EXTENDED CLOUDY BAG MODEL AND NUCLEAR FORCE

By means of the Cloudy Bag Model (CBM), the contributions of pion field to nucleon-nucleon potential are calculated. It is shown that the results given by CBM are in good agreement with One-Pion-Exchange-Potential (OPEP) at large distance. Futhermore, in order to take into account the contributions of vector meson to nuclear force, we extend the CBM to include the vector meson and quark-antiquark-vector meson interaction. Using the Extended Cloudy Bag Model (ECBM) and Breit-Fermi equation of two nucleon system, through Foldy-Wouthuysen transformation, we get the nucleon-nucleon interaction which is in good agreement with naive One-Vector-Meson-Exchange-Potential (OVEP) in nonrelativistic limit, but effects of nucleon structure to nuclear force (form factor) are given.     

The nuclear symmetry energy from relativistic Brueckner-Hartree-Fock model

The microscopic mechanisms of the symmetry energy in nuclear matter are investigated in the framework of the relativistic Brueckner-Hartree-Fock (RBHF) model with a high-precision realistic nuclear potential, pvCDBonn A. The kinetic energy and potential contributions to symmetry energy are decomposed. They are explicitly expressed by the nucleon self-energies, which are obtained through projecting the G-matrices from the RBHF model into the terms of Lorentz covariants. The nuclear medium effects on the nucleon self-energy and nucleon-nucleon interaction in symmetry energy are discussed by comparing the results from the RBHF model and those from Hartree-Fock and relativistic Hartree-Fock models. It is found that the nucleon self-energy including the nuclear medium effect on the single-nucleon wave function provides a largely positive contribution to the symmetry energy, while the nuclear medium effect on the nucleon-nucleon interaction, i.e., the effective G-matrices provides a negative contribution. The tensor force plays an essential role in the symmetry energy around the density. The scalar and vector covariant amplitudes of nucleon-nucleon interaction dominate the potential component of the symmetry energy. Furthermore, the isoscalar and isovector terms in the optical potential are extracted from the RBHF model. The isoscalar part is consistent with the results from the analysis of global optical potential, while the isovector one has obvious differences at higher incident energy due to the relativistic effect.     

对称核物质中核子-核子散射截面的研究

在扩展的Brueckner-Hartree-Fock (BHF)理论框架下, 采用Argonne V₁₄ 两体相互作用势研究了对称核物质中核子-核子散射的总截面和微分截面, 分别讨论了三体核力(TBF)重排效应和基态关联效应对全同和非全同核子散射截面的影响。 计算表明: 低动量区域的基态关联效应会导致介质中核子-核子散射截面的增大; 而随着密度的增加,TBF重排效应的逐渐加强会减小介质中的核子-核子散射截面。 The nucleon nucleon cross sections in symmetric nuclear matter were investigated in the framework of the extended Brueckner-Hartree-Fock(BHF) approach with Argonne V₁₄ two body interaction. The influences of the ground state correlation and the rearrangement contribution of the three body force (TBF) on the cross section have been obtained and discussed separately for identical and non identical nucleon collisions. It is shown that the magnitudes of the cross section are increased by the effects of the ground state correlation in low momentum transfers,and are suppressed in medium with increasing density when the rearrangement contribution of the TBF force is considered.     

△contribution to the parity-violating nucleon-nucleon force

Because the nucleon may be excited and transformed into a virtual △ resonance easily,we consider the decuplet contribution to the parity-violating(PV)nucleon-nucleon interaction in the chiral effective field theory.The effective PV nucleon-nucleon potential is derived without introducing any unknown coupling constants.     

Relations between the Skyrme force parameters and the nucleon-nucleon forces within the quasi-Hartree-Fock-theory

The energy-density for the Skyrme forces in the Hartree-Fock-approximation is compared with the energy-density in the quasi-Hartree-Fock-approximation for two-body nucleon-nucleon forces with hard-core repulsion. From these expressions the magnitudes of the corresponding Hartree-Fock parameters of the nucleon-nucleon force are estimated. As an example, a simple nucleon-nucleon force is constructed, which reproduces the results of the Skyrme force SII. The ambiguities of such an comparison are discussed. The Skyrme forces SII-SIV correspond to potentials with hard-core radii ≈0.5–0.7 F. Smaller hard-core radii correspond to larger exchange contributions.     

Hadronic parity violation in pionless effective field theory

We present results for two-body observables that are sensitive to the parity-violating component of nucleon-nucleon interactions. These interactions are studied using an effective field theory in which the only dynamic degrees of freedom are nucleon fields. The observables we study are cross-section asymmetries in nucleon-nucleon scattering and asymmetries and induced polarizations in the process np??d??.     

Nuclear effective interactions and spin excitations

In view of the one-boson-exchange model for the nucleon-nucleon interaction and the Hartree-Fock (HF) interaction, we formulate the effective interactions for particle-hole states in terms of the exchange of the fields which are confined in the nucleus. This theory, as an extension to the nuclear field theory (NFT), takes into account the propagation of the fields which is neglected in NFT. The effective interactions thus obtained reproduce the energies of a sequence of electric giant resonances and the core polarizabilities associated with the resonances. It is found that the coupling constants of the σ- and ω-fields are suppressed for the particle-hole interaction by 60% with respect to the HF interaction. As for the effective interactions involving nucleon spins, we consider the fields coupled to nucleon spins. The effective interactions obtained, essentially different from those in NFT, have a tensor component. We analyse the energies and cross sections for excitation of stretched spin particle-hole states which are the most sensitive to the tensor force. The effective interaction responsible for the stretched spin states is shown to be consistent with that for the magnetic resonances observed in the (p, n) reactions.     

A time reversal violating interaction in the two-nucleon system

Based on a direct coupling of the nucleon, photon and pion field introduced by Clement and Heller we calculated a time reversal noninvariant nucleon-nucleon potential. It has the pion range, does not conserve total spin and acts only between a neutron and a proton. Its effect on certain nucleon-nucleon observables sensitive to time reversal violation are evaluated.     

The 1/Nc Expansion in Hadron Effective Field Theory

We study the N_c scalings of pion-nucleon and nucleon-nucleon scatterings in hadron effective field theory. By assuming Witten's counting rules are applied to matrix elements or scattering amplitudes, which use the relativistic normalization for the nucleons, we find that the nucleon axial coupling g_A is of order N_c0, and a consistent large N_c counting can be established for the pion-nucleon and nucleon-nucleon scatterings. We also justify the nonperturbative treatment of the low energy nucleon-nucleon interaction with the large N_c analysis and find that the deuteron binding energy is of order 1=N_c.     

On the Coupling of φ Meson to Nucleons and Backward φ Production

Some analyses of the electromagnetic form factors of the nucleon and some analyses of the nucleon-nucleon potential favor a large coupling of the φ meson to nucleons. This is frequently quoted by advocates of a large ss component of the nucleon. It is shown that such large OZI-violating couplings are incompatible with data on φ production in backward direction via nucleon exchange.     

Δ(3, 3) excitations and effective three-nucleon forces in finite nuclei

Contributions from three-body terms with intermediate Δ(3,3) isobar excitations to the ground state energies of nuclei are investigated. These terms can either be understood as three-body clusters in a many-body theory including isobar excitations explicitly or as contributions to an effective three-nucleon force. For the example ¹⁶O the resulting contribution is attractive and its value is typically about ?0.5 MeV per nucleon. This is smaller than the typical values of 1 MeV per nucleon repulsion obtained from the modifications of the effective two-body nucleon-nucleon interaction in nuclei due to intermediate Δ(3, 3) configurations. The gain in energy from the three-body terms including Δ(3,3) configurations, however, is of the same importance as the contribution from three-body terms including nucleons only.     

Proton-neutron and proton-proton correlations in medium-weight nuclei and the role of the tensor force

A detailed analysis of the effect of short-range and tensor correlations on one- and two-nucleon momentum distributions of medium-weight nuclei (12相似文献   

Neutrino propagation in a medium with a magnetic field

Data on the mean multiplicity of strange hadrons produced in minimum bias proton-proton and central nucleus-nucleus collisions at momenta between 2.8 and 400 GeV/c per nucleon have been compiled. The multiplicities for nucleon-nucleon interactions were constructed. The ratios of strange particle multiplicity to participant nucleon as well as to pion multiplicity are larger for central nucleus-nucleus collisions than for nucleon-nucleon interactions at all studied energies. The data at AGS energies suggest that the latter ratio saturates with increasing masses of the colliding nuclei. The strangeness to pion multiplicity ratio observed in nucleon-nucleon interactions increases with collision energy in the whole energy range studied. A qualitatively different behaviour is observed for central nucleus-nucleus collisions: the ratio rapidly increases when going from Dubna to AGS energies and changes little between AGS and SPS energies. This change in the behaviour can be related to the increase in the entropy production observed in central nucleus-nucleus collisions at the same energy range. The results are interpreted within a statistical approach. They are consistent with the hypothesis that the Quark Gluon Plasma is created at SPS energies, the critical collision energy being between AGS and SPS energies.     

Pion multiplicity in nuclear collisions

Data on the mean multiplicity of ^- produced in minimum bias proton-proton, proton-neutron and proton-nucleus interactions as well as central nucleus-nucleus collisions at momenta of 1.4–400 GeV/c per nucleon have been compiled and studied. The results for neutron-neutron and nucleon-nucleon interactions were then constructed. The dependence of the mean pion multiplicity in proton-nucleus interactions and central collisions of identical nuclei are studied as a function of the collision energy and the nucleus mass number. The number of produced pions per participant nucleon in central collisions of identical nuclei is found to be independent of the number of participants at a fixed incident momentum per nucleon. The mean multiplicity of negatively charged hadrons per participant nucleon for central nucleus-nucleus collisions is lower by about 0.12 than the corresponding multiplicity for nucleon-nucleon interactions atp _LAB 15 A·GeV/c, whereas the result at 200 A·GeV/c is above the corresponding nucleon-nucleon multiplicity. This may indicate change of the collision dynamics at high energy.     

A density expansion of the pion-nucleus optical potential. II. Second order terms

The second order terms in a density expansion of the pion optical potential V_opt are evaluated quantitatively. The coefficients of these terms are proportional to various combinations of on- and off-shell nucleon-nucleon T-matrices, averaged over the distribution of two nucleon relative momenta in the Fermi sea. The on-shell contributions can be obtained directly from experimental phase shifts, but the calculation of the off-shell-parts requires a model for the nucleon-nucleon potential. We consider a number of realistic local and nonlocal, separable potentials which fit nucleon-nucleon phase shifts, in order to study the variations in V_opt which arise from differences in the off-shell behavior of T. We find absorptive (imaginary) and dispersive (real) contributions to V_opt which are of comparable magnitude. The dispersive part, which leads to a real energy shift associated with the two nucleon absorptive process, has not been previously estimated quantitatively. We compare our results to empirical potentials obtained by fitting energy level shifts and widths in pi-mesic atoms, as well as theoretical estimates based on threshold cross sections for the processes π + N + N ? N + N.     

Nucleon-nucleon interaction in a “non-symmetric” quark model

We extend the work of Warke and Shanker on the nucleon-nucleon interaction to include colored nucleon clusters. By treating a two-nucleon state as a mixture of a color singlet-singlet component and a color octet-octet component, it is shown that, with a few percent admixture of the octet-octet component, a Coulomb-type quark-quark potential can generate a nucleon-nucleon potential comparable to the phenomenological central-force potentials. It also leads to a color van der Waals potential which is too strong in comparison with experiment.     

Special features of detectors, electronics, and trigger system of the ALICE setup

Special features and parameters of detectors, front-end electronics, and trigger electronics of the ALICE setup, which is intended for investigations of ultrarelativistic nucleon-nucleon collisions at the LHC and for studies of heavy ion collisions, starting from protons to several types of ions, which have 5.5 TeV/nucleon energy in the center of mass, are described. One of the first collisions of lead ions was recorded by the ALICE detector on November 8, 2010.     

Nonlocality in the Nucleon-Nucleon Interaction Due to Minimal-Relativity Factors: Effects on Two-Nucleon Observables and the Three-Nucleon Binding Energy

Minimal-relativity factors, when multiplied to a static local mesonexchange nucleon-nucleon potential, lead to a nonlocal potential. It is shown that the nonlocality can be represented by an analytically given nonlocality function, which has a width inversely proportional to the nucleon mass and which tends towards δ(r − r), the usual locality condition, in the limit of the nucleon mass going to infinity. Consequences of this particular type of non-locality for two-nucleon observables and for the three-nucleon binding energy are investigated along with a Malfliet-Tjjon-type potential. After readjusting the potential parameters of the nonlocal potential such that its two-nucleon properties are well matched to the ones of the static local potential the effect of the nonlocality on the three-nucleon binding energy essentially vanishes. Received October 9, 1995; revised December 30, 1995; accepted for publication March 12, 1996     

Self-consistent and optical potentials for nucleons at positive and negative energies

The properties of the Hartree-Fock potential that coincides with the self-consistent potential and with the real part of the optical potential for nucleons in the case of a global regime of averaging are investigated by consistently taking into account the velocity-dependent components of nucleon-nucleon forces and nonlocality effects. For the first time, the properties of the effective energy-dependent Hartree-Fock potential are analyzed at negative nucleon energies. It is shown that the form of this potential undergoes a significant change upon reversal of the sign of nucleon energy. The conditions of applicability of the semiclassical approximation are found.