentials. Short-range correlations are taken into account phenomenologically by modifying the potentials at short distances. The induced contribution is generated as a self-consistent summation of particle-hole reducible diagrams. The resulting quasi-particle scattering amplitude is fully antisymmetric for all moment 0⩽ q ⩽2 k F . Realistic bounds on the s-wave pairing gap are given and consequences for the dynamics of rotating neutron stars are briefly discussed.

S-Wave ππ Correlations in Dense Nuclear Matter

The s-wave correlated �-� strength distribution in cold nuclear matter is investigated. The in-medium single pions are renormalized by means of the usual p-wave couplings to nucleons and �’s. Two groups of models for the �-� interaction are studied. The first one uses phenomenological models which reproduce the phase shifts but do not respect the soft-pion theorems. With increasing density a strong accumulation of strength in the subhreshold region is observed. Slightly above saturation density there even occurs an instability with respect to s-wave �-�pair condensation. These effects are strongly moderated or disappear altogether if constraints from chiral symmetry are built into the �-� interaction.

RPA ground state correlations in nuclei

Abstract Overcounting in the RPA theory of ground state correlations is shown to be avoided if exact rather than quasiboson commutators are used. Single particle occupation probabilities are formulated in a compact way by the RPA Green function. Calculations with large configuration spaces and realistic interactions are performed with 1p1h RPA and second RPA (SRPA) including 2p2h mixing in excited states. In 41 Ca valence hole states are found to be quenched by about 10% in RPA and up to 18% in SRPA. Contributions from low and high lying excitations and their relation to long and short range correlations in finite nuclei are investigated.

Chirally constraining the ππ interaction in nuclear matter

Abstract A general prescription for the construction of ππ interaction potentials which preserve scattering length constraints from chiral symmetry when iterated in scattering equations is derived. The prescription involves only minor modifications of typical meson-exchange models, so that coupling constants and cut-off masses in the models are not greatly affected. Calculations of s-wave ππ scattering amplitudes in nuclear matter for two models are compared with those for similar models which violate the chiral constraint. While the prescription tends to suppress the accumulation of the near sub-threshold strength of the ππ interaction, an earlier conjecture that amplitudes which satisfy chiral constraints will not exhibit an instability towards ππ s-wave pair condensation appears to be incorrect. At the same time, however, conventional ππ interaction models which fit scattering data well can readily be adjusted to avoid the instability in nuclear matter without recourse to exotic mechanisms.

Double-dipole excitations in 40Ca

Abstract The double-dipole strength distribution in 40 Ca is calculated microscopically within a model space of 1p1h- and 2p2h excitations. Anharmonic effects in the centroid energies of the 0 + - and 2 + components are found to be small, in agreement with experimental observation. Firm conclusions about the width cannot be drawn, as yet, due to computational limitations in the number of 2p2h states.

Quasiparticle properties of protons in 208Pb

Abstract Motivated by recent (e, e′p) data in 208Pb a quasiparticle hamiltonian which includes correlations in a phenomenological way is used to evaluate the energy distributionof quasiparticle pole-strength, effective masses and shell-model occupation numbers for protons in this nucleus. Differences between standard Woods-Saxon and quasiparticle radial wavefunctions are also pointed out.

Investigation of the hadronic transition operator for inelastic proton scattering

Abstract The local part of the hadronic transition operator for inelastic proton scattering is calculated and compared with the electromagnetic transition operator. It is found that there is quite a large difference between the radial shapes of the operators as well as between the transition rates caused by them. The applicability of the electromagnetic sum rules to inelastic proton scattering is discussed.

The onset of color transparency in (e,ép) reactions on nuclei

Abstract Quantum filtering of the ejectile wave packet from hard ep scattering on bound nucleons puts stringent constraints on the onset of color transparency in (e,ep) reactions in nuclei at moderate energies. Based on multiple-scattering theory, we derive a novel formula for nuclear transparency and discuss its energy dependence in a realistic model. With increasing energy a slow onset is found.