Virtual Pion Scattering

We propose a theory which exploits the charge-exchange reactions ($^3$He,$^3$H$\pi^+$) and (p,n$\pi^+$) as effective sources of virtual pions. We consider processes in which the creation of virtual pions is followed by conventional coupled-channel pion scattering to discrete nuclear states. This picture allows us to incorporate successful theories of pion scattering and utilize virtual pions as probes of the nuclear matter. For coherent pion production we clearly demonstrate that the shift of the coherent peak position in the excitation function of $^3$He-A relative to $^3$He-N scattering is determined entirely by the pion nucleus rescattering.Comment: 10 pages, revtex 3, 2 figures attached in file figures.u

Collectivity Embedded in Complex Spectra of Finite Interacting Fermi Systems: Nuclear Example

The mechanism of collectivity coexisting with chaos in a finite system of strongly interacting fermions is investigated. The complex spectra are represented in the basis of two-particle two-hole states describing the nuclear double-charge exchange modes in $^{48}$Ca. An example of $J^{\pi}=0^-$ excitations shows that the residual interaction, which generically implies chaotic behavior, under certain specific and well identified conditions may create strong transitions, even much stronger than those corresponding to a pure mean-field picture. Such an effect results from correlations among the off-diagonal matrix elements, is connected with locally reduced density of states and a local minimum in the information entropy.Comment: 16 pages, LaTeX2e, REVTeX, 8 PostScript figures, to appear in Physical Review

Pluto"+"+ - A Monte Carlo simulation tool for hadronic physics

A versatile package for Monte Carlo simulations of hadronic interactions in C++ is presented, designed for compatibility with the ROOT analysis environment. Realistic models of resonance production, hadronic, and electromagnetic decays are implemented, motivated by the physics program of HADES. Empirical angular-distribution parametrizations for selected processes are utilized as well, such as resonance excitation in hadronic interactions, and nucleon-nucleon elastic scattering. The code comprises a self-contained framework for stand-alone principle simulations, including an extensive database of elementary particles and properties with support for additional user-input data, as well as utilities for the implementation of elementary detector setups and acceptance cuts. A standard interface for further on- and off-line processing of generated events with GEANT is also supplied. User-defined tasks via macros and derived classes are facilitated by the flexible design of the code, which in analysis mode may be employed for on-line fitting of experimental spectra. (orig.)Available from TIB Hannover: RA 3692(2000-03) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Exploring the collective spin-isospin longitudinal response of nuclei with coherent pion production in (He-3, t pi(+))

The SATURNE charge-exchange program explored the spin-isospin response of nuclei in the Delta region. New results on coherent pion production are presented. They give a precise measure of the spin longitudinal component of this response. This component is particularly interesting since theoretical models predict that attractive residual Delta -hole correlations in the longitudinal channel (pion exchange) softens the response