Parton Model & Parton Dynamics

Huan Z HuangDepartment of Physics and AstronomyUniversity of California, Los Angeles

Department of Engineering PhysicsTsinghua University

Oct 2006 @ Tsinghua

Quantum ChromodynamicsQCD: 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

Scattering Process

Momentum Scale and Position Resolution

P x > hc ~ 200 MeV fm

Rutherford

Hofstadter

SLAC

FNAL

CERN

HERA

Discoveries

New Machine and Energy Frontier Discovery

• Exploring nuclear structure - elastic electron-nucleus scattering

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!

12C

Hofstadter, 1953

Form Factors

qdeqFrf

xdxfeqF

qFd

d

d

d

xqi

xqi

Mott

3/23

3/2

22*

)()2(

1)(

)()(

)(

Form Factors

Form Factors

Experimental Data

Charge Distribution

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

Parton Scattering and Bjorken Limit

p

22 qp Q2 = -q2

222

22

QM

Q

Bjorken Limit: Q2, infinite, 2 >> Q2 >> 2

Q2/(2M) = xJ

fraction of the proton momentum carried by the charged parton

fraction of the electronenergy carried by the virtual photon(inelasticity)

(mass)2 of * p system

center of mass energyof ep system

(momentum transfer)2

virtuality of *, Z0, W

(size of the probe)-1

Kinematic Variables

Exploring the Proton structure - inelastic ep scattering

Friedman, Kendall and Taylor

Scattering of electron (Spin 1/2) on proton (Spin 1/2)

Deep-inelastic scattering (DIS)

Nobel Prize 1990

Scattering on point-like objects: Quarks!

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

Parton Model Interpretation of F2

)

Isospin Symmetry

)()(

)()(

12

21

xxFxF

zxqxF

eNeN

iii

eN

F2 Measurements

(small)

Neutrino Scattering

Sketch of Structure Function

Scaling Region !

Experimental Data

From Experimental Measurement to Parton Distribution Function

Model Dependent Result !

Structure Function as a Function of x and Q2

Quark Structure Functions

Sea Quarks Dominate

Zd2/Zu

2 ~ 1/4

u quark in pd quark in n dominate ?

Total Momentum in Quarks

18.0)()(

36.0)()(

1

0

1

0

dxxdxdxP

dxxuxuxP

d

u

Momentum Carried by u, d quarks:

Proton: uud

Quarks carry ~50% of the momentumGluons ~ 50%

Interest in the low x Region

Saturation Region

Gluon pile up at fixed size until

gluons with strength

act like a hard sphere

Once one size scale is filledMove to smaller size scaleTypical momentum scale

grows

Saturation in the Parton PDF

Saturation: Low x region Scale Q2 ~ GeV2

Saturation: The saturation scale changes in nuclei

~ A1/3

EMC Effect

Gluon EMC Effect

Not measured well in electron or muon scattering experiments !

Why?

How to improve the gluon measurement?

Experimental Measurement from lepton Scattering on Polarized Target

Fit Experimental Data for Spin Structure Functions

u +d –sea –gluon ?

Gluon Spin

@Q2 = 1.0 GeV2

Result Model-Dependent !

Spin Physics Program

The Spin Structure of the Proton:

½ = ½ q + G + <L>

q up, down and strange quarksG gluonsL 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>

RHIC Spin Physics

At RHIC we use polarized p+p collisions to study1) Gluon spin structure function q+gq+2) Sea quark spin structure function q+qW boson3) 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.

Meson Spectroscopy

Baryon Spectroscopy

Where Does the Mass Come From?

QCD Masses Dominate !!

Proton Mass ~ 940 MeV three quarks uud each quark ~ 300 MeV

Pion two quarks: Mass ~ 140 MeV, spin=0Rho meson (same quark content as pion):

Mass ~ 776 MeV

Very large component of the mass from Interactions The Interaction energy strongly spin-dependent !

Proton Wave Function

)21,2

1()0,1(31)2

1,21()1,1(3

2)2

1,2

1( duuduujp mJ

2/)0,1(

Proton and Neutron Wave Functions

Magnetic Moments

Magnetic Moments

3

2

p

n

Exp ~ -0.685

Magnetic Moments of Baryons

The END