Parton Model

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Parton Model Parton Dynamics

• Huan Z Huang
• Department of Physics and Astronomy
• University of California, Los Angeles

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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
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Scattering Process
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Momentum Scale and Position Resolution
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Discoveries
New Machine and Energy Frontier ? Discovery
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• Exploring nuclear structure - elastic
  electron-nucleus scattering

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Form Factors
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Form Factors
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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
Q2 -q2
Bjorken Limit Q2,n ? infinite, n2 gtgt Q2
gtgt m2 x ? Q2/(2Mn) xJ
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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
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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
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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
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Parton Model Interpretation of F2
Isospin Symmetry
)
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F2 Measurements
(small)
Neutrino Scattering
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Sketch of Structure Function
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Experimental Data
Scaling Region !
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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 Q2
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Quark Structure Functions
Sea Quarks Dominate
Zd2/Zu2 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 Q2 GeV2
Saturation The saturation scale changes
in nuclei A1/3
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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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Polarized Lepton-Nucleon Scattering
DIS
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Physical meaning of parton spin structure
functions
pQCD processes couple to quarks and gluons
directly !
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?g(x,Q2) from ALL
Relative luminosity of bunch crossings with
different spin configuration
Spin-dependent cross sections
Spin-dependent yields of processes of interest
Beam polarization averaged over bunch crossings
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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
_at_Q2 1.0 GeV2
Result Model-Dependent !
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Spin Physics Program
The Spin Structure of the Proton ½ ½ SDq
DG ltLgt 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 ltLgt and how to measure ltLgt
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RHIC Spin Physics

• At RHIC we use polarized pp collisions to study
• Gluon spin structure function qg?qg
• Sea quark spin structure function qq?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.

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Beam and Polarization

• ?S 200GeV polarized pp beam
• Each beam separated into up to 111 proton
  bunches, each with own spin state
• Spin pattern known for each beam, each bunch
  crossing
• Spin rotators
• rotate beam polarization from transverse
  (longitudinal) to longitudinal (transverse)
  before (after) STAR interaction region
• transverse polarization more stable around ring
• Polarization
• Measured every few hours by pC Coulomb Nuclear
  Interference (CNI) and Hydrogen Jet Target (HJT)
  polarimeters
• HJT calibrated to 5

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Jet ALL measurement
Data Statistics Limited Gluon spin only loosely
constrained!
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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,
spin0 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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Magnetic Moments
Exp -0.685
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Magnetic Moments of Baryons
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The END