Structure functions of bound nucleons: From the EMC effect to nuclear shadowing

We describe the nuclear structure functions in the whole range of the Bjorken variablex, by combining various effects in a many-step procedure. First, we present a QCD motivated model of nucleons, treated, in the limit of vanishingQ ², as bound states of three relativistic constituent quarks. Gluons and sea quarks are generated radiatively from the input valence quarks. All parton distributions are described in terms of the confinement (or nucleon's) radius. The results for free nucleons are in agreement with the experimental determinations. The structure functions of bound nucleons are calculated by assuming that the main effect of nucleon binding is stretching of nucleons. The larger size of bound nucleons lowers the valence momentum and enhances the radiatively generated glue and sea densities. In the small-x region the competitive mechanism of nuclear shadowing takes place. It also depends on the size of the nucleons. By combining stretching, shadowing and Fermi motion effects (the latter confined to very largex), the structure function ratio is well reproduced. Results are also presented for theA-dependence of the momentum integral of charged partons, the nuclear gluon distribution and the hadron-nuclei cross sections.     

INVESTIGATION OF THE NUCLEAR SHADOWING EFFECT IN THE SMALL x REGION

In this paper,we analyse the spatial overlapping behavior of sea quarks of neighboring nucleons in nuclei in the small x region and give a relation between the nuclear shadowing factor R(x,Q²,A) and the spatial overlapping factor ΔV_A(x)/V_A(x).By using this nuclear shadowing factor R(x,Q²,A),we calculate the ratios of the average nuclear structure functions of nuclei ¹²C、⁶³Cu and ¹¹⁶Sn,to the deuteron structure function and obtais a better agreement between the calculated result and the experimental data.     

THE RELATIVISTIC NUCLEAR MODEL AND EMC EFFECT

A correct relativistic nuclear model is presented on the ratios of structure functions F^A(x,Q²) measured in DIS with several different nuclei.We shall assume here that a nucleus consists of nucleons,antinucleons and excess pions,the pions being associate with the meson exchange forces responsible for nuclear binding,only the baryon number is conserved.It is also considered that shadowing-antishadowing and Fermi motion.Our results are consistent with the available recent data.     

THE EFFECTS INDUCED BY A Λ HYPERON ON THE PAIRING CORRELATION IN NUCLEUS

In this paper, the effects induced by a Λ hyperon on the pairing correlation in nucleus are investigated. For simplicity, the δ-type A-N residual interaction is adopted. We have calculated the energy Spectrum and wave function of the low-lying excited states for _Λ¹⁰O. Then using infinite sum approximation, the correlation effect for two nucleons in the high energy shell induced by a Λ-hyperon is discussed.Finally. by means of Bose approximation, the correlation effects for n pairs of nucleons induced by a Λ is analysed.     

Extended Glauber Theory and Its Application in Halo Nucleus Scattering

We introduced an extended Glauber theory for a halo nucleus scattering, where the halo nucleons and the nuclear core were treated separately. Expressions of reaction and interaction cross sections of the halo nucleus scattering were derived. We took the halo structure of the projectile nucleus into account and adopted an improved optical limit approximation. In the framework of the extended Glauber theory, we studied the reaction cross section for the halo nucleus ¹⁴Be scattering on a target ¹²C. For comparison, the reaction cross sections of ¹²Be+¹²C were calculated as well. The density distribution of target ¹²C is taken from experiments, and those of the projectiles ¹²Be and ¹⁴Be were obtained by two methods. One is that the harmonic oscillator wave functions for ¹²Be and ¹⁴Be are used. The length of harmonic oscillator is adjusted to reproduce the reaction cross section of ¹²Be+¹²C at the high energy E=790MeV/u . The density distribution of ¹⁴Be was also calculated self-consistently in the relativistic mean field (RMF) theory, with a long tail wave functions for the two neutrons in ¹⁴Be. It was found that the calculated reaction cross sections for ¹²Be+¹²C at E=790MeV/u and E=56.5MeV/u were in good agreement with the experimental data no matter harmonic oscillator or RMF wave functions were used. In contrast, the experiments of the reaction cross sections for ¹⁴Be+ ¹²C could only be reproduced when the wave functions of two 2s_1/2 neutrons spreaded over with a long tail. It comes to a conclusion that two outside neutrons in ¹⁴Be form a halo structure.     

Nuclear dependence of structure functions in coordinate space

The momentum distributions of partons in bound nucleons are known to depend significantly on the size of the nucleus. The Fourier transform of the momentum (x _Bj) distribution measures the overlap between Fock components of the nucleon wave function which differ by a displacement of one parton along the light cone. The magnitude of the overlap thus determines the average range of mobility of the parton in the nucleon. By comparing the Fourier transforms of structure functions for several nuclei we study the dependence of quark mobility on nuclear size. We find a surprisingly small nuclear dependence (< 2% for He, C and Ca) for displacements t = z ? 2.5 fm, after which a nuclear suppression due to shadowing sets in. The nuclear effects observed in momentum space for x _Bj ? 0.4 can be understood as a reflection of only the large distance shadowing in coordinate space.     

Study of Multi-Quasiparticle Excited States in 139Ce

High-spin states in ¹³⁹Ce have been populated using the ¹³⁰Te(¹²C,3n) reaction at beam energy of 50MeV. The level scheme of ¹³⁹Ce has been revised and extended greatly up to E=5765.0keV. The level structure of ¹³⁹Ce shows typical characteristics of spherical nucleus, and the high-spin states were formed by the excitations of valence nucleons. Energies of the yrast and near yrast high-spin states in ¹³⁹Ce have been calculated by the empirical shell model, and the multi-quasiparticle nature of high-spin excited states has been discussed.     

The anomalous behaviour of nuclear structure functions revisited

The anomalous behaviour of the nuclear structure functions is discussed in the framework of a simple statistical parton model, where the nucleus is treated as a bag of uncorrelated partons. We show that the model reproduces correctly the main features of the effect and, to some extent, it is even successful at the numerical level. The characteristic prediction of the model (to be tested experimentally) is a saturation law: for largeA (=nuclear mass number) the anomalous nuclear behaviour of the structure functions is described by a universal (i.e.A-independent) function of the Bjorken variable.     

The role of nucleon-nucleon short-range correlations in the nuclear structure function in thex > 1 region

By using the Jastrow correlation wave function, the role of nucleon-nucleon short-range correlations in the nuclear structure function in thex > 1 region is reanalyzed. The result shows that, with the proper energy-momentum distribution of nucleons in the nucleus including the effect of nucleon-nucleon short-range correlations, the experimental data of the nuclear structure function in thex > 1 region can not be well explained. It seems that additional components which go beyond the conventional nuclear physics are necessary to describe the effect.     

Shadowing,dipions and the EMC effect

The EMC effect is thought to be a compound phenomenon that appears when nucleons are bound in a nucleus. The shadowing effect shows up in a very low-x range. It is possible that in low- and medium-x ranges there exist weak-binding regions caused by the nucleon distortion in the nucleus as well as by exchange pions among the nucleons and other pions in weak-binding regions. Both these types of pions will be in the dipion state because of the Coulomb field. The dipion effect contributes to the structure function of the nucleus. Nuclear binding energy and Fermi motion in medium- and higher-x ranges cause a shift of the ratio of the nucleus structure function to the deuteron one. Calculations show that better agreement with recent data is obtained if the above three effects are mixed.     

Single Particle Schr dinger Fluid and Moments of Inertia of Deformed Nuclei

We have applied the theory of the single-particle Schrodinger fluid to the nuclear collective motion of axially deformed nuclei. A counter example of an arbitrary number of independent nucleons in the anisotropic harmonic oscillator potential at the equilibrium deformation has been also given. Moreover, the ground states of the doubly even nuclei in the s-d shell ²⁰Ne,²⁴Mg,²⁸Si,³²S and ³⁶Ar are constructed by filling the single particle states corresponding to the possible values of the number of quanta of excitations n_x,n_y, and n_z. Accordingly, the cranking-model, the rigid-body model and the equilibrium-model moments of inertia of these nuclei are calculated as functions of the oscillator parameters ω_x,ω_yand ω_z which are given in terms of the non deformed value ω00 , depending on the mass number A, the number of neutrons N, the number of protons Z, and the deformation parameter β. The calculated values of the cranking-model moments of inertia of these nuclei are in good agreement with the corresponding experimental values and show that the considered axially deformed nuclei may have oblate as well as prolate shapes and that the nucleus ²⁴Mg is the only one which is highly deformed. The rigid body model and the equilibrium model moments of inertia of the two nuclei ²⁰Ne and ²⁴Mg are also in good agreement with the corresponding experimental values.     

How exotic are very-ordinary nuclei?

The nuclear structure physicist does not have to have concern for the structure of the nucleons that make up the complex nucleus although the details of their mutual interactions is a matter of paramount importance. But nucleons indeed have a structure of their own and we must hope to understand the interactions between them in terms of that structure; we must also understand the way in which those interactions modify the intrinsic structures of the nucleons involved in them. Present ways of discussing nucleon structure are reviewed from the standpoint of a nuclear structure physicist with occasional illuminations from that field.     

Alpha-particle reactions

Recent work using the concept of alpha-clustering in studies of nuclear structure and nuclear reactions is reviewed. The high symmetry and binding energy of the alpha-particle makes it likely that nucleons inside the nucleus can condense into alpha particles and live long enough to affect many properties of nuclei and also the cross-sections of nuclear reactions, particularly those with alpha-particles as the projectile or ejectile. The alpha-particles inside the nucleus may escape, and the resulting alpha decay is now quite well understood. An incident projectile may collide with a transient alpha-particle in the nucleus and knock it out, thus providing information on the degree of clustering. This model enables the direct part of the cross-sections of (n, α) reactions to be calculated. The alpha-particle mean field unifies many of the characteristics of alpha-particle structure and alpha-particle scattering. Many properties of light nuclei may be simply explained using the concept of alpha-clustering.     

THE RIGHT-HANDED CURRENTS OF NEUTRINOLESS DOUBLE β DECAY IN 48Ca

Right-handed Currents in 0νββ decay with lepton-number nonconservation is discussed by using two nucleons mechanism and the effective operator approach of nuclear matrix elements in this paper. The relation between the mixing parameter η of right-handed currents and the Majorana neutrino mass m_ν is calculated from the experimental decay lifetime of 0νββ decay in ⁴⁰Ca. Our results show that i) m_ν≤(18~23)eV, if |η|=0;ii) |η|≤(1.6~2)×10^－5, if m_ν=0     

Deep processes in nuclei and in-medium dressing of the nucleons and mesons

K⁺ scattering and quasielastic electron scattering from nuclei are expected to provide information about the nucleons and mesons in the inner regions of nuclei. The K⁺- nucleus cross sections and the quasielastic electron-nucleus response functions have been calculated taking into account the same in-medium dressing of the nucleons and the same coupling of the σ and ω mesons to the polarization of nuclear matter. We obtain a good agreement with experimental data for the two processes.     

Distorted wave theory of scattering of nucleons with intermediate energies on nuclei

In the context of nonrelativistic theory in the distorted wave approximation, a three-dimensional form of analytical expression for the differential cross section of scattering of nucleons with intermediate energies on atomic nuclei is derived. In the context of this theory, the main parameters of elastic scattering of protons with incident energy of 1 GeV on the ²⁰⁸Pb nucleus are determined. For inelastic scattering of protons with nuclear surface vibrations, giant multipole resonances in the excited nucleus are investigated for the collective nucleus model. The energy losses of the scattered proton are calculated together with the energies of giant dipole and quadrupole resonances and nuclear surface vibration energy. This allows the deformation parameter of the excited nucleus to be calculated.     

Systematics of ground state multiplets of atomic nuclei in the delta-interaction approach

Pairing forces between nucleons in an atomic nucleus strongly influence its structure. One of the manifestations of pair interaction is the ground state multiplet (GSM) formation in the spectrum of low-lying excited states of even–even nuclei. The value of GSM splitting is determined by the value of pair interaction of nucleons; for each isotope, it can be estimated on the basis of experimental nuclear masses. The quality of this estimate is characterized by the degree of reproduction of GSM levels in the nucleus. The GSM systematics in even–even nuclei with a pair of identical nucleons in addition to the filled nuclear core is considered on the basis of delta interaction.     

ON THE TRANSFER MECHANISM OF 16O+24Mg AND 20Ne+24Mg ELASTIC SCATTERING

The mechanism of transfering a cluster of nucleons between two colliding nuclei is considered to explain the backward angle oscillatory rise in the differential cross section of the elastic scattering between certain nuclei, such as ¹⁶O+²⁴Mg or ²⁰Ne+²⁴Mg. The nuclear molecular orbit approximation theory is applied. For one-step transfer, if the parameter involved is assumed to be adjustable, the numerical calculations can be made to fit the experimental results naturally.     

核内核子部分子的逸出及共用与核子结构函数的核效应

在核介质中,可能存在非核子自由度,这些非核子自由度是核内核子中逸出的部分子组成的色单态集团.另外,由于核内部分子在核子运动方向上的不确定性,致使较小工的部分子的运动范围进入邻近核子,成为几个核子的共用部分子.在考虑了部分子逸出与共用的因素后,用有效核子质量一个参数,计算了束缚核子的结构函数,所得结果和NMC组最新实验数据符合甚好.