1. The Structure of the Nucleon Introduction Meson cloud effects: two-component model Strange form factors Results Summary and conclusions Roelof Bijker ICN-UNAM

3. 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 the electromagnetic and weak form factors in the space- and time-like regions

4. Form Factors Structure of particles described by form factors Form factors hide our ignorance of how the composite particle is constructed. Elastic Scattering Q 2 = 2M n 

5. Nucleon Electromagnetic Current Dirac and Pauli form factors Sachs form factors

6. The Structure of the Nucleon Introduction Meson cloud effects: two-component model Strange form factors Results Summary and conclusions

7. Photon couples to nucleon via vector meson ( , ,  ) Adjust high Q 2 behaviour to pQCD scaling Include the finite width of  meson Vector Meson Dominance Two-component model: Intrinsic structure (valence quarks) Meson cloud (quark-antiquark pairs)

8. Electromagnetic Currents Isoscalar em current Isovector em current Strange em current

9. Dirac and Pauli Form Factors Intrinsic form factor

10. Iachello, Jackson, Lande, PLB 43, 191 (1973) Iachello, Wan, PRC 69, 055204 (2004) Bijker, Iachello, PRC 69, 068201 (2004)

11. Electromagnetic form factors Static properties

12. The Structure of the Nucleon Introduction Meson cloud effects: two-component model Strange form factors Results Summary and conclusions

13. 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 or down valence quarks New data from SAMPLE, HAPPEX, PVA4 and G0 Collaborations  meson mostly strange quarks

15. Weak Currents Neutral weak vector current Weak axial current

16. Strange form factors Nucleon em form factors Proton weak form factor

17. Charge symmetry Strange form factor Static properties

18. Strange Dirac and Pauli form factors Physical states Ideally mixed states Mixing angle (Jain, 1988)

19. 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)

20. Isoscalar Couplings Four parameters One constraint

21. Strange Couplings One constraint: no net strangeness contribution to the electric charge Reduction of the number of parameters!

22. Summary Two-component model for the electromagnetic and weak vector currents Intrinsic structure Isoscalar mesons Isovector mesons

23. The Structure of the Nucleon Introduction Meson cloud effects: two-component model Strange form factors Results Summary and conclusions

24. Electromagnetic Form Factors 2004 1973

25. Electric Form Factors Andivahis (1994), Christy (2004), Qattan (2004) Herberg (1999), Passchier (1999), Zhu (2001), Golak (2001), Schiavilla (2001), Madey (2003), Bermuth (2003), Warren (2004),

26. Preliminary results from the CLAS collaboration (JLab) Nucleon05: Vineyard Neutron magnetic form factor

27. Scaling Behavior Dimensional analysis (Brodsky, Farrar), pQCD (Brodsky, Lepage) Polarization transfer data Quark orbital angular momentum (Miller,Ralston) Valid in VMD in an intermediate Q 2 region, but not in the asymptotic region (Iachello)

28. pQCD (Belitsky)

29. Strange Form Factors SAMPLE – PLB 583, 79 (2004) HAPPEX – arXiv:nucl-ex/0506010 Nucleon05: Riska, Maas, Kox, Pate, …

30. HAPPEX – PRC 69, 065501 (2004) arXiv:nucl-ex/0506011 PVA4 – PRL 93, 022002 (2004) PRL 95, 152001 (2005)

32. G0 – PRL 95, 092001 (2005)

33. Time-like form factors: Q 2 <0 Experimentally, the time-like form factors can be probed through (Frascati) or the inverse reaction (CERN, FNAL) Theoretically, the space- and time-like form factors are related by analytic continuation

35. Comparison between the 1973 and 2004 VMD calculations IJL (1973) IW (2004) BI (2004) Proton space OK Neutron space X OK Proton time OK +/- Neutron time OK X

36. The Structure of the Nucleon Introduction Meson cloud effects: two-component model Strange form factors Results Summary and conclusions

37. Summary and Conclusions The nucleon has a complex structure: valence quarks and meson cloud Space-like form factors well reproduced Neutron data in the space- and time-like regions seem to be inconsistent (Hammer et al., Tomasi-Gustafsson & Rekalo, Brodsky et al., …) pQCD not reached up to 10 (GeV/c) 2 New experiments to study the nucleon form factors for higher values of Q 2 Extension to strange form factors reduces by one the number of parameters. Excellent description of the SAMPLE, HAPPEX, PVA4 and G0 data.

38. Outlook Extend two-component model to baryon resonances Strange form factors Axial form factors

41. N-  Transition From Factors Two-componet model (Iachello & Wan, IJMPA 20, 1846 (2005)) Generic structure Intrinsic form factors from previous study of baryon resonances (Bijker, Iachello, Leviatan, AP 236, 69 (1994)) Isovector vector meson: 

43. Vector meson dominance Lomon, Iachello, Hammer Relativistic chiral soliton model Goeke, Holzwarth Lattice QCD Schierholz Relativistic constituent quark model Miller, Simula, Giannini, Plessas, Metsch Other theoretical calculations

44. Vector Meson Dominance chargemagnetization proton neutron

45. Relativistic Constituent Quark Models chargemagnetization proton neutron