arXiv subjects
Publications and source records attributed to J. Wambach.
We extend hadronic models for rho-meson propagation in cold nuclear matter via coupling to in-medium pions to include finite three-momentum. Special care is taken to preserve gauge invariance. Consequences for photoabsorption on the proton and on nuclei as well as for the dilepton production in relativistic heavy-ion collisions are discussed.
The present status of the in-medium invariant $π-π$ mass distribution in the sigma-meson channel around the threshold is reviewed from the theory side and contrasted with recent experiments on $(π,2π)$ knock-out reactions. A preliminary investigation indicates that the strongly target-mass dependent invariant mass enhancement of the two pions can be explained theoretically. A more refined reaction theory is needed to confirm this result. In the theoretical description, based on the linear sigma model, emphasis is put on constraints from chiral symmetry.
It is discussed how chiral symmetry restoration manifests itself through mixing of vector and axial-vector correlators. The vector correlator is directly accessible in relativistic heavy-ion collisions. Within models of the vector correlator its implications for low-mass dilepton spectra are reviewed.
We discuss the similarities and differences for the theories of Rapp, Wambach and collaborators (called R/W in short) and those based on Brown-Rho scaling (called B/R), as applied to reproduce the dileptons measured by the CERES collaboration in the CERN experiments. In both theories the large number of dileptons at invariant masses $\sim$~$m_ρ/2$ are shown to be chiefly produced by a density-dependent $ρ$-meson mass. In R/W the medium dependence is dynamically calculated using hadronic variables defined in the matter-free vacuum. In B/R scaling it follows from movement towards chiral symmetry restoration due to medium-induced vacuum change, and is described in terms of constituent (or quasiparticle) quarks. We argue that the R/W description should be reliable up to densities somewhat beyond nuclear density, where hadrons are the effective variables. At higher density there should be a crossover to constituent quarks as effective variables scaling according to B/R. In the crossover region, the two descriptions must be ``dual''.
We present a dynamical study of $e^+e^-$ production in C + C and Ca + Ca collisions at BEVALAC/SIS energies on the basis of the covariant transport approach HSD employing momentum-dependent $ρ$-meson spectral functions that include the pion modifications in the nuclear medium as well as the polarization of the $ρ$-meson due to resonant $ρ- N$ scattering. We find that the experimental data from the DLS collaboration cannot be described within the $ρ$-meson spectral function approach. A dropping $η$-mass scenario leads to a good reproduction of the DLS dilepton data, however, violates the $m_T$-scaling of $π^0$ and $η$ spectra as observed by the TAPS collaboration as well as $η$ photoproduction on nuclei.
An overview is given of chiral symmetry restoration at finite temperature and baryochemical potential. Within hadronic models of the vector correlator its implications for low-mass dilepton spectra in ultrarelativistic heavy-ion collisions are discussed.
We present a dynamical study of $e^+e^-$ and $μ^+ μ^-$ production in proton-nucleus and nucleus-nucleus collisions at CERN-SPS energies on the basis of the covariant transport approach HSD employing a momentum-dependent $ρ$-meson spectral function that includes the pion modifications in the nuclear medium as well as the polarization of the $ρ$-meson due to resonant $ρ$$-N$ scattering. We find that the experimental data from the CERES and HELIOS-3 Collaborations can be described equally well as within the dropping $ρ$-mass scenario. Whereas corresponding dilepton $q_T$-spectra are found to be very similar, the inclusive dilepton yield in the invariant mass range $0.85 \leq M \leq 1.0$ GeV should allow to disentangle the two scenarios experimentally.
We present a non-perturbative method for the study of the O(N+1)-version of the linear sigma-model. Using boson-mapping techniques, in close analogy to those well-known for fermionic systems, we obtain a systematic 1/N-expansion for the Hamiltonian which is symmetry-conserving order by order. The leading order for the Hamiltonian is evaluated explicitly and we apply the method to $ππ$-scattering, in deriving the T-matrix to leading order.
Based on the formalism of thermo-field dynamics a new approach for studying collective excitations in hot finite Fermi systems is presented. Two approximations going beyond the thermal RPA namely renormalized thermal RPA and thermal second RPA are formulated.
A recently developed model for $ρ$-meson propagation in dense hadronic matter is applied to total photoabsorption cross sections in $γ$-proton and $γ$-nucleus reactions. Within the vector dominance model the photon coupling to the virtual pion cloud of the nucleon, two-body meson-exchange currents, as well as $γ$-nucleon resonances are included. Whereas the $γp$ reaction is determined by the low-density limit of the model, higher orders in the nuclear density are important to correctly account for the experimental spectra observed on both light and heavy nuclei over a wide range of photon energies, including the region below the pion threshold. In connection with soft dilepton spectra in high-energy heavy-ion collisions we emphasize the importance of photoabsorption to further constrain the parameters of the model.
Double-dipole excitations in ^{208}Pb are analyzed within a microscopic model explicitly treating 2p2h-excitations. Collective states built from such 2p2h-excitations are shown to appear at about twice the energy of the isovector giant dipole resonance, in agreement with the experimental findings. The calculated cross section for Coulomb excitation at relativistic energies cannot explain simultaneously the measured single-dipole and double-dipole cross sections, however.
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.
Based on a realistic model for the rho meson in free space we investigate it's medium modifications in a hot hadron gas generated by hadronic rescattering processes, i.e. renormalization of intermediate two-pion states as well as direct rho meson scattering off hadrons. Within the vector dominance model the resulting in-medium rho spectral function is applied to calculate $e^+e^-$ spectra as recently measured in heavy-ion collisions at CERN-SpS energies in the CERES experiment.
A meson exchange model of the $ππ$ interaction which fits free $ππ$ scattering data is used to calculate the interactions of pions in nuclear matter as a function of nuclear density. Polarization of the nuclear medium by the pions results in a marked increase in the s-wave $ππ$ attraction at low energy. The influence of this effect on the nucleon-nucleon interaction is a corresponding increase with density of the $NN$ central potential due to the exchange of two correlated pions, resulting in an $NN$ interaction which fails to saturate. A possible mechanism for restoring the theoretical stability of nuclear matter is explored and found to be effective.
The temperature evolution of the quark condensate is studied using three different methods. In the spirit of a many-body approach we make an expansion in the scalar density up to second order. Our result is consistent chiral perturbation theory to two-loop order.
Using a non-perturbative method based on the selfconsistent Quasi-particle Random-Phase Approximation (QRPA) we describe the properties of the pion in the linear $σ$-model. It is found that the pion is massless in the chiral limit, both at zero- and finite temperature, in accordance with Goldstone's theorem.
The fluctuation properties of nuclear giant resonance spectra are studied in the presence of continuum decay. The subspace of quasi-bound states is specified by one-particle one-hole and two-particle two-hole excitations and the continuum coupling is generated by a scattering ensemble. It is found that, with increasing number of open channels, the real parts of the complex eigenvalues quickly decorrelate. This appears to be related to the transition from power-law to exponential time behavior of the survival probability of an initially non-stationary state.
Within the finite-temperature Greens-function formalism we study the equation of state of a hot interacting pion gas at zero chemical potential. Employing realistic $ππ$ meson-exchange interactions we selfconsistently calculate the in-medium single-pion selfenergy and the $ππ$ scattering amplitude in the quasiparticle approximation. These quantities are then used to evaluate the thermodynamic potential, $Ω_π$(T), from which the state variables: pressure-, entropy- and energy-density can be derived. In contrast to earlier calculations based on the low-energy Weinberg Lagrangian we find an overall increase as compared to the free-gas results. We also consider the possibility of a dropping $ρ$-meson mass as suggested by the 'Brown-Rho Scaling' law.