Nuclear matter equation of state and σ-meson parameters

We try to determine phenomenologically the extent of in-medium modification of σ-meson parameters so that the saturation observables of the nuclear matter equation of state (EOS) are reproduced. To calculate the EOS we have used Brueckner-Bethe-Goldstone formalism with Bonn potential as two-body interaction. We find that it is possible to understand all the saturation observables, namely, saturation density, energy per nucleon and incompressibility, by incorporating in-medium modification of σ-meson-nucleon coupling constant and σ-meson mass by a few per cent.     

QCD susceptibilities, nuclear saturation and two-pion processes

We discuss the in-medium behaviour of the QCD scalar susceptibility and of the sigma mass in a chiral relativistic theory which incorporates the scalar response of the nucleon. We also study the many-body effects in the propagation of the scalar meson in the nuclear medium arising from its coupling to two-pion states. The same modification applies to the QCD scalar susceptibility. We conclude with the nuclear physics implications for the in-medium scalar nucleon–nucleon attraction.     

Effect of a meson cloud on the jet nuclear modification factor in <Emphasis Type="Italic">pA</Emphasis> collisions

We study the effect of the nucleon meson cloud on centrality dependence of the jet nuclear modification factor R _pA. We find that the meson–baryon Fock components may lead to a noticeable deviation of R _pA from unity. Our results for R _pA show the same tendency as that observed by ATLAS in p + Pb collisions at √s = 5.02 TeV. The meson cloud suppresses the central jet events and enhances the peripheral jet events. However, quantitatively the effect is somewhat smaller than in the data.     

The spectral function of the rho meson in nuclear matter

We calculate the modification of a rho meson in nuclear matter through its coupling to resonance-hole states. Starting from a recently proposed model, we include all four star resonances up to 1.9 GeV. In contrast to previous works, we include not only resonances that couple to the rho in a relative p-wave, but also those that couple to an s-wave state. In addition, we solve the equation for the rho spectral function self-consistently. We find that s-wave resonances affect the in medium spectral function of the rho strongly. In the transverse channel the rho meson is, especially at non-zero momentum, completely washed out and can in the presence of nuclear matter no longer be viewed as a resonant excitation of the vacuum. Instead, our model shows a continuum of possible excitations with the quantum numbers of a transversely polarized rho. In the longitudinal channel, however, the rho retains its resonant character in our calculation. As a consequence of the self-consistent treatment we also find a strong enhancement of the widths of the included nucleon resonances in medium.     

On the Bosonic Atoms

We investigate the ground state properties of atoms, in which substitute fermions—electrons by bosons, namely, π^?-mesons. We perform some calculations in the frame of modified Hartree–Fock (HF) equation. The modification takes into account symmetry, instead of antisymmetry of the pair identical bosons wavefunction. The modified HF approach thus enhances (doubles) the effect of self-action for the boson case. Therefore, we accordingly modify the HF equations by eliminating the self-action terms “by hand.” The contribution of meson–meson and meson–nucleon non-Coulomb interaction is inessential at least for atoms with low and intermediate nuclear charge, which is our main subject. We found that the binding energy of pion negative ions A_π^-, pion atoms A_π, and the number of extra bound pions ΔN_π increases with the nuclear charge Z. In particular, for Xe ΔN_π = 4. As an example of a simple process with a pion atom, we consider photoionization that differs essentially from that for electron atoms. Namely, it is not monotonic decreasing from the threshold but has instead a prominent maximum above threshold. We study also elastic scattering of pions by pion atoms.     

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.     

现代核子-核子势

核子-核子二体相互作用是联结 QCD与核多体理论计算的桥梁,一直是核物理研究的主线之一.简单回顾了现代核子-核子相互作用理论的新发展,特别是在同位旋相关性方面.Nucleon nucleon interaction is the key point of nuclear physics, bridging the gap between QCD and the effective interaction appropriate for nuclear many body calculations. The older potential models are no longer suitable for describing the present set of more numerous and much more accurate experimental data without refitting the parameters. In 1990s, both older (classical) phenomenological potentials and meson exchange potentials have already had ...     

Nuclear Matter Properties from a Chiral Soliton Model

We present a recent investigation on properties of nuclear matter within a chrial soliton approach which includes the explicit scalar dilaton-quarkonium meson saturating the quark loop contribution to the scale anomaly of QCD. We discuss the binding energy per nucleon, the compressibility of nuclear matter and the behavior of meson–nucleon coupling constants according to the density variations in the system.     

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     

Mesonic and binding contributions to the nuclear Drell-Yan process

We have evaluated the Drell-Yan cross section in nuclei paying special attention to the meson cloud contribution from pions and ϱ-mesons, for which an accurate calculation using the meson nuclear spectral functions is used. Similarly, the nucleonic contribution is evaluated in terms of a relativistic nucleon spectral function. Fair agreement with experiment is found for different nuclei and the results show a sizeable contribution from the renormalized meson cloud. In order to reproduce the experiment a novel element is introduced, consisting of a gradual energy loss of the incoming proton in its pass through the nucleus which produces a strong A dependence at x₁ large.     

Role of four-quark condensates in QCD sum rules

The QCD sum rule approach to the in-medium behavior of hadrons is discussed for the ω meson, nucleon and D meson. Emphasis is placed on the impact of four-quark condensates and on order parameters of spontaneous symmetry breaking.     

Optical potentials in relativistic meson-nucleon model

The relativistic microscopic optical potentials (RMOP) at E < 300 MeV have been derived and investigated based on Walecka's meson-nucleon model. An effective lagrangian including nucleon, σ- and ω-mesons, which is required to reproduce the nuclear matter saturation properties, has been introduced and used to calculate the self-energy of a nucleon in the nuclear medium. Systematical analyses of the scattering data are performed with the RMOP. Finally, several effects, such as the meson-nucleon vertex form factor, isovector meson exchanges, non-linear σ-model are studied.     

On the isovector form factors of the nucleon in meson theory

We present a calculation of the nucleon isovector form factors for low momentum transfer. Our model is based on skeleton diagrams in pseudoscalar πN interaction where unknown parameters describing higher order corrections are fixed such that a ρ meson is generated dynamically with the correct properties. The normalization of the form factors and the radii agree well with experiment. For ?q ² up to 1 GeV² the contributions of the π θ-intermediate state do not fall off rapidly enough to give perfect agreement with the dipole formulae.     

Nonlinear σ-model and nucleon structure

The nonlinear σ-model with the Wess-Zumino action describes the nucleon as a soliton and incorporates the non-abelian chiral anomalies. Several studies have shown that the model works well except for the nucleon mass, which comes out consistently too large. We investigate this question beginning with the more general framework of the linear σ-model, which has besides a pseudoscalar meson sector, a fermion or quark sector, a scalar field and an interaction between the fermions via the scalar field. Using a path integral formulation, we express the fermion measure of the model as the product of a Jacobian and an invariant measure. Identifying this Jacobian as exp[iΓ _wz] , we find that the model breaks up into two parts, when in the pseudoscalar meson sector the scalar field is replaced by its vacuum value. The pseudoscalar part of the model becomes the nonlinear σ-model with the Wess-Zumino actionΓ _wz. The other part involves chiral fermions, the scalar field and their interaction. We continue this part back to the Minkowski space to determine its ground state and energy levels. We find that for a scalar field that vanishes at smallr, but rises sharply to its vacuum value at someR, the ground state energy of the interacting quark-scalar-field system can be lower than the ground state energy of the non-interacting quark system. This means the interaction between quarks and the scalar field can lead to a condensed ground state or vacuum and can reduce the overall energy of the system (a phase transition as in superconductivity). It is, therfore, not surprising that the nonlinear σ-model predicts too large a nucleon mass, since it implicitly assumes a normal non-interacting vacuum in the quark sector. Quarks are now quasiparticles that appear as excitations of the condensed vacuum. The nucleon structure that emerges from this investigation agrees fully with the phenomenological nucleon structure found from analysis of high energy elasticpp and \(\bar p\) p scattering at CERN ISR and SPS Collider.     

Influence of Self-Interaction ofσ Meson to the Equation of State of Nuclear Matter

In the relativistic σ-ωmodel, the influence of the parameters in self-interaction of a meson to the equation of state of normal nuclear matter, especially, to incompressibility, effective mass, and coupling constants, is studied in detail. We find that these parameters have an intense relationship to the property of nuclear matter. At the same time , we study the relation between the binding energy and pressure of relativestic △-resonance nuclear matter and temperature using using above results in the relativistic σ-ω-π model,and it is interesting to compare it to our prior work. In all these studies, the vacuum fluctuation on nucleon, △-isobar, and σmeson is considered.     

PT violating single particle nuclear potential

We calculate aPT-odd one-loop meson radiative correction to the scattering amplitude of a nucleon on the external field of the nucleus. The nonrelativistic limit of this amplitude, Fourier transformed to the configuration space, can be interpreted as a single particlePT-odd potential for a valence nucleon. Bound state effects on this potential are evaluated and found to be negligible.     

A relativistic quantum field theoretical model for nuclear slab collision dynamics

We treat the dynamics of colliding nuclear slabs in a relativistic quantum field theory by using the relativistic mean field approximation. Starting from Walecka's lagrangian, the nucleons are represented by single-particle spinors determined by a Dirac equation that contains a repulsive mean vector meson field and an attractive mean scalar meson field. Both fields satisfy Klein-Gordon equations whose source terms are again determined by the nucleon spinors. The two equal nuclear slabs are translationally invariant in two transverse dimensions and consist of spin and isospin symmetric nuclear matter. By specification of appropriate initial conditions for the collision, we numerically solve the system of coupled Dirac and Klein-Gordon equations for lab energies per nucleon up to 420 MeV. For small energies the results are similar to TDHF results. The time dependence of the density distribution, the mean meson fields, and the damping of the collision are studied. At the highest bombarding energy retardation effects are taken into account.     

An effective lagrangian with broken scale and chiral symmetry IV: Nucleons and mesons at finite temperature

We study the finite temperature properties of an effective chiral Lagrangian which describes nuclear matter. Thermal fluctuations in both the nucleon and the meson fields are considered. The logarithmic and square root terms in the effective potential are evaluated by expansion and resummation with the result written in terms of the exponential integral and the error function, respectively. In the absence of explicit chiral symmetry breaking a phase transition restores the symmetry, but when the pion has a mass the transition is smooth. The nucleon and meson masses as a functions of density and temperature are discussed.