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