Density dependence of nucleon radius and mass in the global color symmetry model of QCD with a sophisticated effective gluon propagator

Making use of the global color symmetry model (GCM) at finite chemical potential and with a sophisticated effective gluon propagator, the density dependence of the bag constant, the total energy and the radius of a nucleon in nuclear matter is investigated. A maximal nuclear matter density for the existence of the bag with three quarks confined within is given as about 8 times the normal nuclear matter density. The calculated results indicate that, before the maximal density is reached, the bag constant and the total energy of a nucleon decrease, and the radius of a nucleon increases, with the increasing of the nuclear matter density. As the maximal nuclear matter density is reached, the mass and the bag constant of the nucleon vanish and the radius becomes infinite suddenly. It manifests that a phase transition from nucleons to quarks takes place. Meanwhile, shortening the interaction range among quarks can induce the phase transition to happen easier.     

Critical analysis of quark-meson coupling models for nuclear matter and finite nuclei

Three versions of the quark-meson coupling (QMC) model are applied to describe properties of nuclear matter and finite nuclei. The models differ in the treatment of the bag constant and in terms of non-linear scalar self-interactions. In two versions of the model the bag constant is held fixed at its free-space value whereas in the third model it depends on the density of the nuclear environment. As a consequence opposite predictions for the medium modifications of the internal nucleon structure arise. After calibrating the model parameters at equilibrium nuclear matter density, binding energies, charge radii, single-particle spectra and density distributions of spherical nuclei are analyzed and compared with QHD calculations. For the models which predict a decreasing size of the nucleon in the nuclear environment, unrealistic features of the nuclear shapes arise.     

LONG RANGE POTENTIAL BETWEEN NUCLEONS

Using hybrid chiral bag model, one gets a model which can be used to calculate potential between two separate nucleons. It is shown that the obtained results, centre and tensor potential, are compatible with phenomenological potentials such as Hamada-Johnston potential and Reid potential. Whether the bag radius for a free nucleon is different from that of a nucleon in the nuclear matter is discussed.     

Influence of the nuclear bulk properties and the MIT bag constant on the phase transition to the quark gluon plasma

We consider the influence of the bulk properties of nuclear matter, namely the ground state incompressibility and the effective nucleon mass, and of the MIT bag constant on the phase transition from hadron matter to quark gluon plasma. It is mainly the effective nucleon mass which determines the stiffness of the equation of state and therefore also the behaviour of the phase transition curves. The energy densities in the coexistence region are found to increase for finite chemical potentials and softer equations of state up to 10 GeV/fm³. For small bag constants and for softer nuclear equations of state the phase boundary exhibits unusual deformations, due to the fact that the phase transition sets in already at pressures not too far from the saturation value. Although this would increase the experimental possibility to create the QGP, it is more likely that one must regard bag constants in the range of the original MIT value as not producing a realistic behaviour of the quark-hadron matter phase transition in the context of an MIT bag equation of state for the quark side.     

THE PROPERTIES OF NUCLEAR MATTER AT LOW TEMPERATURE IN A POSSIBLE QUARK MECHANISM

In this paper,the nucleon is described by the MIT bag model,and the internal quark motion in the nucleon is modified by the scalar and vector meson fields.The Fermi motion of nucleon in nuclear matter is considered.The changes for intrinsic properties of nucleon in nuclear matter at different temperature are calculated as a function of the density.The binding energy per nucleon for different temperature is given.     

Properties of the nucleon in the nuclear medium within a chiral quark-meson theory

The modifications of the nucleon structure due to the presence of an external baryon medium are investigated in a chiral quark meson theory. To that end the Nambu-Jona-Lasinio (NJL) model is combined with the projected chiral soliton model. The medium effects are incorporated using the medium modified values of the pion decay constant and the pion and sigma masses at finite density. These values are evaluated within the NJL model. Using functional integral techniques the latter is solved in a quark continuum at finite density. The effective meson values serve to fix the parameters of the linear chiral sigma model which is solved in a variational projected mean field approach in order to obtain the nucleon properties. All nucleon properties show modifications in the medium except for the pion nucleon coupling constant. The proton radius shows an increase of 19% and the nucleon mass a decrease of 17% if the medium reaches nuclear matter density. The magnetic moments and axial vector coupling constant are less modified. All form factors show remarkable reduction at finite transfer momenta.     

QMC模型的介质相关参数的确定及核物质中的夸克凝聚

通过把QMC模型参数展到σ场的一阶项来引入模型参数的核介质效应,并利用核子袋在介质中的平衡条件自洽地确定展开系数.计算结果表明袋参数及核子半径受介质影响较大,而零点运动参数则保持不变.在此基础上,分析了介质相关的模型参数对核物质状态方程及夸克凝聚的影响.     

EMC and polarized EMC effects in nuclei

We determine nuclear structure functions and quark distributions for ⁷Li, ¹¹B, ¹⁵N and ²⁷Al. For the nucleon bound state we solve the covariant quark–diquark equations in a confining Nambu–Jona-Lasinio model, which yields excellent results for the free nucleon structure functions. The nucleus is described using a relativistic shell model, including mean scalar and vector fields that couple to the quarks in the nucleon. The nuclear structure functions are then obtained as a convolution of the structure function of the bound nucleon with the light-cone nucleon distributions. We find that we are readily able to reproduce the EMC effect in finite nuclei and confirm earlier nuclear matter studies that found a large polarized EMC effect.     

Zero-point motion in the bag description of the nucleon

In the bag model, confinement of quarks is accomplished by introduction of a boundary condition at some definite radius $\text{R}$, where the energy of the total system is a minimum. This minimum is, however, realtively shallow and energies for substantially different bag radii are not much larger than this minimum value. This indicates that the zero-point motion of the bag surface may be important.In this paper, quantization of the bag surface motion is carried out in a somewhat ad hoc fashion, modelled after the generator coordinate theory in nuclear physics. This procedure unifies a number of ideas previously in the literature; it stresses the anharmonicity of the collective motion. As in earlier treatments, the Roper resonance emerges as a breathing-mode type of excitation of the nucleon.The one- and two-pion decays of the Roper resonance are calculated and the widths are found to fall short of the empirical ones. It is pointed out, however, that decays involving intermediate states containing virtual ρ-mesons will enhance the widths. Pion-nucleon scattering in the P₁₁ channel is constructed in our model and found to agree roughly with experiment. A crucial term in the driving force involves the pion coupling to the nucleon through a virtual ρ-meson.With introduction of zero-point motion of the bag surface, the notion of “bag radius” becomes dependent on precisely which moment of the radius is measured. Our development gives a model for cutting off smoothly the pion-exchange term in the nucleon-nucleon interaction.     

ρ-Meson coupling to the nucleon in the chiral bag model

The vector and tensor couplings of the ρ-meson to the nucleon are calculated by assuming the indirect coupling of the ρ-meson to the nucleon, through the pion cloud of the chiral bag model. Qualitative agreement with experiment is found at reasonable bag radii for both versions of the model: the cloudy bag model with pions in the interior, and the standard model with pions excluded from the bag.     

A compressible bag model and equation of state for high density nuclear matter

We propose a compressible bag model, in which a nucleon bag responds microscopic thermal pressure of the other bags. The volume exclusion effect and the particle exchange type interaction ensure the saturation property of the nucleus at normal density and bring about the deconfinement transition in high density region. The critical values of chemical potential and baryon number density for nuclear/neutron matter are estimated. Our equation of state is applied to neutron stars, and shown to be consistent with the observed rotation periods of millisecond pulsars.     

密度相关袋常数的夸克介子耦合模型关于核物质和有限核的研究

由满足手征对称的Nambu-Jona-Lasinio(NJL)模型出发, 计算得到袋常数的密度相关性, 并将其与夸克介子耦合(QMC)模型相结合, 研究核物质和有限核的性质. 结果表明, 该模型能够成功地描述核物质的性质, 但得到的有限核结合能过大, 其中重整势起了非常关键的作用.     

A chiral bag model with a soft surface : structure and solutions of the Fuzzy and the modified Fuzzy Bag Model

In the present work we propose a new bag model for hadrons, called the modified fuzzy bag model (MFBM). The distinguishing feature of this model is the suppression of the pion field, as it enters the bag, by means of a scalar potential for the pions, while still preserving chiral symmetry. The mechanism of pion suppression in the MFBM is similar to the mechanism of quark suppression in the fuzzy bag model (FBM). The standard chiral transformation for the pion field suffers a natural alteration in the MFBM, and as a result the model is chiral invariant. We present also a discussion of the FBM and study, in the quark sector, the implications of the soft surface of the bag on the expectation value of the mass operator. In the pion-quark sector, we study the effects of the suppression of the pion field on the form factor for the pion-nucleon interaction, on the pion-nucleon coupling constant and on the nucleon axial charge . Calculations of the pion-nucleon form factor exhibit, in particular, an improvement over previous results. The pionic axial current induces, in the MFBM, a nonvanishing and orientation dependent contribution to axial charge. An analysis of the asymptotic behaviour of the axial charge shows that the role of the surface is to increase the difference of the contributions associated to different orientations. Received: 10 March 1997 / Revised version: 14 October 1997     

A METHOD OF COMPUTING THE EFFECTIVE COLOR DIELECTRIC CONSTANT AND THE BAG RADIUS OF NUCLEONS IN NUCLEI

Using the parabolic Fermi distribution of the nucleon number density instead of the average density, a new method of computing the effective color dielectric constant and the nucleon bag radius in nuclei is presented, so that the physical picture is made more realistic, and the results thus obtained are better consistent with the data and have two new features.     

On the instanton-induced portion of the nucleon strangeness II: the MIT model beyond the linearized approximation

If instantons are introduced into the MIT bag model in such a way that the bag radii are allowed to vary, the MIT bag interior can accommodate an instanton density which is by an order of magnitude larger than in the case when the radii are fixed (although it is still significantly smaller than in the non-perturbative QCD vacuum). The instanton contribution to the baryon mass shifts is also correspondingly larger. The instanton-induced part of the scalar strangeness of the nucleon MIT bag is an order of magnitude larger than found previously, within the linearized approximation. The decrease of the model radii (which is associated with the increase of the instanton density) is very favorable from the standpoint of nuclear physics. Received: 7 February 2003 / Revised version: 1 April 2003 / Published online: 23 May 2003 RID="a" ID="a" e-mail: klabucar@phy.hr RID="b" ID="b" e-mail: kkumer@phy.hr RID="c" ID="c" e-mail: dmekter@rudjer.irb.hr RID="d" ID="d" e-mail: bp@phy.hr. Present address: Faculty of Civil Engineering, University of Rijeka, HR-51000 Rijeka, Croatia     

Magnetic moments of the nucleon octet calculated in the cloudy bag model

The formalism of the cloudy bag model is used to calculate the pion coupling to the strange members of the nucleon octet (and delta decuplet). We then calculate the magnetic moments of all members of the octet, including lowest-order pionic corrections. Results are presented as a function of the radius of the bag, R, which is the only true free parameter of the model. Excellent agreement is obtained with experiment for bag radii ranging from 0.8 to 1.1 fm.     

Excluded volume,bag constant and hadron-quark phase transition

The model of hadron-quark phase transitions proposed by Cleymans et al. is modified by taking into account the fact that the nuclear repulsive force is operative between a pair of nucleons or a pair of antinucleons but not between a nucleon and an antinucleon. The phase boundary in the temperature (T)-chemical potential (μ) plane is calculated for some values of the bag constantB and the hard core radiusR. Stability of the normal nuclear matter together with the bag picture for the nucleon yields rather stringent bounds forB as functions ofR. The most probable range of values is estimated to beB ^1/4?150～200 MeV being consistent with the estimate from hadron spectroscopy. For this range ofB, it is improbable to realize the broken chiral phase with deconfining constituent quarks and Goldstone pions at someT and μ.     

Delta Excitation Calculation Studies in Compressed Finite Spherical Nucleus 40Ca

With an oscillator basis, the nuclear Hamiltonian is defined in a no core model space. It consists of an effective nucleon nucleon interaction obtained with Brueckner theory from the Reid soft core interaction, a Coulomb potential, nucleon delta transition potentials, and delta delta interaction terms. By performing spherical Hartree Fock (SHF) calculations with the realistic baryon Hamiltonian, the ground state properties of 40Ca are studied. For an estimate of how the delta degree of freedom is excited, SHF calculations are performed with a radial constraint to compress the nucleus. The delta degree of freedom is gradually populated as the nucleus is compressed. The number of Δ’s is decreased by increasing model space. Large amount of the compressive energy is delivered to create massive Δ in the nucleus. There is a significant reduction in the static compression modulus for RSC static compressions which is reduced by including the Δ excitations. The static compression modulus is decreased significantly by en larging the nucleon model space. The results suggest that inclusion of the delta in the nuclear dynamics could head to a significant softening of the nuclear equation of state.