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Single-Spin Asymmetries at CLAS

Single-Spin Asymmetries at CLAS. H. Avakian, L.Elouadrhiri (Jefferson Lab). For the CLAS Collaboration. Conference on Intersections of Particle and Nuclear Physics. Transverse momentum of quarks and spin-azimuthal asymmetries Target single-spin asymmetries

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Single-Spin Asymmetries at CLAS

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  1. Single-Spin Asymmetries at CLAS H. Avakian, L.Elouadrhiri (Jefferson Lab) For the CLAS Collaboration Conference onIntersections of Particle and Nuclear Physics Transverse momentum of quarks and spin-azimuthal asymmetries Target single-spin asymmetries Beam single-spin asymmetries in semi-inclusive pion production exclusive pion production Summary

  2. Orbital Angular Momentum and Transverse momentum of quarks Parton Distribution Functions generalized to contain information not only on longitudinal, but also on the transverse distribution of partons: • Generalized Parton Distributions (GPD) H, E ... • Transverse-momentum dependent (TMD) parton distributions Spin asymmetries + azimuthal dependence new class of DIS measurementsSingle-Spin Asymmetries: SSA are sensitive to the orbital motion of quarks, enable measurements of GPDs and kT-dependent PDFs (TMDs) H. Avakian CIPANP 2003 May21

  3. - Dependent Parton Distributions f1, g1 studied for decades: h1 and others essentially unknown Twist-2 - dependent PDFs by Mulders et al. opens access to mixed hadron-parton and parton-hadron transitions Gauge invariant definition of TMDs discussed by Collins and Belitsky,Ji & Yuan Nucl.Phys. B656 165, 2003 TMDs appear in sinf and sin2fSSA In polarized semi-inclusive DIS H. Avakian CIPANP 2003 May21

  4. Polarized Semi-Inclusive DIS Cross section defined by scale variables x,y,z Hadron-Parton/ Parton-Hadron transitions described by distribution and fragmentation functions. Two fundamental QCD mecha- nisms (Collins and Sivers) identified, to generate SSA: H. Avakian CIPANP 2003 May21

  5. Contributions to σ in ep→e’pX σ for longitudinally polarized leptons scattering off unpolarized protons: Different structure functions can be extracted as azimuthal moments of the total cross section. More spin-dependent azimuthal moments arise for polarized targets H. Avakian CIPANP 2003 May21

  6. The CLAS Detector Scattering of 5.7 GeV polarized electrons off polarized NH3 and unpolarized hydrogen n X • ~8M p+ in DIS kinematics, Q2>1 GeV2, W2>4 GeV2, y<0.85, z>0.5 • beam polarization 73% • target polarization 72% (f≈0.2) H. Avakian CIPANP 2003 May21

  7. Longitudinally Pol. Target: SSA for p+ Target SSA: CLAS (4.3 GeV, and 5.7 GeV ) consistent with HERMES (27.5 GeV) predictions for Sivers effect from BHS Phys.Lett.B53099,2002Sivers only interpretation require large AUT AULz-dependence also consistentboth in magnitude and sign with predictions based on Collins mechanism H. Avakian CIPANP 2003 May21

  8. Longitudinally Pol Target: SSA for p+ P1= 0.0110±0.0024 P2=-0.0065±0.0025 D1 – unpol. FF SSA: x-dependence CLAS (5.7 GeV) is consistent with predictions for Collins effect from Efremov et al. hep-ph/0208124 H1 ┬ - Collins FF • sin2f moment (Kotzinian-Mulders asymmetry) • is a clean source of Collins SSA • x 4 more CLAS data available for analysis H. Avakian CIPANP 2003 May21

  9. Unpolarized target: x,z factorization studies at 5.7GeV The z distribution for different ranges of Bjorken x ep → e’ p+ X ( Ee =5.7 GeV, MX > 1.1) No significant variation observed in z dependence for different x ranges. H. Avakian CIPANP 2003 May21

  10. Factorization studies in CFR at CLAS The separation of CFR with 0.5<z<0.8 is not changing significantly with beam energy. DIS: Q2>1 W2>4 y<0.85 No significant variation observed in z dependence of ALU for different x ranges H. Avakian CIPANP 2003 May21

  11. ALU from CLAS 4.3 and 5.7GeV data Low MX in average correspond to large z 5.7GeV No significant dependence on beam energy 4.3 GeV No sign flip in ALU in the exclusive limit (unlike HERMES target SSA) H. Avakian CIPANP 2003 May21

  12. Beam SSA: sinf Moment ALU Beam SSA ALU from CLAS at 4.3GeV and HERMES (SPIN-2002) at 27.5 GeV Beam SSA measurements for different beam energies are consistent. H. Avakian CIPANP 2003 May21

  13. ALU x-dependence: CLAS 4.3 vs. 5.7 GeV 0.5<z<0.8 5.7GeV 4.3 GeV Photon Sivers Effect Afanasev & Carlson (preliminary) CLAS preliminary Beam SSA analyzed also in terms of Collins effect by Efremov et al. 4.3 GeV and 5.7 GeV data consistent H. Avakian CIPANP 2003 May21

  14. Photon Sivers Effect Target SSA (Brodsky et al.) Essential constituents Light-cone spin wave function of a nucleon Beam SSA (Afanasev & Carlson) Final state interaction from gluon exchange Beam SSA has the same source and provides access to essential parameters with less ambiguities H. Avakian CIPANP 2003 May21

  15. Exclusive pions: CLAS 4.3 vs. 5.7GeV W2>4 GeV2,Q2>1 GeV2 Sign flip at z ~ 0.5 4.3 GeV -t < 0.5GeV2 5.7 GeV At z>0.5 struck quark in pion LUND-MC H. Avakian CIPANP 2003 May21

  16. Beam SSA ALU: x,t-dependence t=(PN-PN’)2 <0.5GeV2 ,W2>5GeV2, Q2>2.5GeV2 (<Q2>=3.0GeV2>) CLAS PRELIMINARY CLAS PRELIMINARY - Beam SSA for exclusive pions may provide access to GPDs H. Avakian CIPANP 2003 May21

  17. Summary • Beam spin asymmetries observed for the first time in exclusive p+ electroproduction in hard scattering kinematics. • No significant dependence on x observed for z dependencies of beam SSA. • Kinematic dependencies of beam and target SSA measured for different beam energies are consistent, and can be described by a variety of theoretical models. • Combined analysis of SSA for polarized beam and target needed to separate contributions from different mechanisms and extract corresponding distribution functions. H. Avakian CIPANP 2003 May21

  18. - Dependent Parton Distributions f1, g1 studied for decades: h1 and others essentially unknown Twist-2 - dependent PDFs by Mulders et al. Gauge invariant definition of TMD PDFs discussed by Collins and Belitsky,Ji & Yuan Nucl.Phys. B656 165, 2003 TMDs appear in sinf and sin2fSSA In polarized semi-inclusive DIS H. Avakian CIPANP 2003 May21

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