A symmetry adapted approach to vibrational excitations in atomic clusters

An algebraic method especially suited to describe the strongly anharmonic vibrational spectra in molecules may be an appropriate framework to study the vibrational spectra of Na _n ⁺ clusters, where nearly flat potential energy surfaces and the appearance of close lying isomers have been reported. As an illustration we describe the model and apply it to the Be₄, H ₃ ⁺ , Be₃ and Na ₃ ⁺ clusters. Presented by A. Frank at the International Conference on “Atomic Nuclei and Metallic Clusters”, Prague, September 1–5, 1997. This work was supported in part by the European Community under contract No. CI1*-CT94-0072, DGAPA-UNAM under project IN101997 and Spanish DGCYT under project PB92-0663.     

Recent Results for the Unquenched Quark Model

We discuss a new generation of unquenched quark models for baryons is presented in which the effects of quark–antiquark pairs are taken into account in an explicit form by means of a ³ P ₀ quark–antiquark creation mechanism. This provides the possibility to address many open problems in baryons structure and spectroscopy. The applications to magnetic moments and to the spin and flavor content of baryons will be reviewed here.     

Two-nucleon transfer reactions uphold supersymmetry in atomic nuclei

The spectroscopic strengths of two-nucleon transfer reactions constitute a stringent test for two-nucleon correlations in the nuclear wave functions. A set of closed analytic expressions for ratios of spectroscopic factors is derived in the framework of nuclear supersymmetry. These ratios are parameter independent and provide a direct test of the wave functions. A comparison between the recently measured 198Hg(d-->,alpha)196Au reaction and the predictions from the nuclear quartet supersymmetry lends further support to the validity of supersymmetry in nuclear physics.     

Interacting boson-fermion model of collective states II. Boson-fermion symmetries connected with the U(5) limit

The boson-fermion symmetries, which are connected with the U^(B)(5) limit of the interacting boson model are discussed. These symmetries arise when the bosons have U(5) symmetry and the fermions occupy a single-particle orbit with spin $\text{j =}\text{1}\text{2}$ (Spin(3) limit), $\text{j =}\text{3}\text{2}$ (Spin(5) limit), or $\text{j =}\text{3}\text{2}\text{,}\text{5}\text{2}$ (U^(B+F)(5) ? U^(F)(2) limit). Closed expressions for energy spectra, electromagnetic transition rates, static moments, and (one and two) nucleon transfer reaction intensities are derived.     

Spectroscopy of pentaquark states

We construct a complete classification of pentaquark states in terms of the spin-flavour SU(6) representations. We find that only some definite SU(3) representations are allowed, that is singlets, octects, decuplets, anti-decuplets, 27-plets and 35-plets. The latter three contain exotic states, which cannot be constructed from three quarks only. This complete classification is general and model independent and is useful both for model builders and experimentalists. The mass spectrum is obtained from a Gürsey-Radicati type mass formula, whose coefficients have been determined previously by a study of qqq-baryons. The ground-state pentaquark, which is identified with the recently observed state, is predicted to be an isosinglet anti-decuplet state. Its parity depends on the interplay between the spin-flavour and orbital contributions to the mass operator.PACS: 14.20.-c Baryons (including antiparticles) - 12.39.-x Phenomenological quark models - 02.20.-a Group theory     

Band structure from random interactions

The anharmonic vibrator and rotor regions in nuclei are investigated in the framework of the interacting boson model using an ensemble of random one- and two-body interactions. We find a predominance of L(P) = 0(+) ground states, as well as strong evidence for the occurrence of both vibrational and rotational band structures. This remarkable result suggests that such band structures represent a far more general (robust) property of the collective model space than is generally thought.