the structure of the nucleon - argentina.gob.arscoccola/iguazu/sp/03_sp03-bijker.pdf · meson cloud...
TRANSCRIPT
The Structure of the Nucleon
• Introduction• Nucleon form factors• Meson cloud effects: two-component model• Strange form factors• Results• Summary and conclusions
Roelof BijkerICN-UNAM
Internal Structure of the Proton
• Anomalous magnetic moment of the proton (Stern, 1930’s)
• Spatial structure of the proton (Hofstadter, 1950’s)• Scaling: quark structure of the proton (Friedman,
Kendall, Taylor, 1960’s)• EMC effect, spin crisis (1988)• Form factor ratio (2000)
• Investigate the structure of the nucleon through theelectromagnetic and weak form factors.
The Structure of the Nucleon
• Introduction• Nucleon form factors• Meson cloud effects: two-componentmodel
• Strange form factors• Results• Summary and conclusion
Form FactorsStructure of particles described by form factors
Form factors hide our ignorance of how the composite particle is constructed.
ER,PRE’, k’
E , k
Electron
ω,q ω2− q2 = -Q2
G(Q2)
M
Target
→
→
→→
→ Time
γ or Z
Elastic ScatteringQ2 = 2Mnω
Nucleon Electromagnetic CurrentDirac and Pauli form factors
Sachs form factors
Rosenbluth Technique
ε linear polarization of the virtual photon
Recoil Polarization Technique
Method: measure ratio of sideways (PT) tolongitudinal (PL) recoil polarization of proton
(MIT-Bates and JLab)
Rosenbluth vs polarization transfer measurements of GE/GM of proton
Jlab/Hall A Polarization data
Jones et al. (2000)Gayou et al. (2002)
SLAC
Rosenbluth data
Two methods, two different results !
Marc Vanderhaeghen
Proton Charge Form FactorJLab spin transfer data have attracted great interest (New York Times, May 6)
However, there is considerable disagreement within the theory community how to interpret the data
Moreover, there is considerable disagreement within the experimental community which data to interpret
Kees de Jager
Jerry Miller
The Structure of the Nucleon
• Introduction• Nucleon form factors• Meson cloud effects: two-component model• Strange form factors• Results• Summary and conclusions
Photon couples to nucleon via vector meson (ρ,ω,φ)Adjust high Q2 behaviour to pQCD scalingInclude the finite width of ρ meson
Vector Meson Dominance
Two-component model:
Intrinsic structure (valence quarks)Meson cloud (quark-antiquark pairs)
Electromagnetic Currents
• Isoscalar em current
• Isovector em current
• Strange em current
Dirac and Pauli Form Factors
Intrinsic form factor
Iachello, Jackson, Lande, PLB 43, 191 (1973)Iachello, Wan, PRC 69, 055204 (2004)
Bijker, Iachello, PRC 69, 068201 (2004)
• Electromagnetic form factors
• Static properties
The Structure of the Nucleon
• Introduction• Nucleon form factors• Meson cloud effects: two-componentmodel
• Strange form factors• Results• Summary and conclusions
Strange Form Factors• The strangeness distribution is a very
sensitive probe of the nucleon’s properties• The strange (anti)quarks come uniquely from
the sea: there is no contamination from up ordown valence quarks
• New data from SAMPLE, HAPPEX, PVA4 andG0 Collaborations
• φ meson mostly strange quarks
Weak Currents
• Neutral weak vector current
• Weak axial current
Strange form factors• Nucleon em form factors
• Proton weak form factor
• Charge symmetry
• Strange form factor
• Static properties
Strange Dirac and Pauli form factors
Physical states
Ideally mixed states
Mixing angle (Jain, 1988)
• Meson-nucleon couplings
• Current-meson couplings Quark in the vector mesons couples only to the quark vector current of the same flavor with a flavor-independent strength
Jaffe – PLB 229, 275 (1989)
Isoscalar Couplings
Four parameters
One constraint
Strange Couplings
One constraint: no net strangeness contributionto the electric charge
Reduction of the number of parameters!
Summary
• Two-component model for theelectromagnetic and weak vector currents
• Intrinsic structure• Isoscalar mesons• Isovector mesons