Baryon Structure in AdS/QCD

We discuss baryon structure in a soft-wall anti-de Sitter/quantum chromodynamics model. From unified point of view we describe nucleon form factors and the electroproduction of the N(1440) Roper resonance. The Roper resonance is identified as the first radially excited state of the nucleon. The obtained results for helicity amplitudes of the Roper electroproduction are in good agreement with the recent results of the CLAS Collaboration at JLab.     

Understanding the proton spin “puzzle” with a new “minimal” quark model

We investigate the spin structure of the nucleon in an extended Jaffe-Lipkin quark model. In addition to the conventional 3q structure, different (3q)(Q ) admixtures in the nucleon wave function are also taken into account. The contributions to the nucleon spin from various components of the nucleon wave function are discussed. The effect due to the Melosh-Wigner rotation is also studied. It is shown that the Jaffe-Lipkin term is only important when antiquarks are negatively polarized. We arrive at a new “minimal” quark model, which is close to the naive quark model, in order to understand the proton spin “puzzle”. Received: 4 November 2000 / Accepted: 23 October 2001     

Highlights from the COMPASS experiment

The Compass experiment at CERN is studying the nucleon spin structure with a 160 GeV polarized muon beam and polarized targets as well as hadron structure with 190 GeV pion, kaon and proton beams. The paper gives an overview of the results for the helicity and transverse spin structure of the nucleon. A first result from the spectroscopy experiments, the observation of a resonance with exotic JPC = 1- + quantum numbers at 1660 MeV is also presented. The paper ends with an outlook to future measurements.     

A hamiltonian approach to the roper resonance

The breathing modes along with the spin-isospin zero modes of the ω-stabilized skyrmion are quantized using Dirac's method. The result is a hamiltonian for the nucleon, the δ-isobar and the Roper with explicit meson couplings. The linear pion coupling is used to evaluate the decay width of the Roper into πN.     

Measurement of the proton spin polarizabilities at MAMI

The spin polarizabilities of the nucleon are fundamental structure constants which describe the response of the nucleon spin to an incident polarized photon. The most model-independent way to measure the nucleon spin polarizabilities is the Compton scattering with polarization degrees of freedom. Three Compton scattering asymmetries on the proton were measured in the Δ(1232) region using a polarized incident photon beam and a polarized (or unpolarized) proton target at the Mainz Microtron (MAMI). These asymmetries are sensitive to values of the spin polarizabilities. Fits to asymmetry data were performed using a dispersion model calculation, and a separation of all four proton spin-polarizabilities in the multipole basis was achieved. The values of the proton spin polarizabilities are presented.     

干涉截面对核子自旋结构函数的影响

利用非相对论和相对论夸克模型分别计算了干涉截面对极化核子自旋结构函数的影响.计算所得到的明显的作用为今后实验上抽取核子自旋结构函数提供了依据.     

The N~* physics program at Jefferson Lab

Recent measurements of nucleon resonance transition form factors with CLAS at Jefferson Lab are discussed. The new data confirm the assertion of the symmetric constituent quark model of the Roper as the first radial excitation of the nucleon. The data on high Q~2 nπ~+ production better constrain the branching ratios β_(Nπ) and β_(Nη). For the first time, the longitudinal transition amplitude to the S_(11)(1535) was extracted from the nπ~+ data. Also, new results on the transition amplitudes for the D_(13)(1520) resonance are presented showing a rapid transition from helicity 3/2 dominance seen at the real photon point to helicty 1/2 dominance at higher Q~2. I also discuss the status of the search for new excited nucleon states.     

Lattice results on nucleon/roper properties

In this proceeding, I review the attempts to calculate the Nucleon resonance (including Roper as first radially excited state of nucleon and other excited states) using lattice quantum chromodynamics (QCD). The latest preliminary results from Hadron Spectrum Collaboration (HSC) with m_n≈380 MeV are reported. The Sachs electric form factor of the proton and neutron and their transition with the Roper at large Q~2 are also updated in this work.     

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 isshown. 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 islessthan 10%. However, the effect on the Roper resonanceis remarkable but is inconclusive.     

Transverse spin structure of the nucleon from lattice-QCD simulations

We present the first calculation in lattice QCD of the lowest two moments of transverse spin densities of quarks in the nucleon. They encode correlations between quark spin and orbital angular momentum. Our dynamical simulations are based on two flavors of clover-improved Wilson fermions and Wilson gluons. We find significant contributions from certain quark helicity flip generalized parton distributions, leading to strongly distorted densities of transversely polarized quarks in the nucleon. In particular, based on our results and recent arguments by Burkardt [Phys. Rev. D 72, 094020 (2005)], we predict that the Boer-Mulders function h(1/1), describing correlations of transverse quark spin and intrinsic transverse momentum of quarks, is large and negative for both up and down quarks.     

Q2 dependence of the neutron spin structure function g2(n) at low Q2

We present the first measurement of the Q2 dependence of the neutron spin structure function g2(n) at five kinematic points covering 0.57 (GeV/c)2 < or = Q2 < or = 1.34 (GeV/c)2 at x approximately = 0.2. Though the naive quark-parton model predicts g2 = 0, nonzero values occur in more realistic models of the nucleon which include quark-gluon correlations, finite quark masses, or orbital angular momentum. When scattering from a noninteracting quark, g2(n) can be predicted using next-to-leading order fits to world data for g1(n). Deviations from this prediction provide an opportunity to examine QCD dynamics in nucleon structure. Our results show a positive deviation from this prediction at lower Q2, indicating that contributions such as quark-gluon interactions may be important. Precision data obtained for g1(n) are consistent with next-to-leading order fits to world data.     

核子中夸克的极化

根据强子张量和向前的虚光子康普顿散射振幅之间的关系,用虚光子总吸收截面表达了核子的自旋结构函数和它的第一矩. 在夸克模型中计算了小动量转移区的虚光子的总吸收截面,进而得到了第一矩的理论结果,导出了质子中夸克的极化. 合理地解释了EMC和SLAC的实验结果.     

Spin Dependent Quark Forces and the Spin Content of the Nucleon

The spin crisis of the nucleon is that the quark spin contribution is only a small fraction of the nucleon spin. A relativistic Dirac equation approach is followed assuming three low mass current quarks in the nucleon described by a (1/2⁺)³ configuration. If the lower component contribution to the normalization of the quark wave function is about 0.18, then the axial charge of the nucleon can be reproduced. However including the same lower component to every quark wave function is not enough to resolve the spin crisis. The net u quark spin z component is predicted as 1.0 and the net d quark spin z component is predicted as –0.25, both in disagreement with experiment. These predictions can be brought into agreement with experiment if flavor independent but spin dependent forces are assumed between the quarks. The strength of the spin dependent force found by empirically fitting the nucleon spin data is shown to be comparable to the spin dependence that can explain the -nucleon mass difference. The spin content of the ⁺ is then predicted using the interactions that reproduce the spin content of the proton.     

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.     

The swelling of nucleons in nuclei and the Roper resonance

Conditions where some swelling of the nucleon occurs, and, in particular the relation of this effect with the attractive character of the force acting on it, are studied. It is found that short range repulsive correlations can turn the swelling into a shrinking, in spite of a globally attractive interaction, whereas repulsive velocity dependent forces can lead to some swelling. The role of the Roper resonance in this nucleon change of size is considered in some detail.     

Tensor Form Factors and Transverse Spin Structures of the Nucleon

We briefly review recent investigations on the transverse spin structure of the nucleon, based on the chiral quark-soliton model. We discuss the results of the tensor and anomalous tensor magnetic form factors and the corresponding transverse quark spin densities inside a nucleon. The results are compared with those of the recent lattice calculation. While their qualitative features are similar to the lattice ones, there are some discrepancies in the momentum dependence of the form factors and accordingly in the strengths of the transverse spin densities.     

Effects of bag surface motion with relativistic kinematics

Radial surface motion of the baryon bag is carried out in a model with relativistic kinematics, with confinement the result of volume energy and surface tension and the pion field coupled to the bag surface. We calculate radial wave functions for the nucleon, the Δ(1233) and the Roper resonance N^*(1450), which is identified as the first radial excitation of the nucleon.

Results are used to calculate form factors and pionic decay widths of the baryons examined. The approximations made in these calculations are discussed in extensio.     

Spin structure of the nucleon—status and recent results

After the initial discovery of the so-called “spin crisis in the parton model” in the 1980s, a large set of polarization data in deep inelastic lepton-nucleon scattering was collected at labs like SLAC, DESY and CERN. More recently, new high precision data at large x and in the resonance region have come from experiments at Jefferson Lab. These data, in combination with the earlier ones, allow us to study in detail the polarized parton densities, the Q² dependence of various moments of spin structure functions, the duality between deep inelastic and resonance data, and the nucleon structure in the valence quark region. Together with complementary data from HERMES, RHIC and COMPASS, we can put new limits on the flavor decomposition and the gluon contribution to the nucleon spin. In this report, we provide an overview of our present knowledge of the nucleon spin structure and give an outlook on future experiments. We focus in particular on the spin structure functions g₁ and g₂ of the nucleon and their moments.     

核子结构

本文介绍了半个世纪来核子结构研究的重要进展，着重介绍了近十年来轻子核子极化深度非弹散射实验导致的有关核子自旋、磁矩结构的新进展，还介绍了当前核子结构研究中的一些重要问题。     

Integrated spin structure function of the nucleon using extended Fock space approach

We study the effect of sea quarks on the integrated spin structure functions, the F/D ratio and the total quark polarization, ∑, of the nucleon. The extended Fock space approach is used to incorporate the sea quarks and its wavefunction is properly antisymmetrized. These configurations give significant contributions to the integrated spin structure functions. In particular, antisymmetrization is found to play an important role in the case of the integrated spin structure function of the neutron and the F/D ratio. We find good agreement with experimental values for all four spin polarization observables.