R. Bijker

Dominance ofJP=0+ground states in even-even nuclei from random two-body interactions

Recent calculations using random two-body interactions showed a preponderance of J(P)=0(+) ground states, despite the fact that there is no strong pairing character in the force. We carry out an analysis of a system of identical particles occupying orbits with j=1/2, 3/2 and 5/2 and discuss some general features of the spectra derived from random two-body interactions. We show that for random two-body interactions that are not time-reversal invariant the dominance of 0(+) states in this case is more pronounced, indicating that time-reversal invariance cannot be the origin of the 0(+) dominance.

Generic Rotation in a Collective SD Nucleon-Pair Subspace

Low-lying collective states involving many nucleons interacting by a random ensemble of two-body interactions (TBRE) are investigated in a collective SD-pair subspace, with the collective pairs defined dynamically from the two-nucleon system. It is found that in this truncated pair subspace collective vibrations arise naturally for a general TBRE hamiltonian whereas collective rotations do not. A hamiltonian restricted to include only a few randomly generated separable terms is able to produce collective rotational behavior, as long as it includes a reasonably strong quadrupole-quadrupole component. Similar results arise in the full shell model space. These results suggest that the structure of the hamiltonian is key to producing generic collective rotation.

Spin-rotor interpretation of identical bands and quantized alignment in superdeformed {ital A}{approx}190 nuclei

The ``identical`` bands in superdeformed mercury, thallium, and lead nuclei are interpreted as examples of orbital angular momentum rotors with the weak spin-orbit coupling of pseudo-SU(3) symmetries and supersymmetries.

Pentaquark spectroscopy: exotic Θ baryons

We propose a collective stringlike model of q4q-bar pentaquarks with the geometry of an equilateral tetrahedron in which the four quarks are located at the four corners and the antiquark in its center. The nonplanar equilibrium configuration is a consequence of the permutation symmetry of the four quarks. In an application to the spectrum of exotic {theta} baryons, we find that the ground state pentaquark has angular momentum and parity Jp 1/2- and a small magnetic moment of 0.382 {mu}N. The decay width is suppressed by the spatial overlap with the decay products.

Transition from the seniority to the anharmonic vibrator regime in nuclei

A recent analysis of experimental energy systematics suggests that all collective nuclei fall into one of three classes {emdash} seniority, anharmonic vibrational, or rotational {emdash} with sharp phase transitions between them. We investigate the transition from the seniority to the anharmonic vibrator regime within a shell model framework involving a single large j orbit. The calculations qualitatively reproduce the observed transitional behavior, both for U(5)-like and O(6)-like nuclei. They also confirm the preeminent role played by the neutron-proton interaction in producing the phase transition. {copyright} {ital 1997} {ital The American Physical Society}