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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. Quantum Chromodynamics. QCD: Field Theory of strong interaction in matter

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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


Field Theory of strong interaction in matter

SUflavor(3) X SUcolor(3)


No free quarks, gluons

(so-called partons)

Color singlet


meson baryon


P Dx > hc ~ 200 MeV fm

Momentum Scale and Position Resolution









New Machine and Energy

Frontier  Discovery


Scattering of electron (Spin 1/2) on point-charge charge (Spin 0): Mott cross-section

  • Take into account finite charge distribution: Form factor


Ignore recoil!

Hofstadter, 1953

  • Exploring nuclear structure - elastic electron-nucleus scattering
charge distribution
Charge Distribution

Data from Electron Scatterings!

How do we measure the neutron distribution?

parton scattering and bjorken limit
Parton Scattering and Bjorken Limit

Q2= -q2

Bjorken Limit:

Q2,n infinite,

n2 >> Q2 >> m2

x  Q2/(2Mn) = xJ


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


center of mass energyof ep system

(mass)2 of *p system


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

structure functions
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

f 2 measurements
F2 Measurements


Neutrino Scattering


Experimental Data

Scaling Region !

quark structure functions
Quark Structure Functions

Sea Quarks Dominate

Zd2/Zu2 ~ 1/4

u quark in p

d quark in n

dominate ?

total momentum in quarks
Total Momentum in Quarks

Momentum Carried by u, d quarks:

Proton: uud

Quarks carry ~50% of the momentum

Gluons ~ 50%

saturation region
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

saturation in the parton pdf
Saturation in the Parton PDF


Low x region

Scale Q2 ~ GeV2


The saturation scale

changes in nuclei

~ A1/3

gluon emc effect
Gluon EMC Effect

Not measured well

in electron or

muon scattering

experiments !


How to improve

the gluon


gluon spin
Gluon Spin

@Q2 = 1.0 GeV2

Result Model-Dependent !


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


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>


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.
qcd masses dominate
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 !

magnetic moments43
Magnetic Moments

Exp ~ -0.685