Electromagnetic Properties of S11 States in a Light Cone Quark Model

Using relativistic spin-flavor wave functions of a Lorentz-covariant light cone quark model, we calculate the electromagnetic form factors of two S11 resonances, N（1535） and N（1650）, and the helicity amplitudes A1/2 and S1/2 for electroexcitation of the S11 resonances from the nucleon. The electromagnetic form factors of these S11 resonances are found to be similar to those of the nucleon in shape, while the charge form factor of neutral N（1650） is nearly zero. The relative peak height of the S11 charge form factors is controlled by the mixing angle common to both resonance wave functions. As in most quark models, there is a systematic overestimate of A1/2 in both N（1535） and N（1650） cases at the photon point. A sizeable S1/2 for all cases is produced as suggested by experiments.     

Approach to Perturbative QCD Results in Transition Amplitudes of Nucleon Negative-Parity Resonances

The scaling behaviors of the nucleon resonance transition amplitudes from perturbative QCD (PQCD) are utilized to parametrize the amplitudes of the first negative-parity nucleon resonance S₁₁(1535). Our analysis indicates that the constraints of the transition amplitude for the S₁₁ resonance at the limit Q²→∞ by QCD sum rule calculations are not applicable at a moderate Q² range of 2.5 ～ 4 GeV² compared with the present available data if the contribution of S₁₁ is dominant in the Q² limit.     

Explicit Confinement Effect on the Helicity Amplitudes of the Low-Lying Nucleon Resonances

Calculations of helicity amplitudes for the low-lying nucleon resonances are displayed based on a non-relativistic constituent quark model with a harmonic oscillator confinement. The explicit effect of quark confinement is shown. Our results show that the effect plays sizable role on some transition amplitudes of S11 (1535) and D13(1520) resonances. The effect on the △(1232) transition amplitudes is less than 10%. However, the effect on the Roper resonance is remarkable but is inconclusive.     

Effect of confined one-gluon-exchange potential and instanton-induced interaction on nucleon–nucleon interaction

The effect of confined one-gluon-exchange potential and instanton-induced interaction potential in the singlet(~1S_0) and triplet(~3S_1) channels for nucleon–nucleon interaction has been investigated in the framework of the relativistic harmonic model using the resonating group method in the adiabatic limit with the Born–Oppenheimer approximation. The contributions of the different components of the interaction potentials have been analyzed.     

Electromagnetic Properties of S11 States in a Light Cone Quark Model

Using relativistic spin-flavor wave functions of a Lorentz-covariant light cone quark model, we calculate the electromagnetic form factors of two S11 resonances, N(1535) and N(1650), and the helicity amplitudes A1/2 and S1/2 for electroexcitation of the S11 resonances from the nucleon. The electromagnetic form factors of these S11 resonances are found to be similar to those of the nucleon in shape, while the charge form factor of neutral N(1650) is nearly zero. The relative peak height of the S11 charge form factors is controlled by the mixing angle common to both resonance wave functions. As in most quark models, there is a systematic overestimate of A1/2p in both N(1535) and N(1650) cases at the photon point. A sizeable S1/2 for all cases is produced as suggested by experiments.     

硫化锂分子的分析势能函数和光谱

应用密度泛函理论的B3LYP方法和6-311++g（d,p）基组,研究Li₂、LiS和Li₂S分子的基态构型.结果表明它们的基电子态分别为X¹Σ_g⁺、X²Π和X¹Σ_g⁺.通过非线性曲线拟合,得到基态LiS和Li₂分子的4参数Murrell-Sorbie分析势能函数,计算它们的光谱参数和力常数.基于多体项展式理论得到了基态Li₂S分子的单重态势能面的分析函数.利用得到的分析势能函数重构基态单重Li₂S分子的旋转图、伸缩图和旋转伸缩图,准确地再现了Li₂S分子的静态特征,如平衡结构,最低能量,合理反应通道.从等值势能面图看出,反应Li+S+Li→Li₂S是一个无阈值反应.S原子攻击Li₂分子的反应通道上,有一个过渡态.Li原子攻击LiS分子通道上也有一个过渡态.     

Lipkin-Meshkov-Glick模型中的能级劈裂与宇称振荡研究

Lipkin-Meshkov-Glick (LMG)模型原本描述的是核物理系统,然而近年来,人们发现它广泛存在于凝聚态物理、量子信息、量子光学中,因此对其研究兴趣正在升温.本文采用精确对角化的方法以及量子微扰理论计算和分析了LMG模型在费米子数量为有限N时的能谱结构.在U(1)极限下给出它的能级精确解,发现其相互交错成渔网结构.而离开U(1)极限,系统的能级总是奇偶宇称成对地分组,形成束缚态,并且宇称会发生振荡,给出了宇称交叉点的临界塞曼场的位置.而达到Z2极限,系统能级则在零塞曼场附近形成劈裂,解析地计算了这些能隙与塞曼场之间关系,并发现对于奇数和偶数的N,各能态宇称的行为有所差别,具体而言,奇数N系统各态在零塞曼场处会发生宇称改变,而偶数N不会.     

Structure Evolution in the Even-Even 124-134Xe with IBM2

We have investigated the structure evolution of the ^124-134Xe isotopic chain in the framework of the proton-neutron interacting model IBM2. The positive parity spectra of the ground state, quasi-β and quasi-γ bands are reproduced well. The staggering in ^124-130Xe are almost completely removed and the γ band agree well with the experiment data, even for the high-spin quasi-γ states. The key quantities of the collective structure evolution, including level energies, the B(E2) transition branching ratios, and the M1 excitations to 1₁⁺ mixer-symmetry states are analyzed by comparing with the experimental data. The parameters for representation of the O_πν(6) and SU_πν^*(3) features in isotopes are examined. Both experimental data and theoretical results show that the shape phase transition of ^124-134Xe isotopic chain is from the SU_πν^*(3) (triaxial rotation) to the U_πν(5) (vibration motion) with a considerable constituent of the O_πν(6) symmetry (γ-unstable rotation), where the shape phase transition rapidly takes place between the neutron number N = 76 and N = 78.     

Extended PCAC Relation and Nucleon Structure

We explore a consistent way to extend the old partially conserved axial vector current (PCAC) relation for a nucleon to a finite q² region. The paper includes an investigation of a chiral Ward-Takahashi identity, the explicit chiral symmetry breaking by a finite current quark mass, the extension of the PCAC relation and the physical consequences. Two explicit relationsbetween the nucleon axial vector form factor g_A(q²), pseudo-scalar form factor g_p(q²) and the pion-nucleon coupling constant g_πNN(q²) are obtained. One of the relations is confirmed, within the experimental error, by observations in the 0 < -q² < 0.2 GeV² region. The other one which. relates g_A(q²) and f_π(q²)g_πNN(q²) is studied by using known empirical facts and dispersion relation. Certain inconsistency, which is in favor of the introduction of a diquark condensation inside a nucleon, is revealed.     

Loop corrections and other many-body effects in relativistic field theories

Incorporation of effective masses into negative energy states (nucleon loop corrections) gives rise to repulsive many-body forces, as has been known for some time. Rather than renormalizing away the three- and four-body terms, we introduce medium corrections into the effective σ-exchange, which roughly cancel the nucleon loop terms for densities _nm, where _nm is nuclear matter density. Going to higher densities, the repulsive contributions tend to saturate whereas the attractive ones keep on growing in magnitude. The latter is achieved through use of a density-dependent effective mass for the σ-particle, m_σ = mσ(), such that m_σ() decreases with increasing density. Such a behavior is seen e.g. in the Nambu-Jona-Lasinio model. It is argued that a smooth transition to chiral restoration implies a similar behavior. The resulting nuclear equation of state is, because of the self-consistency in the problem, immensely insensitive to changes in the mass or coupling constant of the σ-particle.     

Mass and KLamda coupling of the N*(1535)

Using a resonance isobar model and an effective Lagrangian approach, from recent BES results on J/psi-->ppeta and psi-->pK+Lamda, we deduce the ratio between effective coupling constants of N*(1535) to KLamda and peta to be R=gN*(153)KLamda/gN*(1535)peta=1.3+/-0.3. With the previous known value of gN*(1535)peta, the obtained new value of gN*(1535)KLamda is shown to reproduce recent pp-->pK+Lamdanear-threshold cross section data as well. Taking into account this large N*KLamda coupling in the coupled channel Breit-Wigner formula for the N*(1535), its Breit-Wigner mass is found to be around 1400 MeV, much smaller than the previous value of about 1535 MeV obtained without including its coupling to KLamda. The implication on the nature of N*(1535) is discussed.     

Coherent and incoherent η-photoproduction from nuclei

Elastic and inelastic η-photoproduction from complex nuclei is studied in a distorted-wave impulse approximation (DWIA) framework. The elementary operator is obtained by using a dynamical model which employs the reactions πN→πN, πN→ππN and π⁻p→ηn to fix the hadronic vertex as well as the isobar propagators and the process γN→πN to constrain the electromagnetic vertex. The nuclear structure input for the inelastic transitions has been extracted from electron-scattering form factors. The η final-state interaction has been included via a simple optical potential using the ηN t-matrix as an input. We find that coherent η-production is dominated by the D₁₃(1520) isobar while spin-flip transitions to excited nuclear states are sensitive to the S₁₁(1535) resonance. Predictions are given for coherent production on ⁴He, ¹²C and ⁴⁰Ca, as well as incoherent production on ⁶Li, ¹⁰B and ¹²C.     

η-production on the proton via electromagnetic and hadronic probes

The reactions π~-p→ηn and γp→ηp are investigated within a dynamical coupled-channels model of meson production reactions in the nucleon resonance region. The meson-baryon channels included are πN, ηN, π△, σN, and ρN. The direct η-photoproduction process is studied within a formalism based on a chiral constituent quark model approach, complemented with a one-gluon-exchange mechanism, to take into account the breakdown of the SU(6) O(3) symmetry. In the models search, the following known nucleon resonances are embodied: S_(11)(1535), S_(11)(1650), P_(11)(1440), P_(11)(1710), P_(13)(1720), D_(13)(1520), D_(13)(1700), D_(15)(1675), and F_(15)(1680). Data for the π~-p→ηn reaction from threshold up to a total center-of-mass energy of W ≈ 2 GeV are satisfactorily reproduced. For the photoproduction channel, two additional higher mass known resonances, P_(13)(1900) and F_(15)(2000), are also considered. However, reproducing the data for γp→ηp requires, within our approach, two new nucleon resonances, for which we extract mass and width.     

Nuclear effects in the F3 structure function for finite and asymptotic Q

We study nuclear effects in the structure function F₃ which describes the parity violating part of the charged-current neuitrino nucleon deep inelastic scattering. Starting from a covariant approach we derive a factorized expression for the nuclear structure function in terms of the nuclear spectral function and off-shell nucleon structure functions valid for arbitrary momentum transfer Q and in the limit of weak nuclear binding, i.e. when a nucleus can be treated as a non-relativistic system. We develop a systematic expansion of nuclear structure functions in terms of a Q⁻² series caused by nuclear effects (“nuclear twist” series). Basing ourselves on this expansion we calculate nuclear corrections to the Gross-Llewellyn-Smith sum rule as well as to higher moments of F₃. We show that corrections to the GLS sum rule due to nuclear effects cancel out in the Bjorken limit and calculate the corresponding Q⁻² correction. Special attention is paid to the discussion of the off-shell effects in the structure functions. A sizable impact of these effects both on the Q² and x dependence of nuclear structure functions is found.