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A unified description of hadronic form factors Preliminary Results

A unified description of hadronic form factors Preliminary Results. Qian Wan Center for Theoretical Physics, Yale University August 20, 2004. N’. . N. Introduction. Electromagnetic form factors (helicity amplitudes) are closely related to the structure of hadrons.

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A unified description of hadronic form factors Preliminary Results

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  1. A unified description of hadronic form factorsPreliminary Results Qian Wan Center for Theoretical Physics, Yale University August 20, 2004

  2. N’  N Introduction Electromagnetic form factors (helicity amplitudes) are closely related to the structure of hadrons Related Processes: Elastic Electron-Nucleon Scattering, Meson Electroproduction, Photoproduction, … • High Q2 region: pQCD gives out asymptotic behavior • Low Q2 region: Only phenomenological models • Any consistent theory for all hadrons in low Q2 region?

  3. N’ N’ N’  ,,  N N  N Two component Model • Intrinsic structure (algebraic model of baryons) • Meson cloud (vector meson dominance) = + (A) (B)

  4. 1 3 2 Algebraic Structure of Baryons

  5. Algebraic Structure of Baryons

  6. Nucleon EM Form Factors

  7. Nucleon EM Form Factors

  8. Spacelike Region

  9. Timelike Region

  10. Calculations are in perfect agreement with spacelike proton data, but deviate drastically from spacelike neutron data at An analytic continuation of the original model is in excellent agreement with timelike data for both proton and neutron. Nucleon EM Form Factors • The model’s asymptotic behavior agrees with p-QCD. The coupling constants determined by a 1973 calculation indicate significant contributions from meson cloud. • Results agree perfectly with the experiments using the recoil polarization technique. • There might be a discrepancy between spacelike data and timelike data of neutron according to the properties of analytic functions.

  11.  EM Form Factors Helicity Amplitudes:

  12.  EM Form Factors Observables:

  13. Siddle (DNPL 1971) Alder (DESY 1972) Brasse 1976 Burkert (DESY 1979) E19 (SLAC) PDG 2002 Blanpied (LEGS 1997) E89 (SLAC) Bartel (DESY 1968) Beck (DAPHNE 1997) E133 (SLAC 1992) Batzner (BONN 1972) Frolov (TJNAF 1999) Haidan (DESY 1979) Alder (DESY 1972) Kamalov (TJNAF 1999) Stein (SLAC 1975) Mertz (MIT BATES 1998) Frolov (TJNAF 1999) Pospischil (MAMI 2000) Joo (TJNAF CLAS 2001)  EM Form Factors Data:

  14. GAshM(Q2)

  15. REM(Q2)

  16. RSM(Q2)

  17. AT(Q2)

  18. Parameters for  • There is significant contribution from meson cloud • b1=b2 confirms the SU(6) symmetry of the intrinsic part of form factors

  19.  EM Form Factors • Fitting results can perfectly describe experimental data. • The importance of meson cloud is further confirmed by the fitted parameters. • Results show a slightly larger quark core with size parameter  = 0.350 (GeV)-1 than the results for nucleon. • At high Q2, the model predicts REM →Const., RSM →0, although current parameters doesn’t give REM →1.

  20. Future direction • Understand MBB’ vertex better • Explore the choice of other mesons • Use algebraic methods to minimize the number of parameters in the coupling constants • A unified description of EM form factors of all hadrons

  21. Conclusion • EM form factors of baryons may be described by a two component model in terms of an intrinsic q3 structure and a meson cloud. • If the Distributed String Model is used for the intrinsic part and vector mesons are included as the meson cloud, results on N and  agree with experimental data respectively. • It is very likely that a unified description of all EM form factors of hadrons can be obtained based on this two component model.

  22. Thank You!

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