Probing the pion field in nuclei

It is suggested that muon absorption and doubly radiative pion absorption in nuclei can complement each other in probing the pion field in the nuclear medium. The relevant issues we consider are possible pion condensation and nuclear “renormalization” of weak and electromagnetic vertices, the adequate understanding of which is necessary to extract from nuclear experiments useful information on basic form factors in the weak Hamiltonian.     

Spin-isospin density fluctuations in nuclear collisions

The possible build-up of the spin-isospin fluctuation strength in nuclear collisions is investigated. In lack of quantum mechanical methods we rely on assumptions usually applied for heavy ion collisions. Based on a fluid dynamical picture we present our ‘isospin coupling in a nucleon cascade’. Within this framework it is possible to describe the mean isospin value as a function of the number of participants in a heavy ion collision and it is possible to study the effect of neutron excess. Since the pion field couples to the spin-isospin density a substantial size of this quantity gives rise to pion bremsstrahlung, the dominant subthreshold pion production mechanism.     

Intermediate energy bremsstrahlung and the structure of free and bound hadrons: Radiative pion proton scattering,pion decay and muon capture

Experimental data on pion-proton bremsstrahlung, radiative pion decay and radiative muon capture are reviewed to demonstrate that the study of intermediate energy bremsstrahlung processes can reveal additional information on hadron structure inaccessible in their non-radiative counterparts.Dedicated to the memory of M. Gmitro.     

Size of the pion emission region in collisions of relativistic nuclei from intensity interferometry

Radii of the pion emission region in collisions of relativistic nuclei as obtained from intensity interfermometry measurements, have been compiled and shown to be close to the effective nuclear radius of the lighter nucleus. Uncertainties in the data, due to a non-spherical shape of the pion emission region and non-isotropic mission of pion pairs, are discussed.     

Electromagnetic processes in relativistic heavy ion collisions

Electromagnetic effects in relativistic heavy ion collisions with impact parameter larger than the sum of the nuclear radii are studied using the virtual photon method. With increasing value of the relativistic parameter γ the hardness of the virtual photon spectrum increases. This leads to interesting new effects which will also have to be considered in the design of future relativistic heavy ion machines and experiments. The excitation of high-lying giant E1 and E2 multipole resonances is calculated as well as electromagnetic pion production. Coulomb bremsstrahlung is calculated and compared to the bremsstrahlung emitted in the more violent central nuclear collisions. K-shell ionization and electron-positron pair production is studied. The latter process has a very large cross section for heavy ions and contributes significantly to the stopping power of relativistic heavy ions in a dense medium.     

Chiral invariant phase space event generator

An event generator based on the CHIPS model is implemented in the GEANT4 simulation toolkit. Nuclear fragment production in the process of pion capture on nuclei is used to tune the parameters of the CHIPS model describing clusterization of nucleons in nuclei. The spectra of nucleons and nuclear fragments in pion capture and in photonuclear reactions at 60 MeV are compared with experimental data. Received: 30 May 2000 / Accepted: 2 November 2000     

Pions in isospin asymmetric matter and nuclear Drell-Yan scattering

Using a self-consistent delta-hole model the pion propagation in isospin asymmetric nuclear matter is studied. In neutron-rich matter, corresponding to heavy nuclei, a significant difference in positive and negative pion light-cone distributions is obtained leading to a nuclear enhancement of up antiquark distribution compared to the down antiquark one. This means that the sea-quark asymmetry in the free nucleon cannot be extracted model independently from an experiment on a neutron-rich nucleus.     

Evolution of pion phase-space density and Bose-enhancement effects in high-energy heavy-ion collisions

The evolution of the pion phase-space density in the ultrarelativistic heavy-ion collision is investigated on the basis of an analytical solution of the kinetic equation. The rate of pion production is treated as a superposition of binary collisions of the projectile and target nucleons. It is calculated in accordance with the experimentalNN→πX cross section, neglecting the deceleration of nuclei during their interpenetration. Very large occupation numbers of the lowp _T -pions in the central rapidity region are predicted for typical heavy-ion collisions at Brookhaven and CERN energies. The role of the Boseenhancement of pion production is discussed. It is shown that this effect may be essentially suppressed due to the finite formation time of pions.     

Relativistic many-body theory with radioactive ion beams: Surface pion condensation

We critically review the present relativistic mean-field theory from the viewpoint of missing pions. We introduce the interesting experimental data on pionic states taken at RCNP. These data seem to suggest the occurrence of pion condensation in the nuclear surface. Qualitative discussion is made on the consequence of surface pion condensation on Gamow-Teller transitions and spin response functions and others. The radioactive ion beams are the tools of studying the unstable nuclei, which have extended nuclear surfaces. We shall start with radioactive ion beams the nuclear surface science, which includes the surface pion condensation as the important ingredient in addition to spin-orbit splitting and surface pairing. Received: 1 May 2001 / Accepted: 4 December 2001     

Two-pion production in photon-induced reactions

Differences in the photoproduction of mesons on the free proton and on nuclei are expected to reveal changes in the properties of hadrons. Inclusive studies of nuclear photoabsorption have provided evidence of medium modifications. However, the results have not been explained in a model independent way. A deeper understanding of the situation is anticipated from a detailed experimental study of meson photoproduction from nuclei in exclusive reactions. In the energy regime above the Δ(1232) resonance, the dominant double pion production channels are of particular interest. Double pion photoproduction from nuclei is also used to investigate the in-medium modification of meson-meson interactions     

Pion asymmetry in the three-body radiative decays of polarized hyperons

We have calculated the energy dependence of pion asymmetry relative to the direction of hyperon polarization in the three-body radiative decays of polarized hyperons, using the inner bremsstrahlung model. We find that in the low pion energy region, the pion asymmetry in the radiative decay relative to the non-radiative decay is kinematically enhanced. For certain decays, this effect could be exploited experimentally to obtain additional information about the properties of the hyperons.     

Density isomerism and the Primakoff effect

We show that the decay width for π⁰→2γ in nuclear matter could be used as a signal for phase transitions in nuclear matter. The decay width of the π⁰ is experimentally measured using pion photoproduction off heavy nuclei by observing the Primakoff peak in the differential cross sections. We present calculations for the differential cross section with corrections to the γ?γ?π⁰ vertex arising from the nuclear medium within the nuclear radius when the medium is in the abnormal matter phase.     

AN ESTIMATION OF PION YIELDS

Using the spectrum of bremsstrahlung obtained by solving shower equation of electrons in matter and the differential cross section of photoproduction of pions in OPE approximation, we calculate by PDp-11/23 the pion yields results from 1.2 GeV electrons bombarding on different targets, Be, C, Al, Cu and Pt. The optimization is made for pion yields to find the optimistic production angle. In order to choose the thickness of the target, the curves of pion yields vs. Target thickness are given.     

Medium Energy Inelastic Hadron Scattering and Nuclear Structure

Some aspects of medium energy (∼ 150 MeV to 1 GeV) proton and pion scattering theory from complex nuclei are reviewed with emphasis on inelastic scattering as a tool to extract nuclear structure information.     

Investigation of bismuth nuclei fission into three fragments with comparable masses by means of GeV energy photons

We present the results of experimental studies of the process of fission of ²⁰⁹Bi nuclei into three mass-comparable fragments by bremsstrahlung photons with maximum energies of 2 GeV and 4.5 GeV with use of nuclear photoemulsion implanted with bismuth nuclei. We compare the results with those obtained with proton beams.     

Nuclear critical opalescence,a precursor to pion condensation

It is shown that pion condensation in nuclei, a long-range phenomenon, has a precursor in the disordered phase, the local ordering of spins which becomes of infinite range at the critical point. A new physical effect arising from this short-range order is predicted, namely the enhancement of the static nuclear pion field near the critical momentum. This phenomenon is strongly reminiscent of the critical opalescence observed in the scattering of neutrons by antiferromagnetic substances.     

Abnormal nuclear matter and pion condensation

The possibility of nuclear matter undergoing a combined phase transition into abnormal matter and a pion condensate is investigated. Various lagrangians for the meson (σ and π_i) fields, based on the σ-models, are used in mean-field approximation, and the entire system (mesons + nucleons) is treated fully relativistically. Equilibrium conditions of nuclear matter are obtained with NN repulsion, parametrized by the excluded volume approximation. It turns out that the formation of abnormal matter depends crucially on the choice of the σ-model lagrangian and considerably less on the additional pion condensate.