The shape of polarized gluon distributions

The recent high precision SMC data on polarized μp scatterings have again confirmed that very little of the proton spin is carried by quarks. To unravel the mystery of the proton spin structure, it is quite important to know the behavior of the polarized gluon distribution. By using the positivity condition of distribution functions together with the unpolarized and polarized experimental data, we restrict thex dependence of the polarized gluon distribution.     

Phenomenological determination of polarized quark distributions in the nucleon

We present a fit to spin asymmetries which gives polarized quark distributions. These functions are closely related to the ones given by the Martin, Roberts and Stirling fit for unpolarized structure functions. The integrals of polarized distributions are discussed and compared with the corresponding quantities obtained from neutron and hyperon??-decay data. We use the combination of proton, neutron and deuteron spin asymmetries in order to determine the coefficients of our polarized quark distributions. Our fit shows that phenomenologically there is no need for taking polarized gluons into account.     

Phenomenological determination of polarized quark distributions in the nucleon

We present a fit to spin asymmetries which gives polarized quark distributions. These functions are closely related to the ones given by the Martin, Roberts and Stirling fit for unpolarized structure functions. The integrals of polarized distributions are discussed and compared with the corresponding quantities obtained from neutron and hyperonβ-decay data. We use the combination of proton, neutron and deuteron spin asymmetries in order to determine the coefficients of our polarized quark distributions. Our fit shows that phenomenologically there is no need for taking polarized gluons into account.     

Recent results from COMPASS spin program

COMPASS is a fixed target experiment at CERN’s Super-Proton-Synchrotron. Part of its physics program is dedicated to the spin structure of the nucleon, which it studies with a polarized muon beam and polarized targets. An overview of its recent results along this line is given. In particular, the first results of our 2011 longitudinally polarized proton run, a report on our progress towards the extraction of the gluon polarization, ΔG, at NLO and an update on our measurements of transverse spin and k _T -dependent processes, from our 2010 transversely polarized proton data, and of hadron multiplicities.     

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.     

COMPASS results on transverse spin asymmetries in identified two-hadron production in SIDIS

COMPASS is a fixed target experiment at CERN where nucleon spin structure and hadron spectroscopy are investigated using a 160 GeV/c polarized μ⁺ beam. An important part of its physics program are the measurements of single spin asymmetries (SSA) in semi-inclusive deep inelastic scattering (SIDIS) on transversely polarized targets. Data on a deuteron target (⁶LiD) were taken in 2002–04. After taking the first data on a transversely polarized proton target (NH₃) in 2007, a full year of data taking followed in 2010 to increase precision. The SSA of identified hadron pairs consisting of charged pions and/or kaons from the 2010 data are shown for the first time and compared to model predictions and results from HERMES.     

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.     

Spin-dependent parton distributions from polarized structure function data

In the past year, polarized deep inelastic scattering experiments at CERN and SLAC have obtained structure function measurements off proton, neutron and deuteron targets at a level of precision never before achieved. The measurements can be used to test the Bjorken and Ellis-Jaffe sum rules, and also to obtain information on the parton distributions in polarized nucleons. We perform a global leading-order QCD fit to the proton deep inelastic data in order to extract the spin-dependent parton distributions. By using parametric forms which are consistent with theoretical expectations at large and smallx, we find that the quark distributions are now rather well constrained. We assume that there is no significant intrinsic polarization of the strange quark sea. The data are then consistent with a modest amount of the proton's spin carried by the gluon, although the shape of the gluon distribution is not well constrained, and several qualitatively different shapes are suggested. The spin-dependent distributions we obtain can be used as input to phenomenological studies for future polarized hadron-hadron and lepton-hadron colliders.     

Active polarized target: Measurement of proton spin polarizabilities

The static (electric and magnetic) polarizabilities of the proton are known from measurements of low-energy Compton scattering of real photons. However, no experimental data are so far available about its spin polarizabilities, which describe the response of the proton spin to a varying electromagnetic field. These fundamental structure constants can be obtained in the most model-independent way from measurements of double-polarized Compton scattering at energies below the pion threshold using a polarized photon beam and a polarized proton target. An active polarized target based on polystyrene doped with a scintillator allows recoil protons to be detected directly in the target material, background from the coherent Compton effect on heavier nuclei (¹²C etc.) to be eliminated and precise data on spin polarizabilities to be obtained.     

Transverse single spin asymmetries in polarized proton-proton collisions at PHENIX

The PHENIX experiment probes the transverse spin structure of the proton with asymmetries of particle production in polarized proton collisions. This talk summarizes the measurements of A _N in hadron production at forward and at mid rapidities.     

Hadronic colliders as probes of the proton spin structure

We perform a systematic analysis of different processes with high energy polarized proton beams: jets, direct photon, lepton pairs (Drell-Yan) andWZ production. Different sets of polarized partonic densities are used that fit EMC and SLAC polarized deep inelastic scattering data with variable amount of quark and gluon components of the proton spin. The case of the future Relativistic Heavy Ion Collider (RHIC) used as a polarized collider at a maximum energy of \(\sqrt s = 500\) GeV is analyzed in detail.     

Phenomenological determination of polarized quark distributions in the nucleon

We present a fit to spin asymmetries which gives polarized quark distributions. These functions are closely related to the ones given by the Martin, Roberts and Stirling fit for unpolarized structure functions. The integrals of polarized distributions are discussed and compared with the corresponding quantities obtained from neutron and hyperonβ-decay data. We use the combination of proton, neutron and deuteron spin asymmetries in order to determine the coefficients of our polarized quark distributions. Our fit shows that phenomenologically there is no need for taking polarized gluons into account. Work supported in part by the KBN-Grant 2-P302-143-06     

QCD Predictions for Spin Dependence of Parton Distributions

We introduce the polarized valon distributions to describe the spin dependence of parton distributions. The polarized valon distributions in the proton and polarized parton distributions inside the valon can help to obtain polarized parton distribution in a proton. In order to be able to obtain the spin contribution of sea quarks, we need to improve the valon model. We employ the Bernstein polynomial averages to obtain the unknown parameters which exist in our calculations. Our results for the polarized proton structure function, xg^p₁, are in good agreement with the experimental data for some values of Q².     

Spin correlation coefficients for neutron-proton radiative capture

We use a theoretical model for deuteron photodisintegration below the pion production threshold, which has previously been used to calculate the observables for which experimental data is available, in order to calculate the spin correlation coefficients for radiative capture of a polarized neutron beam by a polarized proton target. We give results for the coefficients whose measurement is possible in the near future and explain how such experimental results could improve our understanding of the reaction mechanism. We also comment on exciting measurements of the neutron analyzing power at very low energies.     

Simple model for spin-crisis

Starting from the additive quarkSU(6) model and taking into account the pion and kaon (η) emission during the initial stage of evolution (fromq ²～m _quark ² toq ²～1 GeV²) we convert the main fraction of the proton spin into the orbital momentum of the pseudoscalar mesons. The results are in a good agreement with the experimental data. We get ∫g ₁(x)dx=0.152 for proton and +0.007 for the Λ hyperon (i.e. the quarks carry out only about 39% of the Λ spin). In the same model we explain the violation of the Gottfried sum rule ?∫(F ₂ ^p(x)?F ₂ ⁿ(x))dx=0.23 due to the charged meson exchanges at the beginning of the evolution. The Gribov-Lipatov relation between the polarized structure functions in DIS ande ⁺ e ^?-annihilation is discussed. It gives the possibility to study the spin structure of Λ hyperon produced through the decay of the jet originated by a polarized quark ine ⁺ e ^?-annihilation or in DIS.     

Spin effects in diffractive reactions

Double spin asymmetries in the diffractive \(Q\bar Q\) and J/ψ leptoproduction are discussed. It is shown that the asymmetries for longitudinally polarized lepton and longitudinally or transversely polarized proton can be used to study spin dependent gluon distribution of the proton at small x.     

Measurement of spin polarizabilities of the proton

Spin polarizabilities are as-yet experimentally unknown fundamental structure constants that describe the response of the nucleon spin to the action of a changing electromagnetic field. The A2 Collaboration at the Institute for Nuclear Physics in Mainz (Institut für Kernphysik, Mainz) performed the first measurements of the energy and angular dependences of spin asymmetries of the cross section for doubly polarized (polarized-photon beam incident to a polarized proton target) Compton effect in the Δ-resonance region, these asymmetries being sensitive to values of the spin polarizabilities. The preparation of spin-asymmetry measurements at energies below the pion-production threshold with an active (scintillating) polarized target is in progress. These measurements will make it possible to determine individual spin polarizabilities of the proton in a model-independent way.     

Prompt photon A N with the PHENIX MPC-EX detector

The PHENIX MPC-EX detector is a Si-W preshower extension to the existing Muon Piston Calorimeters (MPC). The combined detectors will make possible a measurement of the prompt photon single spin asymmetry A _N in 200 GeV transversely polarized p + p collisions, which will help elucidate the correlation of the transverse motion of valence partons in the proton with the proton spin.     

Effects of electric dipole,anapole, and neutral weak magnetic moments of a proton in the polarized electron-proton electroweak scattering

We have derived and investigated analytical expressions for the cross sections and spin asymmetries in the elastic electromagnetic and electroweak scattering of unpolarized/longitudinally polarized electrons on a polarized/unpolarized proton target with allowance for the C-, P-, and T/CP-invariance violating anapole and electric dipole moments, as well as the neutral weak electric, magnetic, and axial form factors of the proton. The dependence of the spin asymmetries on the energy and form-factor parameters is studied.     

Measurement of the total cross section difference ΔσT(pp) in the energy range from 0.43 to 2.4 GeV

The SATURNE II polarized proton beam and the Saclay frozen spin polarized proton target were used to measure the total cross section difference Δσ_T = −2σ_{1 tot} at 26 energies between 0.43 and 2.4 GeV. Here Δσ_T is the total cross section difference for transverse beam and target spins parallel and antiparallel, respectively, and σ_1tot is one of spin-dependent terms in the total cross section σ_tot. The energy dependence of Δσ_T below 1 GeV shows similar structures as for Δσ_L. An additional minimum appears at about 1.3 GeV, which involves a structure in singlet spin partial waves.