Nucleon spin structure in a relativistic constituent quark model

The spin structure of the nucleon is analyzed using a relativistic constituent quark model in light-front formulation. We investigate, in particular, relativistic effects on the axial vector coupling constants. Electromagnetic and axial form factors are constructed in terms of quark form factors that reflect the possible non-trivial structure of the constituent quarks. We study the influence of flavour mixing effects on axial constants and discuss the extent to which such effects can renormalize the singlet axial constant g _A ⁰ from its SU(6) quark model value.     

The quark spin structure of the nucleon

Some non-perturbative aspects of the nucleon quark spin structure are reviewed. The first part is a brief summary of early theoretical developments in the field of polarized deep inelastic scattering of electrons on polarized nucleons and an illustration of the non-perturbative power of lattice QCD 25 years later. The second part is a short pedagogical introduction to the analysis of high energy scattering in the complex angular momentum plane, with particular emphasis on spin-dependent deep inelastic electron-nucleon scattering. The third part comprises a brief introduction to lattice QCD and its applications in the non-perturbative determination of the spin-dependent structure functions.     

Second-class axial-vector current of the nucleon in a relativistic constituent quark model

The nucleon is described as a relativistic three-quark system in light-cone dynamics. One of its three wave function invariants, I₁, is shown to generate a second-class axial-vector current. When the transverse quark motion is negligible I₁ is not present. For some separable, scalar, three-body confinement interactions the I₁ admixture also remains small.     

Strangeness content and structure function of the nucleon in a statistical quark model

The strangeness content of the nucleon is determined from a statistical model using confined quark levels, and is shown to have a good agreement with the corresponding values extracted from experimental data. The quark levels are generated in a Dirac equation that uses a linear confining potential (scalar plus vector). With the requirement that the result for the Gottfried sum rule violation, given by the New Muon Collaboration (NMC), is well reproduced, we also obtain the difference between the structure functions of the proton and neutron, and the corresponding sea quark contributions. Received: 15 March 1999 / Revised version: 15 April 1999 / Published online: 14 October 1999     

The spin structure of the constituent quarks and of the nucleon

We define a constituent quark within QCD. It is shown that the spin of such a quark and hence also the spin of the nucleon is a complex phenomenon. We discuss it in view of the experimental data.Received: 31 March 2005, Revised: 13 May 2005, Published online: 28 July 2005     

The (spin) structure of the nucleon

A selection of new data obtained by the HERMES experiment at DESY is presented, which provides new insight into the QCD structure of the nucleon. Using polarized lepton beams and polarized targets, the spin-dependent structure function g ₁(x) has been determined for ¹ H, ² H and ³ He. By also observing one of the produced hadrons it has been possible to extract the polarization distribution of individual quark flavours in the nucleon as well. Further information on nucleon structure has been obtained by observing (almost) exclusive reactions, which can be interpreted in terms of the recently introduced generalized parton distributions (GPDs). As an example of such data measurements of both the beam-spin and beam-charge asymmetries resulting from deeply virtual Compton scattering (DVCS) are presented. By embedding the deep-inelastic scattering process in the nuclear environment additional information can be obtained on nucleon structure and some QCD effects. The potential of this technique is illustrated by showing new results on tagged structure functions and hadronization in nuclei.Received: 30 September 2002, Published online: 22 October 2003PACS: 13.60.Hb Total and inclusive cross-sections (including deep-inelastic processes) - 13.88. + e Polarization in interactions and scattering - 14.20.Dh Protons and neutrons - 24.85. + p Quarks, gluons, and QCD in nuclei and nuclear processesG. van der Steenhoven: For the HERMES Collaboration     

Quark-hadron duality of nucleon spin structure function

Bloom-Gilman quark-hadron duality of nuclear spin structure function is studied by comparing the integral of g₁ from perturbative QCD prediction in the scaling region to the moment of g₁ in the resonance region. The spin structure function in the resonance region is estimated by the parametrization forms of non-resonance background and of resonance contributions. The uncertainties of our calculations due to those parametrization forms are discussed. Moreover, the effect of the (1232)-resonance in the first resonance region and the role of the resonances in the second resonance region are explicitly shown. Elastic peak contribution to the duality is also analyzed.     

Model for the nucleon structure function F2(x,Q2)

A particular j-plane singularity, leading to long-range effects in hadron scattering, is attributed to hard constituent quark interactions. A definite prediction for the nucleon structure function, including scaling violations consistent with QCD. is obtained, which is confronted with the available data.     

Weak hyperon nucleon interaction in a quark model

The nonmesonic decay moded of Λ in nuclear matter, ΛN → NN, and production of Λ in proton-neutron scattering, pn → Λn, are studied theoretically. The weak transition of baryon is described by the one meson exchange mechanism and the direct quark-quark interaction mechanism.     

The spin structure of the nucleon

An introduction to deep inelastic scattering experiments investigating the spin structure of the proton and the neutron is given. Discussed are measurements with longitudinally polarized targets to determine the spin structure functiong ₁ and sum rules and to do pQCD analyses with inclusive data, while a flavour separation and first extractions of the gluon polarization are performed with semi-inclusive data.     

Photoproduction of pions in the constituent quark model

Threshold photoproduction of pions has been calculated for composite systems, both for the nucleon in terms of constituent quarks and the ³H/³He isodoublet in terms of nucleons. The results of the low energy theorems are recovered only if both size effects and internal excitation of the composite system are properly taken into account. In particular, the ground state has to be non-degenerate and the excitation spectrum has to have the proper symmetries of the many-body system.     

Longitudinal spin structure of the nucleon at COMPASS (SPS CERN)

We present new results on longitudinal inclusive double-spin asymmetry A _1,p (x) in deep-inelastic muon-proton scattering and proton spin-dependent structure function g ₁ ^p (x, Q ²). New COMPASS data on longitudinal polarised NH₃ target were collected during the year 2011 with beam of positive muons with energy E = 200 GeV. Kinematical threshold Q ² ≥ 1 (GeV/c)² and the fractional energy 0.1 < y < 0.9 allow us to cover low x region down to 0.025.     

Electromagnetic radii and quark structure of the nucleon

By using a diagrammatical technique it is shown that in scalar theories the stochastic quantization method of Parisi and Wu gives the usual perturbation series in Feynman diagrams.