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Photo- and Electro-Production of Mesons on the Nucleon

Photo- and Electro-Production of Mesons on the Nucleon. … powerful tools to explore:. Nucleon. Equipment. Electron scattering Longitudinally polarised beams Longitudinally and transversely polarised targets Polarisation transfer Photo-production Circularly and linearly polarised beams

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Photo- and Electro-Production of Mesons on the Nucleon

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  1. Photo- and Electro-Productionof Mesons on the Nucleon Erice School, 22/9/07

  2. … powerful tools to explore: Nucleon Equipment Electron scattering Longitudinally polarised beams Longitudinally and transversely polarised targets Polarisation transfer Photo-production Circularly and linearly polarised beams Longitudinally and transversely polarised targets Polarisation transfer 4 detector • Structure • Sum rules • Transition form factors • Spin structure • Semi-inclusive DIS • Hard exclusive processes • Spectroscopy • In higher resonance regions • Search for new resonances • MAMI-A2, ELSA-GDH, JLab-CLAS, DESY-HERMES Erice School, 22/9/07

  3. GDH sum rule • Proton: 205 b • Neutron: 233 b Erice School, 22/9/07

  4. GDH @ MAMI Erice School, 22/9/07

  5. GDH @ ELSA Erice School, 22/9/07

  6. Proton excitation function D13(1520) D(1232) F15(1680) F35(1905) Erice School, 22/9/07

  7. GDH integral On proton On neutron  MAMI C Erice School, 22/9/07

  8. Generalised GDH integral DIS pQCD G1 operator product expansion quark models Lattice QCD? ChPT 1 Q2 (GeV2) GDH sum rule Elastic contribution not included The spin-dependent virtual photon Compton amplitudes S1(n,Q2) and S2(n,Q2) are calculable and can be related to measurable structure functions via dispersion relations calculablemeasurable Variation with Q2: Parton Hadron Erice School, 22/9/07

  9. Hall B @ JLab Photon Tagger  Erice School, 22/9/07

  10. Proton G1 = g1(x,Q2)dx • Shows expected trend toward DIS result at high Q2 • At low Q2 we observe a negative slope as expected from GDH Sum Rule. • Agreement with PT at the lowest points. Erice School, 22/9/07

  11. G1d(Q2) (per nucleon) • Like the proton: shows expected behavior at low and high Q2 • Agreement with PT at the lowest points Erice School, 22/9/07

  12. Bjorken Sum: Γ1p-n(Q2) • Agreement with PT up to Q2 = 0.25 (GeV/c)2. • NNLO pQCDin reasonable agreement with the data •  Higher twists are small, even down to Q2 = 0.75 (GeV/c)2 MENU2007, FZ Juelich, 14/9/07

  13. Resonance Transition Form Factors • Improve understanding of nucleon structure in confinement region • γNΔ or γNN* transition amplitudes at various distance scales (Q2)  wave functions of excited states • Diff. cross sections & polarisation observables in full angular range  partial wave analysis • Isospin amplitudes of final state decomposition  resonance flavour e’ , K γv e N*,△* N’,△’, L N A1/2, A3/2, S1/2 Erice School, 22/9/07

  14. NΔ Transition Form Factor GM* • Exclusive π0electro-production data • Meson contributions play an important role even at relatively high Q2. Erice School, 22/9/07

  15. ND Multipole Ratios REM, RSM • Precision measurement of baryon form factors:REM,  RSM < 0.01 • REM remains small and negative at -2% to -3.5% up to 6 GeV2 • NoREM sign change or asymptotic behavior. Helicity conservation:REM→ +100(%)! • RSM negative and increases in magnitude. Helicity conservation:RSM→ constant! K. Joo, et al., PRL88 (2002); J. Kelly et al., PRL95 (2005); M. Ungaro, et al., PRL97 (2006) Erice School, 22/9/07

  16. Comparison with Theory • Quenched Lattice QCD • E1+/M1+: Good agreement within large errors. • S1+/M1+: Undershoots data at low Q2 • Linear chiral extrapolations may not be adeqate or dynamical quarks required • Dynamical Models • Pion cloud model:reasonable description of quadrupole ratios over large Q2 range. Erice School, 22/9/07

  17. Transitions to 2nd resonance region • Poorly understood in nrCQMs • Alternative models: • Light front kinematics (relativity) • Hybrid baryon with gluonic excitation |q3G> • Quark core with large meson cloud |q3m> • Nucleon-sigma molecule |N> • Dynamically generated resonance P11(1440) Hard form factor (slow fall-off with Q2) Not a quark resonance, but KΣ dynamical system? S11(1535) Change of helicity structure with increasing Q2 from λ=3/2 to λ=1/2 dominance, predicted in nrCQMs, pQCD D13(1520) Measure Q2 dependence of Transition FF Erice School, 22/9/07

  18. P11(1440) CQM Comparison @ low Q2 nrCQM nrCQM LC CQM LC CQM • Non-relativistic CQ Models do not reproduce sign of A1/2 at Q2=0, and show no zero-crossing. • Relativistic CQ Models (LC) give correct sign and show zero-crossing, but miss strength at Q2=0, possibly due to missing meson cloud contributions.  Check at higher Q2 Erice School, 22/9/07

  19. P11(1440) Transition FF @ high Q2 Nπ, pπ+π- nπ+ DR UIM pπ0 nπ+ pπ0 • Analysis with • Unitary Isobar Model (UIM) • Fixed-t Dispersion Relations (DR) First zero-crossing observed for a nucleon form factor! Erice School, 22/9/07

  20. S11(1535) in pη and N New CLAS results n+ p0 CLAS 2007 CLAS 2002 pη n+ , p0 previous results pη preliminary PDG 2006 CQM PDG (2006): S11→ N (35-55)% → ηN (45-60)% A1/2 from both pand N agree Erice School, 22/9/07

  21. Transition γ*p  D13(1520) A3/2 Previous pπ0 based data preliminary preliminary A1/2 Q2, GeV2 Q2, GeV2 N, p+- n+ n+ , p0 p0 nrCQM: Erice School, 22/9/07

  22. Helicity Asymmetry for γ*pD13 A21/2 – A23/2 Ahel = A21/2 + A23/2 Ahel D13(1520) CQMs and pQCD Ahel→ +1 at Q2 → ∞ Helicity structure of transition changes rapidly with Q2 from helicity 3/2(Ahel= -1) to helicity 1/2 (Ahel= +1) dominance! Erice School, 22/9/07

  23. Search for “missing” resonances • (Nearly) complete, measurements in p→N, ηN, K+Y and n→N, K0Y by using various combinations of beam, target and recoil polarisations • Differential cross sections with unpolarised, circularly polarised and linearly polarised photon beams • longitudinally or transversely polarised proton and neutron (deuteron) targets • recoil polarisation for hyperons • Other reactions: p → ρN, ωp, N with linearly polarised beams or targets Erice School, 22/9/07

  24. “Missing” Resonances Really missing, or just not observed yet ? Erice School, 22/9/07

  25. Photoproduction of K+Λ and K+Σ0 P11 K exchange P13 P13 Coupled channel fit: Bonn-Gatchina group, Anisovich et al., 2007 Erice School, 22/9/07

  26. Angular distributions Fit with P13(1900) KΛ KΣ Coupled channel fit: Bonn-Gatchina group, Anisovich et al, 2007 Erice School, 22/9/07

  27. Pseudoscalar meson production • Bilinear combinations of 4 complex amplitudes: • 16 observables • Required:8 observables • differential cross section d/d • 3 single polarization observables: P, T,  • 4 double polarization observables • CLAS: • linearly + circularly polarised photons • longitudinal + transverse target polarisation • recoil polarisation in hyperon production I. S. Barker, A. Donnachie, J. K. StorrowNucl. Phys. B 95(1975)347 Erice School, 22/9/07

  28. Polarisation transfer γp  K+Λ K+ proton proton transferred polarization along x  induced polarization along y  transferred polarization along z  density matrix , Pauli spin matrix   Erice School, 22/9/07

  29. Polarisation transfer γp  K+Λ w/o P13(1900) with P13(1900) Quark-Diquark Model (E. Santopinto, 2005) Includes *** / **** states Coupled channel 2007 fit: Bonn-Gatchina group, Anisovichet al. Fit favours existence of second P13state  Evidence against quark-diquark model Erice School, 22/9/07

  30. Beam & target polarisation Polarised HD-Target Frozen Spin Target P(D) P(H) FROST Linearly polarised photons Erice School, 22/9/07

  31. CLAS search for new resonances Erice School, 22/9/07

  32. Polarised semi-inclusive DIS • Flavour separation of • Quark & antiquark helicity distributions by • Detecting hadron h in coincidence with scattered lepton • Flavour content of final state hadrons related to • Flavour of struck quark via fragmentation functions D. • In LO QCD: • Spin-independent purity matrix P from Monte-Carlo Erice School, 22/9/07

  33. HERMES @ DESY Erice School, 22/9/07

  34. Flavour-tagged q’s • First 5-flavour fit to q(x) • Input: • Sizeable positive polarisation of u quarks • Negative polarisation of d quarks • No significant polarisation • In 0.023 < x < 0.6 : Erice School, 22/9/07

  35. Generalised Parton Distributions Nucleon matrix element of quark energy-momentum tensor Formfactors M2(t): Mass distribution inside the nucleon in transverse space J(t): Angular momentum distribution d1(t): Distribution of forces and pressure These form factors are moments of GPD’s, e.g. Ji’s sum rule: Erice School, 22/9/07

  36. GPD’s Erice School, 22/9/07

  37. Quark total angular momentum HERMES 2002-2004 data: • Unpolarised beam, transversely polarised target • Best VGG fit for0.2< Ju< 0.4(assuming u-quark dominance) DVCS Erice School, 22/9/07

  38. Exclusive + production information on polarised GPD’s • no L/T separation, but T suppressed as 1/Q2 • LO VGG X-sections too low • LO VGG X-sections + power correction to space-like pion form factor overestimate data Erice School, 22/9/07

  39. 0transverse target-spin asymmetry • TTSA: access to GPD E • E related to Jq [curves: Vinnikov et al. (2005)] Erice School, 22/9/07

  40. Exclusivity with HERMES 4 detector Erice School, 22/9/07

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