Theoretical physicists of the Nuclear Physics Institute are active in intermediate energy physics, such as hypernuclear physics, interactions of hadrons with nuclei, non-nucleonic degrees of freedom in nuclei. Further, they work on the problems of interacting boson model and its microscopic derivation, mathematical physics, and elementary particle theory.
The group of hypernuclear physics has been contributing significantly to the development of our knowledge of the production, structure and decay of strange baryonic systems. Among topics studied recently, the following deserve to be mentioned:
Polarization phenomena in a hypernuclear production in (pi+, K+), (K-, pi-) and (gamma, K+) reactions were estimated in literature for the first time. A possibility to use them in a study of mesonic and non-mesonic weak decays of polarized hypernuclei was pointed out [1.1].
Spectroscopy and strong decays of 1p shell hypernuclei were studied. It was shown that new information on Lambda N residual interaction can be learnt from hypernuclear gamma spectroscopy. We constructed a new set of parameters of the residual Lambda N interaction and explained recent hypernuclear spectroscopic data [1.2].
Photo- and electroproduction of strangeness on elementary target and complex nuclei were calculated . It was demonstrated that photons (real or virtual) may excite natural and unnatural parity low- and high-spin hypernuclear states with comparable strengths. The process affords a unique tool for the high-energy resolution hypernuclear spectroscopy [1.3].
Lambda, Sigma and Xi hypernuclei, as well as multi-strange baryonic systems were investigated within the relativistic mean field theory. The spin orbit splittings and magnetic moments are very sensitive to the value of the anomalous coupling f_omega_Y. A selfconsistent treatment together with elimination of the hyperon self-coupling contribution is crucial for determining the rho meson contribution to the hyperon binding. Calculations predict existence of bound baryonic systems with a considerable amount of hyperons [1.4]. The production cross section of the lightest hypernucleus 3-H_Lambda in high energy nuclear collisions was explained in the framework of the coalescence model. Production rates of heavy hypernuclei in the stopped antiproton annihilation were studied, as well. It was shown that two step processes with intermediate K and pi mesons dominate the process [1.5].
A consistent description of the elementary antinucleon-nucleon process and the antinucleon-nucleus elastic scattering has been achieved within the multiple scattering formalism. Influence of the off-shell effects in the antinucleon-nucleon t-matrix and of the isospin effects has been studied. The method of fully-off-energy-shell extension of the t-matrix was tested and particularly the influence of the nucleon-nucleon bound state on the proton-nucleus elastic scattering observables has been investigated. The role of the 2h omega shell-model configurations was shown to be important for reactions on the 13-C nucleus [2.1].
The p-anti p, annihilation at rest has been investigated in the diquark model. The calculated branching ratios of two mesons final states compare with the experimental data and with other more sophisticated approaches quite well [2.2].
An extensive theoretical study of pionic atoms has been performed using the newly developed calculational technique in the momentum space. Electromagnetic corrections due to the finite size of interacting charges and due to the vacuum polarization were included. The strong-interaction characteristics of the lowest pi-atomic states have been studied with the pion-nucleus optical potential constructed within the framework of the multiple scattering theory. The second order optical potential has been suggested for the treatment of the long-range two-nucleon correlations. Except for the very light nuclei, the general description of the existing data for the 1s and 2p levels of pionic atoms is fairly good [2.3].
Recently, an important progress has been reached in understanding spin phenomena in pion scattering by polarized nuclei. In collaboration with experimentalists in Paul Scherrer Institute in Villigen, where the first data on pion scattering by polarized 6,7-Li and 15-N were obtained, behaviour of vector and tensor analysing power was accounted for [2.4] in terms of coupled channel model. The model [2.5] was developed in the Department of Theoretical Physics , NPI, and has also been succesfully used in interpreting the data on elastic scattering and charge-exchange (\pi+, pi0) reaction on polarized 13-C at LAMPF [2.6].
In the high-energy deuteron breakup pd --> ppn reaction, the differential cross section and the tensor asymmetry for the backward emission of the protons have been shown to be sensitive to the non-spectator contributions at medium energies. The anomaly observed in the inclusive cross section in the region q=300 MeV/c has been well reproduced using the Paris deuteron wave function. The secondary interactions in a dibaryon system can lead to narrow oscillating structures which could be seen in the mass spectrum [2.7].
In studies of multiparticle production in heavy-ion and high-energy reactions, the entropy scaling of multiplicity distributions has been shown to be a special case of a more general set of multifractal measures. An alternative approach to the interpretation of the development of the clan structure arising formally from the natural decomposition of the negative binomial multiplicity distribution was elaborated. In the model, the effects of the coherent cluster disintegration and collapses are included in addition to the birth and death processes inspired by the QCD branching and the gluon splitting or fussion [2.8].
Non-nucleonic degrees of freedom in nuclei are studied in the electro-weak interactions of leptons and photons with nuclei. A new technique - called the extended S-matrix technique - for the construction of the nuclear effective operators, i.e. the nuclear Hamiltonian and electromagnetic and weak nuclear currents, has been developed. All operators were derived in the one boson exchange approximation within the framework of the v/c expansion. The resulting operators were applied to estimation of the relativistic effects in the deuteron electrodisintegration, muon capture on the deuterium, and tritium beta-decay [3.1].
A proper separation of the center-of-mass motion of the nuclear system is important for getting reliable and conclusive results in studies of electromagnetic and weak interactions in nuclei. In this respect, advanced calculations with the full inclusion of the retardation and boost effects have been initiated [3.2].
The boson mapping techniques have been studied with the aim of applying them in a derivation of the collective nuclear models. It was shown that different mapping procedures, being exact and equivalent in the full physical space, may differ considerably as concerns the behaviour of the unphysical states and properties under the truncation. The seniority boson mapping appears to be a convenient tool for deriving the parameters of the Interacting Boson Model (IBM) microscopically from the shell model. The microscopic sdg-IBM Hamiltonian has proved to give a good agreement with experiment particularly in the region of the spherical and gamma-soft nuclei. The relation between the Fermion Dynamical Symmetry Model and the IBM has also been prospected [4.1].
The applications of the energy independent effective interaction approach of Suzuki and Lee to boson mappings was investigated. As a result, a general energy independent effective operator has been derived for both hermitian and non-hermitian effective interactions [4.2].
In studies of non-perturbative aspects of the standard model of subnuclear physics, nonminimal interaction of the gluon field with a massive antisymmetric order parameter was shown to yield the vanishing tree-level dielectric function. The London-type description of a perfect diaelectric suggests tractable non-Abelian Ginzburg-Landau-type macroscopic chromodynamics suitable for describing the confinement-deconfinement phase transition. It was demonstrated that the dynamical mass generation is possible without the Higgs field provided that the leptons and quarks renormalizably interact, besides the standard gauge fields, with a new massive Abelian vector boson [5.1].
[5.1]. J. Hosek: Phys. Rev. D46(1992)3645.
The mathematical--physics group made important contributions to the understanding of properties of quantum systems. Among recent results based on the theory of standard as well as nonstandard Schrodinger operators the following ones may be mentioned.
The existence of bound states in curved quantum waveguides was discovered and geometrically induced spectral and scattering properties of such systems have been analysed. The results help to understand conductivity profiles of the so--called quantum wires [6.1].
A new class of exactly solvable models with contact--type interactions have been constructed. They allow us to explain, in particular, various decay and resonance phenomena [6.2].
Existence of systems has been demonstrated which are classically solvable but exhibit a quantum chaos. New properties of quantum chaotic systems have been found [6.3].
Unexpected spectral properties have been found for systems which model graph superlattices in external electric fields [6.4].
Methods for obtaining the exact and quasiexact solutions of the stationary Schrodinger equation as well as for the computation of the bound states energy levels are developed. Various new schemes of the perturbation theory are proposed. The results are represented by the following topics.
Certain systems with singular anharmonicities were described in terms of the analytic continued fractions [6.5].
A numerically inspired new version of the degenerate Rayleigh-Schrodinger perturbation theory was proposed, illustrated and thoroughly tested [6.6].
In connection with the limited applicability of perturbative methods, several new non-perturbative techniques were proposed and developed in the context of the so called delta expansions and iterative (Pade and continued-fraction) re-summation techniques [6.7].
The constructive method of Hill determinants was given a relatively complete and rigorous formulation [6.8].
Problems of the classical field theory are also investigated. A relation between the existence of the self-duality limit and the extended supersymmetry in the Chern-Simons systems in 2+1 dimensions has been studied. It was shown for the case of non-Abelian Chern-Simons theory with the matter fields that the requirement of N=2 supersymmetry leads to a system with nontopological soliton solutions [6.9].
Possibility of existence of a relativistic Fermi accelerator, i.e. of an unbounded energy transfer from the periodically moving wall to the scalar massless field or to the relativistic particle confined by the wall, has been shown [6.10].