The pion reabsorption effect has been exploited, through a new analysis technique, to study the topological distribution of nuclear matter in the course of a heavy-ion collision at intermediate energies. The azimuthal angular distribution of pions with respect to the reaction plane and the angular correlations between pions and projectilelike fragments have been investigated. Quantitative estimations of the pion production time scale and of the impact parameter range involved are provided. The experimental results are successfully compared with the predictions of a microscopic theoretical model based on the solution of the Boltzmann-Nordheim-Vlasov transport equation.

The correlation between the couples of high-energy photons ${(E}_{\gamma }>30\mathrm{}\mathrm{MeV})$ detected in the reactions induced by a ${}^{36}\mathrm{Ar}$ beam on ${}^{27}\mathrm{Al},$ ${}^{112}\mathrm{Sn},$ and ${}^{197}\mathrm{Au}$ targets at 95 MeV/nucleon has been analyzed with the intensity interferometry technique. Both the size and lifetime of the emitting source have been quantitatively evaluated. Results support the nucleon-nucleon picture as the dominant hard-photon production mechanism.

Pion-nucleus elastic scattering at energies above the Δ(1232) resonance is studied using both ${\pi }^{+}$ and ${\pi }^{-}$ beams on ${}^{12}\mathrm{C},$ ${}^{40}\mathrm{Ca},$ ${}^{90}\mathrm{Zr},$ and ${}^{208}\mathrm{Pb}.$ The present data provide an opportunity to study the interaction of pions with nuclei at energies where second-order corrections to impulse approximation calculations should be small. The results are compared with other data sets at similar energies and with four different first-order impulse approximation calculations. Significant disagreement exists between the calculations and the data from this experiment.

Single and multinucleon transfer yields for the ${}^{136}{\mathrm{X}\mathrm{e}+}^{64}\mathrm{Ni}$ reaction at a scattering energy $\approx 5\%$ above the Coulomb barrier energy are studied using particle–γ-ray coincidence data. $Q$-value and scattering-angle distributions are extracted for the stronger channels. A fast transfer mechanism dominates the yields to these channels over an extended $Q$-value range, leading to a concentration of the cross section near the grazing angle. Analysis of the angular distributions based on a semiclassical barrier penetration model suggests that the single-nucleon and two-neutron exchange channels are dominated, respectively, by direct and two-step sequential transfer from the ground or low-lying excited states of the participating nuclei. The multiproton transfer channels have angular distributions that indicate a more complex mechanism, although direct cluster transfer from an excited configuration cannot be fully discounted. In a separate analysis, the relative population of different mass channels is found to be in general agreement with the expectations of a “random walk” model of particle exchange.

The stacked foil activation technique has been employed for the investigation of $\alpha$-particle induced reactions on the target elements yttrium and terbium up to 50 MeV. Six excitation functions for the $(\alpha ,xn)$ type of reactions were studied using high-resolution HPGe $\gamma$-ray spectroscopy. A comparison with Blann’s geometric dependent hybrid model has been made using the initial exciton number $n_0=4\mathrm{}\left(4p0h\right)\mathrm{}$ and $n_0=5\mathrm{}\left(5p0h\right)\mathrm{}$. A broad general agreement is observed between the experimental results and theoretical predictions with an initial exciton number $n_0=4\mathrm{}\left(4p0h\right)\mathrm{}$.

The absorption of ${\pi }^{+}$ on ${}^3$He in the $\Delta$ region is evaluated with exact inclusion of the final state interaction among the three emerging protons. The absorption is described by a $\pi N\to \Delta$ vertex and an $N\Delta$-$\mathrm{NN}$ transition $t$ matrix which are calculated from a phenomenological model for $\mathrm{NN}$ and $\pi d$ reactions. In a calculation where the initial pion scattering effects are neglected, the predicted peaks of the pion absorption cross sections for ${}^2$H and ${}^3$He lie too high in energy in relation to the data. The effect of the final-state three-nucleon interaction turns out to be too small for changing the magnitude and shifting the peak position of the total absorption cross section for ${}^3$He. We demonstrate that the adjustment of the peak position for the deuteron cross section by small modifications of the $\Delta$ parameters automatically leads to the correct peak position in ${}^3$He.

We study coherent $\eta$ photoproduction from nuclei in a relativistic impulse approximation approach. For the elementary production amplitude we use a standard relativistic parametrization based on a set of four Lorentz- and gauge-invariant amplitudes. The photonuclear amplitude is evaluated without recourse to a nonrelativistic reduction; the full relativistic structure of the amplitude is maintained. On general arguments we show that the coherent process is sensitive to only one of the elementary amplitudes. Moreover, we show that the nuclear structure information is fully contained in the ground-state tensor density. The tensor density is evaluated in a mean-field approximation to the Walecka model and it is shown to be sensitive to relativistic effects. Distortion effects are incorporated through an $\eta$-nucleus optical potential that is computed in a simple “$t\rho$” approximation.

The full integral representation of the eikonal approximation to the scattering amplitude of spin-1 elastic scattering in the presence of central, spin-orbit and tensor interactions is derived. Eikonal calculations of the cross section and polarization observables in deuteron-nucleus scattering at intermediate energies using a relativistic model are found to reproduce successfully the partial wave calculations.

An energy-dependent amplitude analysis of ${\pi }^{\pm }{-}^{12}\mathrm{C}$ elastic-scattering data in the pion energy range from 486 to 870 MeV was carried out. Using a careful treatment of Coulomb effects to describe the Coulomb nuclear interference together with the constraints from the ${\pi }^{\pm }{-}^{12}\mathrm{C}$ forward amplitude $f_0$ by dispersion relations allows a stringent consistency check of the data. In general a satisfactory description of the data was achieved, but recent ${\pi }^{-}{-}^{12}\mathrm{C}$ elastic-scattering data need considerable normalization factors to fit into the framework. The still not measured ${\pi }^{+}{-}^{12}\mathrm{C}$ data as well as the ${\pi }^{\pm }{-}^{12}\mathrm{C}$ data at 870 MeV are predicted. Phase shifts are calculated by partial wave projection of the amplitudes. The partial waves are summed up to obtain the total, total-elastic and total-inelastic cross sections.

The scattering of heavy ions gives clear evidence of resonances which may be grouped in families like the rotational bands. The classical Breit-Wigner theory makes use of fixed poles which describe locally the resonances, but the global character of the rotational sequences is completely lost. Furthermore, the phenomenology shows that the rotational sequences evolve into surface waves. Again the classical Breit-Wigner theory, in view of its local character, cannot describe this evolution. In this paper we describe the resonances by the use of poles of the scattering amplitude in the complex angular-momentum plane: moving poles. However, in order to interpolate a sequence of resonances belonging to the same family we must add to the poles a term which takes into account the repulsive forces due to the Pauli principle and to the hard core. This term describes the downward crossing, through π/2, of the phase shifts after each resonance. At higher energies the effect of the exchange forces tends to vanish and simultaneously the resonances evolve towards diffractive effects: we have the surface waves creeping around the target. This phenomenon is described in our theory by the moving poles as the imaginary part of the angular momentum increases for increasing energy. Besides a detailed study of this theory we present here an extensive analysis of the α-${}^{40}\mathrm{Ca}$ elastic scattering which gives clear phenomenological support to the model.

We report measurements of proton emission at target rapidities for minimum bias and central collisions of $14.6A\mathrm{GeV}/c$ ${}^{28}\mathrm{Si}$ with Al, Cu, and Au nuclei as well as minimum bias and central collisions of $11.7A\mathrm{GeV}/c$ ${}^{197}\mathrm{Au}$ with Au nuclei. Results for deuteron emission are also reported for the $\mathrm{S}\mathrm{i}+\mathrm{A}\mathrm{u}$ reaction. The spectra span the laboratory angular range of $50°<~\theta <~130°$ and kinetic energy range of $40\mathrm{MeV}<~E_{\mathrm{kin}}<~225\mathrm{MeV}.$ Inverse slopes of proton spectra and proton $dN/d\eta$ values in the kinetic energy range $50\mathrm{MeV}<~E_{\mathrm{kin}}<~110\mathrm{MeV}$ are reported. The inverse slopes are 40–80 MeV for the various systems, generally increasing with increasing pseudorapidity. The $dN/d\eta$ values for $A+A$ collisions within the restricted kinetic energy interval are compared to those for protons from $p+\mathrm{Au}$ in the literature. All pseudorapidity distributions have very similar shapes. The experimental results have been compared to the predictions of the nucleon-nucleon collision models ARC and RQMD. The predictions made by these two models for the distribution of protons at target rapidities are very similar to each other. However, there are significant differences between the model predictions and the experimental results in the details of the spectral slopes and the proton yields for different trigger conditions.

Recently we have found that the integral of the squared Coulomb wave function describing a system composed of charged pion and central charged fragment $Z_{\mathrm{eff}}$ protons, $|{\psi }_r\mathrm{}\left(r\right)\mathrm{}{|}^2$, times pion source function $\rho \mathrm{}\left(r\right)\mathrm{}$ (of the size $\beta )$, $\int \mathrm{dr}|{\psi }_r\mathrm{}\left(r\right)\mathrm{}{|}^2\rho \mathrm{}\left(r\right)\mathrm{}$, shows a quasiscaling behavior. This is approximately invariant under the following transformation: $\left(\beta {,Z}_{\mathrm{eff}}\right)\to (\lambda \beta ,\lambda Z_{\mathrm{}\mathrm{eff}})$; $\lambda >0\mathrm{}$. We called such behavior $\beta$-$Z_{\mathrm{}\mathrm{eff}}$ quasiscaling. We examine this quasiscaling behavior in detail. In particular we provide a semianalytical examination of this behavior and confirm it for the exponential pionic source functions in addition to the Gaussian ones and for the production of $K$ mesons as well. When combined with the results of the Hanbury-Brown Twiss effect a result of the yield ratio allows us to estimate the size of the central charged fragment to be $125<~Z_{\mathrm{}\mathrm{eff}}<~150$ for Pb+Pb collisions at energy $158$ GeV/nucleon. From our estimation, the baryon number density $0.024<~n_B<~0.036$ [1/fm${}^3$] is obtained.

The anomalous $J/\psi$ suppression in Pb-Pb collisions at 158$A$ GeV observed recently by NA50 can be explained as due to the transition to a phase of strong $J/\psi$ absorption, which sets in when the local energy density exceeds about 3.4 GeV/fm${}^3$.

In the present work we discuss the effect of $N$-body processes on pion multiplicity in relativistic heavy-ion reactions. This effect is analyzed in the energy range from the pion threshold up to 2 GeV/nucleon, for several projectile-target systems. The analysis is carried out in the context of intranuclear cascade calculations. It is shown that the inclusion of multibaryonic collisions is a crucial element in the study of the pion production mechanisms, being strongly dependent on the adopted correlation range for the particles involved in the $N$-body processes.

Polarization observables in $\eta$ production induced by $\mathrm{NN}$ collisions $p+p\to p+p+\eta$, $n+p\to n+p+\eta$, and $n+p\to d+\eta$, are derived. The unique $S$ wave spin structure of the threshold amplitude for $p+p\to p+p+\eta$ allows us to calculate all polarization observables in an universal form. Asymmetries in $n\to p\to$ collisions, characterized by noninterfering singlet and triplet amplitudes, are calculated. From the large experimental ratio of the differential cross sections of the processes $n+p\to n+p+\eta$ and $p+p\to p+p+\eta$, near threshold, one predicts large spin correlation coefficients $C_{\mathrm{xx}}$ ${(C}_{\mathrm{yy}})$. The spin structure of $S$- and $P$- wave contributions is derived.

The quark-meson coupling model, based on a mean-field description of nonoverlapping nucleon bags bound by the self-consistent exchange of $\sigma$, $\omega$, and $\rho$ mesons, is extended to investigate the change of hadron properties in finite nuclei. Relativistic Hartree equations for spherical nuclei have been derived from a relativistic quark model of the structure of bound nucleons and mesons. Using this unified, self-consistent description of both infinite nuclear matter and finite nuclei, we investigate the properties of some closed-shell nuclei and study the changes in the hadron masses of the nonstrange vector mesons, the hyperons, and the nucleon in those nuclei. We find a new, simple scaling relation for the changes of the hadron masses, which can be described in terms of the number of nonstrange quarks in the hadron and the value of the scalar mean field in a nucleus.

A confining, Goldstone theorem preserving, separable ansatz for the ladder kernel of the two-body Bethe-Salpeter equation is constructed from phenomenologically efficacious $u$, $d$, and $s$ dressed-quark propagators. The simplicity of the approach is its merit. It provides a good description of the ground-state flavor-octet pseudoscalar, vector, and axial-vector meson spectrum facilitates an exploration of the relative importance of various components of the two-body Bethe-Salpeter amplitudes, showing that subleading Dirac components are quantitatively important in the flavor-octet pseudoscalar meson channels, and allows a scrutiny of the domain of applicability of ladder truncation studies. A color-antitriplet diquark spectrum is obtained. The shortcomings of separable Ansätze and the ladder kernel are highlighted.

We investigate the modification of the pion cloud in the nuclear medium and its effect on the nuclear Drell-Yan process. The pion's in-medium self-energy is calculated in a self-consistent delta-hole model, with particle-hole contributions also included. Both the imaginary and real parts of the pion's and delta's self-energy are taken into account and related through a dispersion relation assuring causality. The resulting in-medium pion light-cone momentum distribution shows only a slight enhancement compared to the one of the free nucleon. As a consequence the ratio of the cross section for Drell-Yan scattering on nuclear matter and nucleonic targets is close to unity, in agreement with experiment.

We study the interactions of an elementary pion with a nucleon made of constituent quarks and show that the enforcement of chiral symmetry requires the use of a two-body operator, whose form does not depend on the choice of the pion-quark coupling. The coordinate space NN effective potential in the pion exchange channel is given as a sum of terms involving two gradients, which operate on both the usual Yukawa function and the confining potential. We also consider an application to the case of quarks bound by a harmonic potential and show that corrections due to the symmetry are important.

A test of parity-conserving, time-reversal noninvariance (PC TRNI) has been performed in 5.9 MeV polarized neutron transmission through nuclear spin aligned holmium. The experiment searches for the $T$-violating fivefold correlation via a double modulation technique—flipping the neutron spin while rotating the alignment axis of the holmium. Relative cross sections for spin-up and spin-down neutrons are found to be equal to within $1.2\times {10}^{-5}$ (80% confidence). This is a two orders of magnitude improvement compared to traditional detailed balance studies of time reversal, and represents the most precise test of PC TRNI in a dynamical process, to our knowledge.