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Parton Model & Parton Dynamics

Parton Model & Parton Dynamics. Huan Z Huang Department of Physics and Astronomy University of California, Los Angeles Department of Engineering Physics Tsinghua University Oct 2006 @ Tsinghua. Quantum Chromodynamics. QCD: Field Theory of strong interaction in matter

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Parton Model & Parton Dynamics

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  1. Parton Model & Parton Dynamics Huan Z Huang Department of Physics and Astronomy University of California, Los Angeles Department of Engineering Physics Tsinghua University Oct 2006 @ Tsinghua

  2. Quantum Chromodynamics QCD: Field Theory of strong interaction in matter SUflavor(3) X SUcolor(3) Confinement: No free quarks, gluons (so-called partons) Color singlet Hadrons: meson baryon

  3. Scattering Process

  4. P Dx > hc ~ 200 MeV fm Momentum Scale and Position Resolution

  5. Rutherford Hofstadter SLAC FNAL CERN HERA Discoveries New Machine and Energy Frontier  Discovery

  6. Scattering of electron (Spin 1/2) on point-charge charge (Spin 0): Mott cross-section • Take into account finite charge distribution: Form factor 12C Ignore recoil! Hofstadter, 1953 • Exploring nuclear structure - elastic electron-nucleus scattering

  7. Form Factors

  8. Form Factors

  9. Form Factors

  10. Experimental Data

  11. Charge Distribution Data from Electron Scatterings! How do we measure the neutron distribution?

  12. Parton Scattering and Bjorken Limit Q2= -q2 Bjorken Limit: Q2,n infinite, n2 >> Q2 >> m2 x  Q2/(2Mn) = xJ

  13. Kinematic Variables (momentum transfer)2 virtuality of *, Z0, W  (sizeof the probe)-1 fraction of the proton momentum carried by the charged parton fraction of the electronenergy carried by the virtual photon (inelasticity) center of mass energyof ep system (mass)2 of *p system

  14. Scattering on point-like objects: Quarks! Exploring the Proton structure - inelastic ep scattering • Scattering of electron (Spin 1/2) on proton (Spin 1/2) Deep-inelastic scattering (DIS) Nobel Prize 1990 Friedman, Kendall and Taylor

  15. Structure Functions • Structure function measurement: Formalism • In terms of laboratory variables: • Formulate this now in relativistic invariant quantities: • Instead of W1 and W2, use: F1 and F2: Longitudinal structure function: FL

  16. Parton Model Interpretation of F2 Isospin Symmetry )

  17. F2 Measurements (small) Neutrino Scattering

  18. Sketch of Structure Function

  19. Experimental Data Scaling Region !

  20. From Experimental Measurement to Parton Distribution Function Model Dependent Result !

  21. Structure Function as a Function of x and Q2

  22. Quark Structure Functions Sea Quarks Dominate Zd2/Zu2 ~ 1/4 u quark in p d quark in n dominate ?

  23. Total Momentum in Quarks Momentum Carried by u, d quarks: Proton: uud Quarks carry ~50% of the momentum Gluons ~ 50%

  24. Interest in the low x Region

  25. Saturation Region Gluon pile up at fixed size until gluons with strength act like a hard sphere Once one size scale is filled Move to smaller size scale Typical momentum scale grows

  26. Saturation in the Parton PDF Saturation: Low x region Scale Q2 ~ GeV2 Saturation: The saturation scale changes in nuclei ~ A1/3

  27. EMC Effect

  28. Gluon EMC Effect Not measured well in electron or muon scattering experiments ! Why? How to improve the gluon measurement?

  29. Experimental Measurement from lepton Scattering on Polarized Target

  30. Fit Experimental Data for Spin Structure Functions u + d – sea – gluon ?

  31. Gluon Spin @Q2 = 1.0 GeV2 Result Model-Dependent !

  32. Spin Physics Program The Spin Structure of the Proton: ½ = ½ SDq + DG + <L> q  up, down and strange quarks G  gluons L  angular momentum of quarks and gluons Experimentally: 1) total spin in quarks ~ 30% 2) sea quarks are polarized too 3) little info about the gluon polarization 4) even less know about <L> and how to measure <L>

  33. RHIC Spin Physics • At RHIC we use polarized p+p collisions to study • Gluon spin structure function q+gq+g • Sea quark spin structure function q+qW boson • Quark transverse spin distribution • Essential to measure photons, electrons and jets ! • STAR: TPC and Electromagnetic Calorimeter- • Lead/Plastic Scintillator sandwich, • Shower Max Detector for electron/hadron • separation.

  34. Meson Spectroscopy

  35. Baryon Spectroscopy

  36. Where Does the Mass Come From?

  37. QCD Masses Dominate !! Proton Mass ~ 940 MeV three quarks uud each quark ~ 300 MeV Pion two quarks: Mass ~ 140 MeV, spin=0 Rho meson (same quark content as pion): Mass ~ 776 MeV Very large component of the mass from Interactions The Interaction energy strongly spin-dependent !

  38. Proton Wave Function

  39. Proton and Neutron Wave Functions

  40. Magnetic Moments

  41. Magnetic Moments Exp ~ -0.685

  42. Magnetic Moments of Baryons

  43. The END

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