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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

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

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Scattering Process

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Momentum Scale and Position Resolution P x > hc ~ 200 MeV fm

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

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

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Form Factors

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Experimental Data

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

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Parton Scattering and Bjorken Limit Q 2 = -q 2 Bjorken Limit: Q 2, infinite, 2 >> Q 2 >> 2 Q 2 /(2M ) = x J

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fraction of the proton momentum carried by the charged parton fraction of the electron energy carried by the virtual photon ( inelasticity ) (mass) 2 of * p system center of mass energy of ep system (momentum transfer) 2 virtuality of *, Z 0, W ( size of the probe) -1 Kinematic Variables

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

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Structure Functions Structure function measurement: Formalism –In terms of laboratory variables: –Formulate this now in relativistic invariant quantities: –Instead of W 1 and W 2, use: F 1 and F 2 : Longitudinal structure function: F L

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Parton Model Interpretation of F 2 ) Isospin Symmetry

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F 2 Measurements (small) Neutrino Scattering

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Sketch of Structure Function

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Scaling Region ! Experimental Data

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From Experimental Measurement to Parton Distribution Function Model Dependent Result !

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Structure Function as a Function of x and Q 2

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Quark Structure Functions Sea Quarks Dominate Z d 2 /Z u 2 ~ 1/4 u quark in p d quark in n dominate ?

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

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Interest in the low x Region

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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

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Saturation in the Parton PDF Saturation: Low x region Scale Q 2 ~ GeV 2 Saturation: The saturation scale changes in nuclei ~ A 1/3

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EMC Effect

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

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Experimental Measurement from lepton Scattering on Polarized Target

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

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Gluon Spin @Q 2 = 1.0 GeV 2 Result Model-Dependent !

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Spin Physics Program The Spin Structure of the Proton: ½ = ½ q + G + 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 and how to measure

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RHIC Spin Physics At RHIC we use polarized p+p collisions to study 1)Gluon spin structure function q+g q+ 2)Sea quark spin structure function q+q W boson 3)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.

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Meson Spectroscopy

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Baryon Spectroscopy

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Where Does the Mass Come From?

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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 !

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Proton Wave Function

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Proton and Neutron Wave Functions

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Magnetic Moments

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Exp ~ -0.685

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Magnetic Moments of Baryons

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The END

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