Intermediate rho mesons in pi-nuclear scattering

The role of rho meson exchange between nucleons is examined by solving the Low equations which couple the pi-nuclear channels to the rho-nuclear channels. The result is in the form of a linear coupled-channel equation of the Lippmann-Schwinger type but including a propagator modification which strongly damps the effects of virtual rho mesons. In particular, the contribution of rho exchange to the Lorentz-Lorenz effect is found to be negligible.     

An effective dynamic interaction for the two-particle propagator

In nuclear structure calculations the collective excitations of the core introduce dynamic effects in the interaction between particles. Under the restriction of including forward-going contributions only, it is shown that the two-particle propagator which yields the spectra and two-particle transfer strengths of nuclei with two nucleons outside a closed shell, can be written in a Dyson-like equation in which a two-particle self-energy or dynamic effective interactionΔ is introduced. An expression forΔ is given in terms of the irreducible vertex part of the Bethe-Salpeter equation and partial summations forΔ using phonon exchange interactions to represent the core-polarization diagrams, are discussed. The single-particle propagators are also dressed with phonon-exchange contributions.     

Latest results from GRAAL collaboration

The GRAAL experimental set-up consists of a polarized and tagged photon beam that covers an energy range from a minimum of 600 MeV up to a maximum of 1500 MeV, of a liquid Hydrogen or Deuterium target and of the 4π Lagrange detector optimized for photon detection. It allows the study of pseudo-scalar and vector meson photoproduction on the nucleon in the energy range corresponding to the second and the third resonance regions. In the following, the Σ beam asymmetries in η and π~0 photoproduction on quasi-free nucleon are shown. Also single and double polarization observables in K~+A photoproduction on free proton are shown; they are important to confirm the role of new or poorly known resonances in the 1900 MeV mass region.     

Correlations and the Dirac structure of the nucleon self-energy

The Dirac structure of the nucleon self-energy in symmetric nuclear matter as well as neutron matter is derived from a realistic meson exchange model for the nucleon-nucleon (NN) interaction. It is demonstrated that the effects of correlations on the effective NN interaction in the nuclear medium can be parameterized by means of an effective meson exchange. This analysis leads to a very intuitive interpretation of correlation effects and also provides an efficient parametrization of an effective interaction to be used in relativistic structure calculations for finite nuclei. Received: 29 January 2001 / Accepted: 5 May 2001     

Odderon and photon exchange in electroproduction of pseudoscalar mesons

We investigate the reaction where PS denotes a pseudoscalar meson , , , or and X either a proton or resonance or continuum state into which the proton can go by diffractive excitation. At high energies photon and odderon exchange contribute to the reaction. The photon exchange contribution is evaluated exactly using data for the total virtual photon-proton absorption cross section. The odderon exchange contribution is calculated in nonperturbative QCD, using functional integral techniques and the model of the stochastic vacuum. For the proton we assume a quark-diquark structure as suggested by the small odderon amplitude in pp and forward scattering. We show that odderon exchange leads to a much larger inelastic than elastic PS production cross section. Observation of our reaction at HERA would establish the soft odderon as an exchange object on an equal footing with the soft pomeron and would give us valuable insight into both the nucleon structure and the mechanism of high energy diffractive scattering. Received: 2 February 1999 / Revised version: 22 March 1999 / Published online: 28 May 1999     

不变振幅的不同投影选择对核子自能与碰撞截面的影响

本文利用Dirac-Brueckner-Hartree-Fock方法,对不同散射道的核子在核物质中的相互作用进行了细致分析,重点分析了不变振幅的不同投影选择对核物质中核子自能和核子碰撞截面的影响.计算结果显示:1)在确定的核子动量下,自旋三重态所对应的自能总是大于自旋单态的自能;同位旋三重态所对应的自能大于同位旋单态的自能值.2)在完全赝矢(CPV)选择下,不同散射道的核子自能对于动量的依赖均明显弱于在赝标(PS)选择下的相应值,而这种差别主要来自于总角动量J较小的分波态.3)在核子相对动量较小时,两种不同选择所对应的核子微分截面较大.4)在确定的入射能量下,质心系中散射角较小时,不同选择下的微分截面差别较为明显.5)在低入射能区,CPV选择下所对应的微分截面大于PS下的微分截面值.6)在CPV选择下的总截面总是大于PS选择下的总截面值.这些差别均随着随入射能量的增大而消失.     

The meson propagator in nuclear matter

We present a formal device for calculation of a meson propagator in infinite nuclear matter, through the calculation of the ground state energy of the system in the presence of external, static meson fields, using a pseudo-Hamiltonian simply related to the actual Hamiltonian. This approach is particularly well adapted for dealing with some effects that are not taken into account at all in multiple scattering approaches, for example, the effects of nuclear forces on the intermediate states which are involved in the meson-single-nucleon scattering. A theorem is proved about limiting conditions under which the charged pion propagator is independent of the nuclear forces. These conditions are nearly enough realized in π-nucleus scattering (sufficiently below the energy of the first resonance) so that we can evaluate the corrections perturbatively. The (virtually unknown) two-body force between the 3-3 resonance and nucleons is shown to be of considerable importance in determining the π-meson propagator.     

Diffractive meson production from virtual photons with odd charge-parity exchange

We calculate the cross section of diffractive charge-parity neutral meson production in virtual photon proton collision at high energies. Due to the opposite -parities of photon and meson () this process probes the -channel odderon exchange which is described here as noninteracting three–gluon exchange. Estimates for the cross section of inelastic diffractive process are presented. The total cross section of diffractive meson photoproduction is found to be 47 pb. The cross sections for the diffractive production of light mesons () in collisions are of the same order if the photon virtuality is . Received: 30 July 1997     

A simple model for nuclear forces which exhibits bound states

A repulsive core force is derived which, assuming mesons are the field particles, gives binding energies in good agreement with binding energies per nucleon of heavy nuclei. The physical model consists of a field of relatively short range, in which emission of a meson by a nucleon and subsequent absorption by a neighboring nucleon is equivalent to a potential well. The binding energy at the equilibrium spacing of the nucleons is the self-energy of the mesons, which is calculated by analogy with classical electric fields.     

Baryon number violating nuclear decay

The expressions for baryon number violating nuclear partial decay widths are derived from the interactions as predicted by grand unified theories. Theory predicts that the baryon number violating proton decay inside the nucleus is hindered relative to the free proton decay rate. In the case of closed shell nuclei, the meson spin-isospin dependence of the partial width is the same as that for the nucleon decay. The branching ratios of decay amplitudes depend on the nuclear binding energies. Nuclear structure introduces lepton energy spread of ±49.5 MeV for light closed shell nuclei, while it does not affect the back to back emission of lepton-meson pair.     

Photoproduction of mesons off the deuteron: quasifree and coherent processes

Photoproduction of mesons off the deuteron has been investigated at a tagged photon beam of the Bonn ELSA accelerator with the combined Crystal Barrel - TAPS electromagnetic calorimeter for incident photon energies up to 2.5 GeV. The mesons have been detected in coincidence with recoil protons, neutrons and deuterons. This allow the measurement of meson production reactions off the quasifree nucleons bound in the deutron, as well as the coherent production off the deuteron. The comparison of quasifree proton reactions to free proton reactions can confirm or invalidate possible nuclear effects on the extracted cross section reactions. Furthermore the isospin composition of a resonance can be estimated from the comparison of quasifree proton and neutron reactions. The quasifree photoproduction of the η' and π~°η mesons off nucleons and the coherent photoproduction of π~°η-pairs off the deuteron are discussed.     

核子电磁形状因子的渐近行为与Callan-Symanzik方程

本文论证了对赝标介子与核子耦合的理论,齐次Callan-Symanzik方程可用来得到电磁形状因子的渐近行为。由此得到这个理论中核子电磁形状因子的微扰论领头项,并讨论了较一般的情形。     

Temperature Dependent Relativistic Microscopic Optical Potential and Mean Free Paths of Nucleons

The microscopic optical potential,mean free paths and Schrodinger equivalent potential of nucleons at finite temperature in nuclear matter are studied based on Walecka's model and thermo field dynamics.We let only the Hartree-Fock self-energy of nucleon represent to be the real part of the microscopic optical potential and the fourth order of meson exchange diagrams,i.e. the core polarization represent the imaginary part of microscopic optical potential in nuclear matter.The microscopic optical potential of finite nuclei is obtained with the local density approximation.     

Threshold π0 photoproduction from the Skyrmion

The Skyrme model for nucleons is used to calculate threshold π⁰ photoproduction off a proton. It was found that the anomalous part of the electromagnetic current yields an important contribution to the photoproduction amplitude, roughly of the same magnitude normally ascribed to ω meson exchange. Agreement with the experimental threshold cross section can thus be achieved without invoking ω exchange.     

On one pion exchange potential with quark exchange in the resonating group method

The effect of quark exchange between different nucleons on the one pion exchange potential is studied in the framework of the resonating group method. The calculated phase shifts including the one pion exchange potential with quark exchange in addition to the one gluon plusσ meson exchange are shown to be consistent with experiments. Especially thep-wave phase shifts are improved by taking into account the quark exchange on the one pion exchange potential.     

Role of excited nuclear states in photoabsorption shadowing at medium energies

The effect of intranuclear motion of nucleons on the photoabsorption shadowing due to the conversion of a γ-ray photon into a vector meson in nuclear matter is analyzed. It is shown within the method of time-correlation functions that, in comparison with the results of the standard approach, the correction to the total photoabsorption cross section due to the nuclear shadowing is determined by two additional factors: the Fermi motion of nucleons in a nucleus and nucleon recoil.     

Nuclear multifragmentation by 700–1500 MeV photons: New data of GRAAL experiment

The cross sections of carbon nucleus photodisintegration into protons and neutrons with high multiplicity for photon energies from 700 to 1500 MeV were measured. The experiment was performed at the tagged photon beam of the GRAAL setup using the wide-aperture detector LAGRANγE. It was shown that multifragmentation up to complete disintegration into separate nucleons is initiated by elementary reactions of meson photoproduction with a subsequent intranuclear cascade.     

Intra-Nuclear Cascade (INC) code for simulation of meson photoproduction in nuclei

η meson photoproduction in the ¹⁴N nucleus was studied using simulations done with INC (Intra-Nuclear Cascade) code. The probability for mesons and nucleons to escape the nucleus on different steps of the reaction was evaluated. These results predict the possibility of studying elastic and inelastic η meson interactions with intra-nuclear nucleons using the recoil proton as a tag for the case of slow η meson and fast recoil proton.     

Massive Meson Fluctuation in NJL Model

Based on the self-consistent scheme beyond the mean-field approximation in the large N_c expansion, including current quark mass explicitly, a general scheme of SU(2) NJL model is developed. To ensure the quark self-energy expanded in the proper order of N_c,an approximate internal meson propagator is deduced, which is in order of O(l/N_c). In our scheme, adopting the method of external momentum expansion, all the Feynman diagrams are calculated in a unified way by only expanding the quark propagator. Our numerical results show that, different &om the mean-field approximation in which the explicitly chiral symmetry breaking is invisible, the effect of finite pion mass can be seen clearly when beyond the meanfield approximation.     

Properties of nucleons in the relativistic quark model with strong binding

Baryons are considered as the strongly bound system of three heavy quarks. The dynamics of the model is described by the Bethe-Salpeter equation with harmonic oscillator interaction. Two different spin structures of the interaction kernels (taken from the meson case) are considered. It is found that one of them fails to reproduce the spectrum of baryons in the ground state. The second spin structure reproduces the spectrum. The model gives good results for the magnetic moments of nucleons and for the weak currents matrix element between the neutron and proton. It is also shown that the properties of quark in our model are similiar to those of the quark model for mesons and thus both these models can be treated together.