Chiral symmetry for positive and negative parity nucleons

We investigate the role of chiral symmetry in the properties of positive and negative parity nucleons (N and N^*). When we consider two kinds of nucleons, two distinctive types of chiral assignment for N^* can be introduced. There are interesting differences between the two assignments in the properties of nucleons, which are studied using linear sigma models based on the two assignments. Behavior of N and N^* in the nuclear medium is also investigated using these linear models.     

Proton optical potentials at intermediate energies from Brueckner theory

We have analysed proton scattering cross sections and polarisations from ⁴⁰Ca and ²⁰⁸Pb at intermediate energies using optical potentials calculated from Brueckner theory with internucleon potentials and ground state densities both different from those used previously. We show that the calculated results are very sensitive to the assumed densities and that agreement with experiment is only obtained using phenomenological densities and is not obtained using shell-model densities. We are not able to obtain agreement with the experimental data with a particular soft-core potential, Urbana V-14.     

Proton inelastic scattering at intermediate energies and Dirac-equation-based optical potentials

The Dirac-equation-based (DEB) optical potential is extended to the study of inelastic excitation of collective states using intermediate-energy protons. Comparison is made with standard calculations using Woods-Saxon potentials. In the energy range 150–800 MeV, the two models give cross sections in close agreement when equal deformation lengths are used. The inelastic polarizations and analyzing powers also agree (to a lesser degree) near or below 200 MeV and at 800 MeV, but show differences at 300 and 500 MeV. We also point out the importance of the deformed spin-orbit terms in inelastic cross section calculations.     

Electromagnetic form factors for nucleons and pions at positive and negative q 2 in the model of quark-gluon strings

The electromagnetic form factors for pions and nucleons are considered within the model of quark-gluon strings, where the momentum-transfer dependence of hadronic form factors is determined by the intercepts of the corresponding Regge trajectories and by the Sudakov form factor. Analytic expressions found for form factors in the timelike region admit an analytic continuation to the spacelike region. The resulting form factors for pions and nucleons comply well with experimental data both for positive and for negative values of the squared momentum transfer q ². It is shown that the distinctions between the absolute values of the pion and nucleon form factors F _π(q ²), G _m (q ²), and F ₂(q ²) at positive values of q ² and those at negative values of this variable are associated with the analytic properties of the double-logarithmic term in the exponent of the Sudakov form factor. The spin structure of the amplitudes for quark transitions into hadrons that is proposed in the present study makes it possible to describe fairly well available experimental data on the Pauli form factor F ₂ and on the ratio G _e /G _m .     

Microscopic approach to calculating cross sections and optical potentials for nucleus-nucleus interaction at intermediate energies

Nucleus-nucleus scattering is considered in the high-energy approximation and on the basis of Glauber-Sitenko microscopic theory in the optical limit. Analytic expressions for eikonal phase shifts are given for the case of Fermi-type realistic potentials and nuclear-density distributions. The effect of taking into account the distortions of the trajectories of the nuclei involved and the nuclear-density dependence of nucleon-nucleon forces on the total reactions cross sections is illustrated. The sensitivity of the reaction cross sections to the choice of model for the ⁶He projectile nucleus, which involves a neutron halo, is explored. Semimicroscopic optical potentials are introduced in order to describe differential cross sections for elastic scattering. The results of the present calculations are compared with available experimental data.     

Hadronization of excited nucleons in nuclear collisions at ISR energies

Calorimeters downstream of the intersection of pα and αα beams in the ISR have been used to study the hadronization of excited nucleons. These data extend and support the conclusions from previous studies of the A-dependence of particle multiplicities in ultrarelativistic p-nucleus collisions, which provided evidence that the proton hadronizes outside the nuclear volume.     

Coupled QCD sum rules for positive and negative-parity nucleons

A new approach of the QCD sum rule is proposed in which positive and negative-parity baryons couple with each other. With positive and negative-parity states explicitly taken into account, sum rules are derived by means of the dispersion relation in energy. The method is applied to the nucleon channel and the parity splitting of the nucleon resonance states is studied. It is found that the obtained sum rules have a very good Borel stability. This suggests that the ansatz for the spectral function in the present sum rule approximates the physical spectrum better than the usual lowest pole plus continuum ansatz. The predicted masses of the positive and negative nucleons reproduce the experimental ones fairly well. Especially, the mass difference is extremely close to the experimental value.     

Asymptotic properties of regge trajectories and elastic pseudoscalar-meson scattering on nucleons at high energies

A phenomenological Regge-eikonal model featuring nonlinear monotonic parametrizations of vacuum Regge trajectories, where their asymptotic behavior in the perturbative sector is taken explicitly into account, is proposed for describing the elastic diffractive scattering of light pseudoscalar mesons on nucleons. In analyzing available experimental data on angular distributions, it is shown that, at collision energies in the region √s > 13 GeV, the diffraction pattern of the processes π ^± p → π ^± p and K ^± p → K ^± p at low momentum transfers can be described qualitatively by using the same phenomenological approximations to vacuum Regge trajectories as in the case of nucleon-nucleon scattering. This fact is indicative of the possibility of explicitly relating Regge phenomenology of various hadron-hadron processes to fundamental results obtained within QCD.     

The effective interaction for unbound nucleons and the optical potential

The effective interaction for an unbound nucleon in a complex nucleus is studied with particular emphasis upon the relationship of the unbound state interaction with the interaction for bound states. A successful unified treatment of unbound and bound states is given, and the unbound state effective interaction is related to the standard “optical model”. The attractive force for an unbound nucleon is shown to be of an exchange type and is best represented by a non-local, energy-independent potential which can be calculated, at least for not too high incident nucleon energy, from the attractive interaction between bound nucleons in a nucleus. We relate the unbound state potential to the bound state effective force as calculated by Negele using Hartree-Fock theory.

A non-local potential used by Elton, Webb, and Barrett in a successful analysis of electron scattering is shown, with minor modification, to work remarkably well in an analysis of elastic scattering of protons. Excellent agreement with experiment is obtained for the differential cross section and the polarization of protons with an incident energy up to 60 MeV scattered from ⁴⁰Ca, ⁵⁸Ni, ¹²⁰Sn and ²⁰⁸Pb. The real potential parameters are shown to be independent of the incident energy and physically reasonable; the imaginary potential parameters agree with expectations from Fermi gas and collective models. Bound state energy levels are also calculated with no readjustment of the real potential parameters in order to check the consistency of the interaction with the bound states, and the results are in good agreement with Negele.

The interaction of Skyrme as modified by Vautherin and Brink is also used in an analysis of elastic proton scattering. Good results are obtained for the differential cross section and the polarization for 20 MeV protons scattered from ⁵⁸Ni. At higher incident proton energy, and for larger nuclei, the results are poorer.     

Distribution of wounded nucleons and of colored strings in α-α collisions at high energies

Using classical probability calculus we estimate and compare the distribution of wounded nucleons and of colored strings in α-α collisions at high energies. Although these distributions show quite remarkable differences, the corresponding multiplicity distributions of produced particles are hardly distinguishable.     

光波段多频负折射率超材料

从光波段圆孔形双鱼网结构的负折射材料模型出发,采用基于有限积分技术的CST软件系统研究了原胞结构的改变对负折射行为的影响.数值仿真结果表明,对原胞结构做微小调节也可获得负折射率频带的增大效应.将双鱼网结构改为阶梯形孔洞和半球形孔洞结构,可以在更多的频段里出现负折射率,并且谐振频率发生了一定的红移.半球形孔洞的双鱼网结构可以方便地用化学模板法制备,这为从实验上实现红外及可见光波段的多频负折射材料提供了一种简单可行的方法. 关键词： 负折射率 多频段 双鱼网结构     

Scattering by singular potentials of the general type at high energies

The problem of high-energy scattering by strongly singular potentials, having no convincing general solution yet, is attacked by a generalization of the phase approach. The asymptotically correct expression obtained is recognized as the standard WKB phase-shift formula.