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.     

Sivers effect for pion and kaon production in semi-inclusive deep inelastic scattering

We study the Sivers effect in the transverse single spin asymmetries (SSA) for pion and kaon production in semi-inclusive deep inelastic scattering (SIDIS) processes. We perform a fit of which, by including recent high-statistics experimental data for pion and kaon production from HERMES and COMPASS Collaborations, allows a new determination of the Sivers distribution functions for quarks and antiquarks with u , d and s flavours. Estimates for forthcoming SIDIS experiments at COMPASS and JLab are given.     

Exclusive meson production at HERMES

Hard exclusive production of mesons in deep-inelastic scattering allows one to probe the so-far unknown Generalized Parton Distributions (GPDs) of the nucleon. The HERMES experiment has measured several different observables in exclusive meson production by scattering the 27.6GeV HERA lepton beam off an internal fixed gaseous target. Recent results on exclusive π ⁺, ρ ⁰ and pion pair production will be presented.     

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.     

Longitudinal and transverse target-spin asymmetries associated with DVCS on the proton at HERMES

Measurements of azimuthal cross-section asymmetries from deeply virtual Compton scattering on transversely and longitudinally polarized hydrogen and longitudinally polarized deuterium targets at HERMES are reported. By comparing the HERMES results on the transverse target-spin asymmetry with theoretical calculations based on a phenomenological model of generalized parton distributions, a model-dependent constraint on the total angular momentum carried by quarks in the nucleon is obtained.     

Spin structure function of deuteron g<Subscript>1</Subscript><Superscript>d</Superscript> from COMPASS

New results on the longitudinal inclusive spin asymmetry A ₁ ^d in the range 1 < Q ² < 100(GeV/c)^2 and 0.004 < x < 0.7 are presented. From these results we derive the spin-dependent structure function g ₁ ^d which we use to evaluate the scale-invariant flavor-singlet axial charge . The contribution of the measured region is evaluated by a QCD fit of the world data. The data were obtained by the COMPASS experiment at CERN using a 160GeV polarized muon beam scattered off a large double-cell polarized ⁶LiD target. The results are in agreement with those from previous experiments and improve considerably the statistical accuracy in the region x < 0.03.     

Linearly polarised photon beams at ELSA and measurement of the beam asymmetry in \pi^{0}_{} photoproduction off the proton

At the electron accelerator ELSA a linearly polarised tagged photon beam is produced by coherent bremsstrahlung off a diamond crystal. Orientation and energy range of the linear polarisation can be deliberately chosen by accurate positioning of the crystal with a goniometer. The degree of polarisation is determined by the form of the scattered electron spectrum. Good agreement between experiment and expectations on the basis of the experimental conditions is obtained. Polarisation degrees of % are typically achieved at half of the primary electron energy. The determination of is confirmed by measuring the beam asymmetry, , in photoproduction and by a comparison of the results to independent measurements using laser backscattering.     

Measurement of the beam asymmetry in η photoproduction off the proton

The beam asymmetry, Σ , was measured at ELSA in the reaction p → ηp using linearly polarised tagged photon beams, produced by coherent bremsstrahlung off a diamond. The crystal was oriented to provide polarised photons in the energy range E _γ = 800 to 1400MeV with the maximum polarisation of P _γ = 49 % obtained at 1305MeV. Both dominant decay modes of the η into two photons and 3π⁰ were used to extract the beam asymmetry from the azimuthal modulation of the cross-section. The measurements cover the angular range Θ_cm ≃ 50 -150 degrees. Large asymmetries up to 80% are observed, in agreement with a previous measurement. The eta-MAID model and the Bonn-Gatchina partial wave analysis describe the measurements, but the required partial waves differ significantly.     

Electromagnetic radiative corrections in parity-violating electron-proton scattering

QED radiative corrections have been calculated for leptonic and hadronic variables in parity-violating elastic ep scattering. For the first time, the calculation of the asymmetry in the elastic radiative tail is performed without the peaking-approximation assumption in hadronic variables configuration. This method has been validated in a comparison with the PVA4 data. It has been also used to evaluate the radiative corrections to the parity-violating asymmetry measured in the G⁰ experiment. The results obtained are here presented.     

Polarization in hyperon photo- and electro-production<Superscript>⋆</Superscript>

Multiple polarization observables must be measured to access the amplitude structure of pseudoscalar meson photoproduction off the proton. The hyperon-producing reactions are especially attractive to study, since the weak decays allow straightforward measurement of the induced and recoil polarization observables. In this paper we emphasize γ + p → K ⁺ + Λ , discussing recent measurements of C_x, C_z, and P for this reaction. An empirical constraint on the helicity amplitudes is obtained. A simplified model involving spin-flip and spin-non-flip amplitudes is presented. Finally, a semi-classical model of how the polarization may arise is presented.     

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.     

IR-renormalon contributions to the structure functions g 3 and g 5

We calculate the leading 1/N _f perturbative contributions to the polarized nonsinglet structure functions g ₃ and g ₅ to all orders in α_s. The contributions from the first renormalon pole are determined. It is a measure for the ambiguity of the perturbative calculation and is hypothetically assumed to dominate the power corrections. The corrections Δg ₃ and Δg ₅ are given as functions of the Bjorken variable x and turn out to be negligible. The anomalous dimensions of the leading twist operators are obtained in the next-to-leading order. Received: 10 June 1998 / Revised version: 25 September 1998     

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.     

Regge approach to charged pion photoproduction at invariant energies above 2 GeV

A Regge model with absorptive corrections is employed in a global analysis of the world data on positive and negative pion photoproduction for photon energies from 3 to 8GeV. In this region resonance contributions are expected to be negligible so that the available experimental information on differential cross-sections and single polarization observables at - t ⩽ 2 GeV^2 allows us to determine the non-resonant part of the reaction amplitude reliably. The model amplitude is then used to predict observables for photon energies below 3GeV. Differences between our predictions and data in this energy region are systematically examined as possible signals for the presence of excited baryons. We find that the data available for the polarized photon asymmetry show promising resonance signatures at invariant energies around 2GeV. With regard to differential cross-sections the analysis of negative pion photoproduction data, obtained recently at JLab, indicates likewise the presence of resonance structures around 2GeV.     

φ-meson photoproduction from nuclei

We study coherent and incoherent φ-meson photoproduction from nuclei. The available data are analyzed in terms of single- and coupled-channel photoproduction. It is found that the data on coherent photoproduction can be well reproduced within a single-channel optical model and show only little room for ω-φ mixing. These data indicate a normal distortion of the φ-meson in nuclei, which is compatible with the results obtained through the vector meson dominance model. The data on incoherent φ-meson photoproduction show an anomalous A-dependence resulting in a very strong φ-meson distortion. These data can be explained by a coupled-channel effect through the dominant contribution from the ω↦φ or the π↦φ transition or, more speculative, through the excitation of a cryptoexotic B _φ-baryon.     

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.     

Hard collisions of spinning protons: Past,present and future

There will be a review of the history of polarized proton beams, and a discussion of the unexpected and still unexplained large transverse spin effects found in several high-energy proton-proton spin experiments at the ZGS, AGS and Fermilab. Next, there will be a discussion of present and possible future experiments on the violent elastic collisions of polarized protons at IHEP-Protvino's 70GeV U-70 accelerator in Russia and the new high-intensity 50GeV J-PARC facility being built at Tokai in Japan.     

Nucleon resonance decay by the K0Σchannel

Hyperon production off the proton in the K ⁰Σ⁺ channel has been studied at the tagged-photon beam facility at the ELSA electron accelerator in Bonn. This experiment was part of a series of neutral meson production experiments on various targets. For this purpose, the Crystal Barrel and TAPS photon spectrometers have been combined to provide a 4π detector for multi-neutral-particle final states. A high-quality excitation function, recoil polarizations, and angular distributions from threshold up to 2.3GeV center-of-mass energy were obtained. The results are compared with predictions of recent coupled-channel calculations within the K -matrix formalism and are interpreted by the partial-wave analysis within the Bonn-Gatchina model.