parton model & parton dynamics huan z huang department of physics and astronomy university of...
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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