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J. Wambach

Publications and source records attributed to J. Wambach.

89 records · Page 5Linked to original sources
arXiv1995

Medium Modifications of the Rho Meson at CERN/SPS Energies

Rho meson propagation in hot hadronic matter is studied in a model with coupling to $ππ$ states. Medium modifications are induced by a change of the pion dispersion relation through collisions with nucleons and $Δ's$ in the fireball. Maintaining gauge invariance dilepton production is calculated and compared to the recent data of the CERES collaboration in central S+Au collisions at 200 GeV/u. The observed enhancement of the rate below the rho meson mass can be largely accounted for.

hep-ph↗
arXiv1995

Chirally Constraining the $ππ$ Interaction in Nuclear Matter

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.

nucl-th↗
arXiv1995

Thermal Properties of a Hot Pion Gas beyond the Quasiparticle Approximation

Within the Matsubara formalism we derive expressions for the pion self-energy and the two-pion propagator in a hot pion gas. These quantities are used to selfconsistently calculate the in-medium $ππ$ amplitude beyond the quasiparticle approximation (QPA). The results are shown to differ significantly from QPA-based calculations. We also examine the impact of chiral constraints on the $ππ$ interaction in a chirally improved version of the Jülich $ππ$ model.

nucl-th↗
arXiv1994

Double-Dipole Excitations in 40Ca

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 spreading width cannot be drawn, as yet, due to computational limitations in the number of 2p2h states.

nucl-th↗
arXiv1994

Where the Nuclear Pions are

Three recent experiments, which looked at pionic effects in nuclei have concluded that there are no excess pions. This puts into serious question the conventional meson-exchange picture of the nucleon-nucleon interaction. Based on arguments of partial restoration of chiral symmetry with density we propose a resolution to this problem.

nucl-th↗
arXiv1994

Phase-Space Exploration in Nuclear Giant Resonance Decay

The rate of phase-space exploration in the decay of isovector and isoscalar giant quadrupole resonances in $^{40}$Ca is analyzed. The study is based on the time dependence of the survival probability and of the spectrum of generalized entropies evaluated in the space of 1p-1h and 2p-2h states. If the 2p-2h background shows the characteristics typical for chaotic systems, the isovector excitation evolves almost statistically while the isoscalar excitation remains largely localized, even though it penetrates the whole available phase space.

nucl-th↗
arXiv1994

Temperature Dependence of the Chiral Condensate from an Interacting Pion Gas

By exploiting the fact that the chiral condensate is related to the derivative of the free-energy density with respect to the bare quark mass we calculate the temperature dependence of the condensate ratio $\ave{\bar qq}_T/\ave{\bar qq}_0$ from an iteracting pion gas. When using the Weinberg Lagrangian at the Hartree level we find a depression of the condensate with temperature. For $T < 100$ MeV the results are in good agreement with chiral perturbation theory in the three loop approximation. Near $T_c$, however, there are marked differences due to non-perturbative nature of our approach.

nucl-th↗
arXiv1994

S-Wave Pion-Pion Correlations in Dense Nuclear Matter

The s-wave correlated $π$-$π$ 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 $Δ$'s. Two groups of models for the $π$-$π$ 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 $π$-$π$ pair condensation. These effects are strongly moderated or disappear altogether if constraints from chiral symmetry are built into the $π$-$π$ interaction.

nucl-th↗
arXiv1994

Theoretical Interpretation of the NE18 Experiment on Nuclear Transparency in A(e,e'p) Scattering

The spectral function, measured in $A(e,e'p)$ reactions, is distorted by the final-state interaction of the struck proton with the residual nucleus. This causes a broadening of the observed transverse-momentum distribution which is large even in the $d(e,e'p)$ reaction. We discuss the effects of this $p_{\perp}$-broadening on the nuclear transparency measured in the recent NE18 experiment. Within conventional Glauber theory we can describe the measurements. Transparency effects are thus small in agreement with our earlier predictions.

nucl-th↗
arXiv1993

Chaos Driven Decay of Nuclear Giant Resonances: Route to Quantum Self-Organization

The influence of background states with increasing level of complexity on the strength distribution of the isoscalar and isovector giant quadrupole resonance in $^{40}$Ca is studied. It is found that the background characteristics, typical for chaotic systems, strongly affects the fluctuation properties of the strength distribution. In particular, the small components of the wave function obey a scaling law analogous to self-organized systems at the critical state. This appears to be consistent with the Porter-Thomas distribution of the transition strength.

nucl-th↗
arXiv1993

Pion Properties in a Hot $ΠN Δ$ Gas

Based on a recent meson-exchange model for the vacuum $ππ$ interaction we compute selfconsistently the in-medium $ππ$ scattering amplitude and pion selfenergy in a hot $πNΔ$ gas. The contributions to the pion selfenergy are calculated from the $ππ$ T-matrix as well as from p-wave interaction with nucleons and thermally abundant $Δ$'s. Results are presented for two scenarios believed to be realized in the relativistic heavy ion collisions performed at the GSI-SIS and the CERN-SpS. Possible implications for the observed soft pion enhancement at both SIS and SpS are indicated.

nucl-th↗
arXiv1993

Selfconsistent Description of a Thermal Piongas

We examine a hot pion gas by including medium modifications of the two-body scattering amplitude as well as mean-field effects selfconsistently. In contrast to earlier calculations, the in-medium T-matrix is rather close to the free one while the mean-field potential agrees well with lowest-order estimates. We also discuss the validity of the quasiparticle approximation. It is found that it is reliable for temperatures up to $\sim 150$ MeV. Above this temperature off-shell effects in the pion selfenergy become important, especially if the pions are strongly out of chemical equilibrium.

nucl-th↗
arXiv1993

Correlation Effects in the Final-State Interaction for Quasielastic $(e,e'p)$ Scattering

Color transparency predicts that, in $(e,e'p)$ reactions at large $Q^2$, the final-state interaction becomes weaker than the reference value predicted from the free-nucleon cross section. This reference value is usually evaluated in the dilute-gas approximation to Glauber's multiple-scattering theory. We derive the leading-order correction taking into account two-body correlations. Large cancellations are found so that the overall correlation effect is small.

nucl-th↗
arXiv1993

The Onset of Color Transparency in $(e,e'p)$ Reactions on Nuclei

Quantum filtering of the ejectile wave packet from hard $ep$ scattering on bound nucleons puts stringent constraints on the onset of color transparency in $(e,e'p)$ 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 terms of a color transparency sum rule.

nucl-th↗
arXiv1993

M1 Resonances in Unstable Magic Nuclei

Within a microscopic approach which takes into account RPA configurations, the single-particle continuum and more complex $1p1h\otimes phonon$ configurations isoscalar and isovector M1 excitations for the unstable nuclei ${56,78}$Ni and ${100,132}$Sn are calculated. For comparison, the experimentally known M1 excitations in ${40}$Ca and $^{208}$Pb have also been calculated. In the latter nuclei good agreement in the centroid energy, the total transition strength and the resonance width is obtained. With the same parameters we predict the magnetic excitations for the unstable nuclei. The strength is sufficiently concentrated to be measurable in radioactive beam experiments. New features are found for the very neutron rich nucleus ${78}$Ni and the neutron deficient nucleus ${100}$Sn.

nucl-th↗
arXiv1993

Modeling Complex Nuclear Spectra - Regularity versus Chaos

A statistical analysis of the spectrum of two particle - two hole doorway states in a finite nucleus is performed. On the unperturbed mean-field level sizable attractive correlations are present in such a spectrum. Including particle-hole rescattering effects via the residual interaction introduces repulsive dynamical correlations which generate the fluctuation properties characteristic of the Gaussian Orthogonal Ensemble. This signals that the underlying dynamics becomes chaotic. This feature turns out to be independent of the detailed form of the residual interaction and hence reflects the generic nature of the fluctuations studied.

nucl-th↗