Twist-3 single-spin asymmetries in semi-inclusive deep-inelastic scattering

The single-spin asymmetries for a longitudinally polarized lepton beam or a longitudinally polarized nucleon target in semi-inclusive deep-inelastic scattering are twist-3 observables. We study these asymmetries in a simple diquark spectator model of the nucleon. Analogous to the case of transverse target polarization, non-vanishing asymmetries are generated by gluon exchange between the struck quark and the target system. It is pointed out that the coupling of the virtual photon to the diquark is needed in order to preserve electromagnetic gauge invariance at the twist-3 level. The calculation indicates that previous analyses of these observables are incomplete.     

Extraction of G<Subscript>M</Subscript><Superscript>n</Superscript> from inclusive electron scattering on D, <Superscript>4</Superscript>He

We use the relation between Structure Functions (SFs) of nuclei A and nucleons N in order to fomulate a criterion which isolates the QE part out of the total inclusive cross-section. From data points around the QEP we extract the reduced neutron magnetic form factor 〈α _n = G _M ⁿ/μ _n G _d〉. The latter shows an unexpected decrease up to Q ² = 10GeV^2, the largest measured.     

Search for exotics in γD↦φnp reactions near threshold<Superscript>⋆</Superscript>

We discuss coherent and incoherent φ -meson photoproduction off the deuteron at low energy and small momentum transfer with the aim to check whether the recent experimental data need for their interpretation an inclusion of exotic channels. Our analysis of the differential cross-section and spin-density matrix elements shows that new data on the γD → φX reaction at E _γ ∼ 2 GeV may be understood on the basis of conventional dynamics.     

Determination of the gluon polarization at RHIC and COMPASS

The most recent determinations of the gluon polarization in the nucleon, , obtained at RHIC and COMPASS experiments, are reviewed. The former accesses the gluon polarization mainly through the production of neutral pions (PHENIX) or jets (STAR) in polarized proton collisions. The latter uses the photon-gluon fusion in polarized lepton-nucleon scattering, tagged either by open charm or high-p_T hadrons production. All the results are in good agreement, and favour values of ΔG roughly between 0 and 0.5 at a few (GeV/c)² , thus in contradiction with what could be derived from the Ellis-Jaffe sum rule and the axial anomaly. Much stronger constraints will be obtained in a near future by both programs, helping us to clarify the role of gluons in the nucleon spin.     

Leading and higher twists in proton,neutron and deuteron unpolarized structure functions F<Subscript>2</Subscript>

We summarize the results of a recent global analysis of proton and deuteron F₂ structure function world data performed over a large range of kinematics, including recent measurements done at JLab with the CLAS detector. From these data the lowest moments (n≤10) of the unpolarized structure functions are determined with good statistics and systematics. The Q² evolution of the extracted moments is analyzed in terms of an OPE-based twist expansion, taking into account soft-gluon effects at large x. A clean separation among the leading- and higher-twist terms is achieved. By combining proton and deuteron measurements the lowest moments of the neutron F₂ structure function are determined and its leading-twist term is extracted. Particular attention is paid to nuclear effects in the deuteron, which become increasingly important for the higher moments. Our results for the non-singlet, isovector (p-n) combination of the leading-twist moments are used to test recent lattice simulations. We also determine the lowest few moments of the higher-twist contributions, and find these to be approximately isospin independent, suggesting the possible dominance of ud correlations over uu and dd in the nucleon.     

Constituent quarks and g 1

We review the theory and present status of the proton spin problem with emphasis on the transition between current quarks and constituent quarks in QCD. Received: 7 January 1999 / Revised version: 12 February 1999     

Semi-Inclusive Deep Inelastic Scattering processes from small to large P<Subscript>T</Subscript>

We consider the azimuthal and P_T-dependence of hadrons produced in unpolarized Semi-Inclusive Deep Inelastic Scattering (SIDIS) processes, within the factorized QCD parton model. It is shown that at small P_T values, P _T ≲ 1GeV/c, lowest-order contributions, coupled to unintegrated (transverse-momentum-dependent) quark distribution and fragmentation functions, describe all data. At larger P_T values, P _T ≳ 1GeV/c, the usual pQCD higher-order collinear contributions dominate. Having explained the full P_T range of available data, we give new detailed predictions concerning the azimuthal and P_T-dependence of hadrons which could be measured in ongoing or planned experiments by HERMES, COMPASS and JLab Collaborations.     

Contribution of boundness and motion of nucleons to the EMC effect

The kinematical corrections to the structure function of the nucleon in the nucleus due to the boundness and motion of nucleons arise from the excitation of the doorway states for one-nucleon transfer reactions in the deep inelastic scattering on nuclei.     

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.     

Spin physics at COMPASS

Results for the spin structure of the nucleon from the COMPASS data taking periods 2002 to 2004 are presented. The quark contribution to the nucleon spin, following from a QCD fit to the new data, turns out to be significantly larger than it was derived from the previous world data. The new data favour, on the other side, a comparatively small gluon polarisation in the range x _g ≈ 0.1. In the data taken with the deuteron target polarised transversely the related asymmetries are found to be small on the level of accuracy reached so far, indicating a cancellation of the proton and neutron contributions. This is in agreement, for both the Collins and the Sivers asymmetry, with recent theoretical calculations. Also, a step towards the understanding of angular-momentum contributions with COMPASS is taken by the evaluation of asymmetries in exclusive vector meson production.     

Measurement of the beam asymmetry Σ in πeta; production off the proton with the CBELSA/TAPS experiment?

In photoproduction experiments, a large number of final states yielding various resonance contributions are accessible. To extract resonance parameters via partial-wave analysis not only the measurement of differential cross-sections is necessary, but also the determination of polarization observables. At the electron accelerator ELSA (Bonn) the coherent bremsstrahlung method was used to generate a linearly polarized photon beam. Using the CBELSA/TAPS detector setup, the beam asymmetry Σ in the reaction γp → pπ⁰η was determined as a function of various masses and angles for photon energies between 970MeV and 1650MeV.     

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     

Measurement of πphotoproduction on the proton from 550 to 1500 MeV at GRAAL

Neutral pion photoproduction has been measured from 550 to 1500 MeV with the GRAAL facility, located at the ESRF in Grenoble. Differential cross-section and beam asymmetry have been measured over a wide angular range. These high-precision data improve significantly the database for the beam asymmetry in the second and third resonance regions covering for the first time the energy domain 1100-1500 MeV. New partial-wave analyses including these data are presented for which the beam asymmetry brings crucial constraints.     

Measurement of η photoproduction on the proton from threshold to 1500 MeV

Beam asymmetry and differential cross-section for the reaction γp → ηp were measured from production threshold to 1500MeV photon laboratory energy. The two dominant neutral decay modes of the η -meson, η → 2γ and η → 3π⁰ , were analyzed. The full set of measurements is in good agreement with previously published results. Our data were compared with three models. They all fit satisfactorily the results but their respective resonance contributions are quite different. The possible photoexcitation of a narrow state N(1670) was investigated and no evidence was found.     

The effect of quark exchange in A = 3 mirror nuclei and neutron/proton structure functions ratio

By using the quark-exchange formalism, the realistic Faddeev wave function and the Fermi motion effect, we investigate the deep inelastic electron scattering from A = 3 mirror nuclei in the deep-valence region. The initial valence quark input is taken from the GRV's (Glück, Reya and Vogt) fitting procedure and the next-to-leading-order QCD evolution on F₂^p(x, Q²) which gives a very good fit to the available data in the (x, Q²)-plane. It is shown that the free neutron to proton structure functions ratio can be extracted from the corresponding EMC ratios for ³He and ³H mirror nuclei by using the self-consistent iteration procedure and the results are in good agreement with the other theoretical models as well as the present available experimental data and especially the projected data expected from the proposed 11GeV Jefferson Laboratory in the near future.     

Leading-order 2πγ exchange NN interaction: Central potentials proportional to g<Subscript>A</Subscript><Superscript>0</Superscript> and g<Subscript>A</Subscript><Superscript>2</Superscript>

We calculate at two-loop order in chiral perturbation theory the electromagnetic corrections to the leading-order 2π exchange NN interaction proportional to g _A ⁰ and g _A ². The resulting 2πγ exchange potential contains isospin-breaking components which reach up to about -2% of the corresponding isovector 2π exchange potential. With a value of only -17keV at r = m _π ^-1 = 1.4fm the charge-independence breaking central potential obtained here is negligibly small in comparison to the one generated by the isoscalar c₃ contact vertex. Our calculation confirms that the largest long-range isospin-violating NN potentials arise from the 2πγ exchange diagrams involving the large low-energy constants c ₄ ≃ - c ₃ ≃ 3.3GeV^-1 representing the important Δ(1232) dynamics.     

A fitter code for Deep Virtual Compton Scattering and Generalized Parton Distributions

We have developed a fitting code based on the leading-twist handbag Deep Virtual Compton Scattering (DVCS) amplitude in order to extract Generalized Parton Distribution (GPD) information from DVCS observables in the valence region. In a first stage, with simulations and pseudo-data, we show that the full GPD information can be recovered from experimental data if enough observables are measured. If only some of these observables are measured, valuable information can still be extracted, with certain observables being particularly sensitive to certain GPDs. In a second stage, we make a practical application of this code to the recent DVCS Jefferson Lab Hall-A data from which we can extract numerical constraints for the two H GPD Compton form factors. An erratum to this article can be found at     

Polarization observable measurements for γp → K+Λ and γp → K+Σfor energies up to 1.5 GeV

Beam asymmetries and hyperon recoil polarizations for the reactions γp → K ⁺Λ and γp → K ⁺Σ⁰ have been measured from the threshold production to 1500MeV with the GRAAL facility located at the ESRF in Grenoble. These results complement the database for the beam asymmetry, covering for the first time the production threshold region. Recent theoretical analyses are presented for which the beam asymmetry data bring interesting new information and allow to better determine some resonance parameters. Most importantly, these results strengthen the need of a new D₁₃ state around 1900MeV.     

Meson photoproduction on the nucleon with polarized photons

Meson photoproduction with polarized photons has proved to be a powerful tool to identify contributions of baryon resonances that are not evident in the differential cross-sections. It provides information that are complementary to those extracted using pion-nucleon scattering data. Extensive results have been produced in the past on beam asymmetries by the Graal collaboration for η and π ⁰ on the proton. New results are now available for the same reactions on the quasi-free neutron and for the K⁺ photoproduction on the proton. Contributions from hitherto undetected baryon resonances may be important to understand the results.