We discuss a multistep variational approach for the study of many-body correlations. The approach is developed in a boson formalism (bosons representing particle-hole excitations) and based on an iterative sequence of diagonalizations in subspaces of the full boson space. The purpose of these diagonalizations is that of searching for the best approximation of the ground state of the system. The procedure also leads us to define a set of excited states and, at the same time, of operators which generate these states as a result of their action on the ground state. We examine the cases in which these operators carry one-particle one-hole and up to two-particle two-hole excitations. We also explore the possibility of associating bosons to Tamm-Dancoff excitations and of describing the spectrum in terms of only a selected group of these. Tests within an exactly solvable three-level model are provided.

For the calculation of polarization transfer observables for quasielastic scattering of protons on nuclei, a formalism in the context of the relativistic plane wave impulse approximation is developed, in which the interaction matrix is expanded in terms of a complete set of 44 independent invariant amplitudes. A boson-exchange model is used to predict the 39 amplitudes that were omitted in the formerly used five-term parameterization, the SPVAT (scalar, pseudoscalar, vector, axial-vector, tensor) form of the nucleon-nucleon scattering matrix. Use of the complete set of amplitudes eliminates the arbitrariness of the five-term representation.

Measurements of cross section, vector analyzing power $A_y,$ and tensor analyzing powers $A_{\mathrm{zz}}$ and $A_{\mathrm{xz}}$ over the angular range $10°<~{\theta }_{\mathrm{lab}}<~40°$ have been performed at ${E(}^6\mathrm{Li})=34\mathrm{}\hspace{0.5em}\mathrm{MeV}$ for the ${}^{58}\mathrm{Ni}{(}^6\mathrm{Li}\to {,d)}^{62}\mathrm{Zn}$ and ${}^{40}\mathrm{Ca}{(}^6\mathrm{Li}\to {,d)}^{44}\mathrm{Ti}$ reactions leading to the ground state and first excited state of both residual nuclei. The reactions are described by distorted-wave Born approximation calculations, assuming a direct $\alpha$-particle transfer mechanism. The asymptotic $D/S$ state ratio $\eta$ for the $d+\alpha$ relative wave function in ${}^6\mathrm{Li}$ is determined. In this one-step analysis, the best fit to the tensor observables leads to a value of $\eta =+0.0003\mathrm{}\pm \mathrm{0.0009.}$ This value is in disagreement with most of the previous theoretical and empirical determinations of $\eta .$ An investigation of two-step reaction mechanisms is performed, allowing the $J^{\pi }{=3\mathrm{}}^{+},$ $2^{+},$ and $1^{+}$ states in ${}^6\mathrm{Li}$ to contribute to the transfer reaction channel. Reasonable agreement is achieved with the cross section and vector analyzing power data for several possible two-step amplitudes. It is found that the fitted magnitude of $\eta$ increases with increasing two-step amplitude, giving $\eta =-0.0030\pm 0.0022$ for unit amplitude, therefore not changing significantly from our one-step result.

Theoretical possibilities for exotic cluster emissions from ${}^{249}\mathrm{Cf}$ and ${}^{252}\mathrm{Cf}$ parents are explored on the basis of the very much used preformed cluster model of one of us (R.K.G.) and collaborators. The calculated α-decay half-life time for ${}^{252}\mathrm{Cf}$ match the experiments exactly and the same for ${}^{249}\mathrm{Cf}$ within an order of 3. The most probable heavy cluster decay for both the parents is predicted to be ${}^{46}\mathrm{Ar}$ or ${}^{48}\mathrm{Ca},$ with the predicted decay half-life times far more than the presently available experimental upper limiting values. In other words, the predicted heavy-ion emission probabilities for both the parents are so small that there seem to be very little chance for their exotic cluster decays to be observed in the very near future.

An angular momentum dependence of the parity splitting in the alternating parity bands of nuclei with strong octupole correlations is considered based on the model of the octupole motion in a one-dimensional potential well conserving axial symmetry. A sign reversal of the parity splitting at higher values of the angular momentum is interpreted as a result of the Coriolis coupling to $K=1\mathrm{}$ octupole mode. Experimental data on the spectra of the alternating parity octupole bands are used.

A nuclear physics example of the U(6/2) supersymmetry group is considered. It is shown that this group contains a supersymmetric subgroup with a structure similar to the supersymmetry model of the quantum field theory (QFT). A comparison of two models helps to clarify the relation between the supersymmetry schemes of QFT and of nuclear physics. Using this similarity relation between the numbers of the bosonic and fermionic states similar to the fundamental relation in QFT is obtained. For those supermultiplets with at least two fermions the numbers of the bosonic and fermionic states are equal as in QFT.

The dressing of nucleons as embodied in single-particle spectral functions is incorporated in the description of nucleon-nucleon scattering in nuclear matter at a density corresponding to $k_F=1.36\mathrm{}\hspace{0.5em}{\mathrm{fm}}^{-1}.$ In order to clarify the new features associated with the complete off-shell behavior of the single-particle motion, results involving mean-field particles are also presented with special emphasis on the behavior of the phase shifts when bound pair states occur. Both the ${}^1{S}_0$ and ${}^3{S}_1{-}^3{D}_1$ channels exhibit this feature at the considered density for mean-field particles at zero temperature. An important tool to assess the effect of the dressing of the particles is the two-particle density of states. A sizable reduction with respect to the mean-field density of states is obtained. At $2{\epsilon }_F$ this reduction corresponds to $z_{k_F}^2,$ where $z_{k_F}$ is the strength of the quasiparticle pole at $k_F,$ and it can therefore be as large as 0.5. This reduction has significant consequences for the strength of pairing correlations both in the ${}^3{S}_1{-}^3{D}_1$ channel where it leads to a dramatic decrease of the attraction at the Fermi energy and for the ${}^1{S}_0$ channel which no longer shows a pairing signal. Phase shifts and cross sections for dressed particles are determined based on expressions which fold the effective interaction with the dressed but noninteracting two-particle spectral function. This folding procedure yields similar results to an “on-shell” prescription reminiscent of the result for free or mean-field particles, except for cross sections deep in the Fermi sea. Comparison of phase shifts and cross sections to the case of mean-field particles indicates that smaller phase shifts in an absolute sense and considerable reductions of the in-medium cross sections for dressed particles are obtained. It is shown that while in many cases these results imply a weakening of the effective interaction, this is not the case for ${}^1{S}_0$ interactions deep in the Fermi sea.

The 180 MeV ${p+}^{27}\mathrm{Al}$ reaction is investigated using antisymmetrized molecular dynamics, i.e., AMD and AMD-V, to investigate properties of fragment formations in medium energy nucleon-induced reactions. After statistical decay calculations, both AMD and AMD-V almost reproduce experimental mass distributions, differential, and double differential spectra of fragments in the ${p+}^{27}\mathrm{Al}$ reaction. However, the inclusion of the diffusion and the deformation of wave packets in AMD-V has drastic effects and, then, aspects in dynamical calculations of AMD-V are very different from AMD calculations. It is shown that various aspects of nucleon-induced reactions, in which residual nuclei decay into several fragments, appear in AMD-V calculations. The decay into fragments proceeds slowly, and thus, differs from the multiple fragmentation in heavy ion collisions. The possibility that AMD-V dynamical calculations describe the processes of statistical decays is discussed.

The reaction rates of neutron capture reactions on light nuclei are important for reliably simulating nucleosynthesis in a variety of stellar scenarios. Neutron capture reaction rates on neutron-rich C, N, and O isotopes are calculated in the framework of a hybrid compound and direct capture model. The results are tabulated and compared with the results of previous calculations as well as with experimental results.

We consider the parity nonconserving (PNC) mixing in the ground state of exotic (halo) nuclei caused by the PNC weak interaction between outer neutron and nucleons within the nuclear interior. For the nucleus ${}^{11}\mathrm{Be}$ as an example of a typical nucleus with a neutron halo, we use an analytical model for the external neutron wave functions to estimate the scale of the PNC mixing. The amplitude of the PNC mixing in the halo state is found to be an order of magnitude bigger than that of typical PNC mixing between the “normal” nuclear states in nearby nuclei. The enhanced PNC mixing in a halo cloud is proportional to the neutron weak PNC potential constant $g_n^W$ only.

The energy and nuclear mass dependences of the total hadronic cross section in the energy range 0.5–2.6 GeV have been measured at Bonn using the SAPHIR tagged photon beam. The measurement, performed on C, Al, Cu, Sn, and Pb, provides the first photoabsorption data in the region 1.2–1.7 GeV. The results show a significant reduction of the photoabsorption strength on the bound nucleon compared to the free-nucleon case in the whole energy region. Above 1.2 GeV this reduction decreases with the average nuclear density.

A model based on the hadronic fluctuations of the real photon is developed to describe the total photonucleon and photonuclear cross sections in the energy region above the nucleon resonances. The hadronic spectral function of the photon is derived including the finite width of vector-meson resonances and the quark-antiquark continuum. The shadowing effect is evaluated considering the effective interaction of the hadronic component with the bound nucleons within a Glauber-Gribov multiple scattering theory. The low energy onset of the shadowing effect is interpreted as a possible signature of a modification of the hadronic spectral function in the nuclear medium. A decrease of the $\rho$-meson mass in nuclei is suggested for a better explanation of the experimental data.

The H dibaryon, a single, maximally symmetric bag containing two up, two down, and two strange quarks, has long been sought after in a variety of experiments. Its creation has been attempted with $K^{-}$’s or protons as projectiles and most recently in relativistic heavy ion induced reactions. We concentrate on the latter, but our conclusions are more generally applicable. The two baryons coalescing to form the single dibaryon, likely $\Lambda \Lambda$ in the case of heavy ions, must penetrate the short range repulsive barrier which is expected to exist between them. We find that this barrier can profoundly affect the probability of producing the H state, should it actually exist.

We employ variational Monte Carlo methods to study the transition to strange matter in a simple one-dimensional string-flip model with two flavors and two colors of quarks. The dynamics of the system are described in terms of a many-body potential that confines quarks within hadrons, yet enables the hadrons to separate without generating unphysical long-range van der Waals forces. The model has “natural” low- and high-density limits: it behaves as a system of isolated hadrons at low density and as a Fermi gas of quarks at high density. We show that the system exhibits a transition to strange matter characterized by an increase in the length scale for confinement. Yet the small increase at the transition region—of only 10%—suggests that clustering correlations remain strong well into the strange-matter domain. Our results put into question descriptions of strange matter in terms of noninteracting, or weakly interacting, quarks.

High-spin structures in the neutron-rich nuclei ${}^{145}\mathrm{Ce}$ and ${}^{147}\mathrm{Ce}$ produced in the spontaneous fission of ${}^{252}\mathrm{Cf}$ have been investigated by prompt $\gamma$-ray spectroscopy. A collective band structure in ${}^{145}\mathrm{Ce}$ is identified. Several sidebands along with the new high-spin states in ${}^{147}\mathrm{Ce}$ are also identified. Particle-plus-rotor model calculations indicate that the yrast bands in ${}^{145}\mathrm{Ce}$ and ${}^{147}\mathrm{Ce}$ most probably originate from coupling of the $\nu i_{13/2}$ orbital to the ground states of ${}^{144}\mathrm{Ce}$ and ${}^{146}\mathrm{Ce}.$ The ground state configurations of ${}^{145,147}\mathrm{Ce}$ are $(\nu h_{9/2}+\nu f_{7/2})$ and $\nu h_{9/2},$ respectively.

It is shown that data on $E2\mathrm{}$ transition rates in ${}^{152}\mathrm{Sm}$ can be reproduced in the geometric collective model and that the calculations support the phase/shape coexistence interpretation of this pivotal nucleus. The coexistence is further supported by microscopic calculations, as is the concept of a sudden onset of deformation in this region.

We present channel by channel calculations for the transition matrix elements of both coherent and incoherent rates of the exotic neutrinoless ${\mu }^{-}\to e^{-}$ conversion in ${}^{27}\mathrm{Al}.$ This nucleus has recently been employed as a target in the MECO experiment at Brookhaven with the aim to search for lepton flavor violating events with a sensitivity of ${10}^{-17}$ leading to very severe constraints for the flavor violation parameters when using our calculated transition matrix elements.

A microscopic combinatorial approach is used to calculate the state and level densities with fixed exciton numbers, in some actinide nuclei. The deformed Woods-Saxon shell model was used as a basis from which all possible configurations were generated. The pairing interaction was taken into account by applying the BCS theory to each configuration. Both the spin and parity distributions were obtained, considering the deformation of the nucleus. Relevance of the result to parity nonconservation studies involving epithermal neutrons on ${}^{238}\mathrm{U}$ and ${}^{232}\mathrm{Th}$ is discussed.

Effective density-dependent pairing forces of zero range are adjusted on gap values in $T=0,1\mathrm{}$ channels calculated with the Paris force in symmetric nuclear matter. General discussions on the pairing force are presented. In conjunction with the effective k mass the nuclear pairing force seems to need very little renormalization in the $T=1\mathrm{}$ channel. The situation in the $T=0\mathrm{}$ channel is also discussed.

A study of the ${}^{136}\mathrm{Nd}$ nucleus has been performed with the EUROBALL III multidetector to establish the decay-out of the yrast superdeformed (SD) band. Three discrete linking transitions (754, 1456, and 1493 keV) have been discovered, establishing the position of the SD band at 7.03 MeV excitation energy with proposed spin and parity ${17}^{\left(-\right)}$ for the lowest observed SD state. Neutron pairing gap parameters for SD shapes have been extracted in Nd isotopes, using the strong-coupling model and odd-even mass difference formulas. The major conclusion of our phenomenological analysis is that the pairing correlations do subsist in the SD configurations of nuclei in the $A=130\mathrm{}$ mass region.