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g 1 (x) and g 2 (x) in the Meson Cloud Model

g 1 (x) and g 2 (x) in the Meson Cloud Model. Tony Signal Institute of Fundamental Sciences Massey University New Zealand. West Island . Overview. Meson Cloud model Meson cloud contributions to spin structure functions Conclusion. Meson Cloud Model.

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g 1 (x) and g 2 (x) in the Meson Cloud Model

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  1. g1(x) and g2(x) in the Meson Cloud Model Tony Signal Institute of Fundamental Sciences Massey University New Zealand

  2. West Island

  3. Overview • Meson Cloud model • Meson cloud contributions to spin structure functions • Conclusion

  4. Meson Cloud Model • Crucial observation (Sullivan 72) - • Pion cloud contribution to DIS scales • Implies quark dists of proton modified • Convolution

  5. Meson Cloud Model

  6. Meson Cloud Model • Fock expansion of proton waveftn. • Bare states are SU(6) symmetric • p  BM vertices described by Lint plus form factor • FF constrained by elastic cross-sec • Quick convergence  Small prob. of high mass states • Model incorporates structure + interactions • Can investigate high-energy  low-energy pictures, symmetry breaking etc.

  7. Gottfried Sum Rule • Isospin broken in sea • MCM gives good fit to

  8. Spin dependent Quark Dists. • Extend MCM to include vector mesons • p cloud dilutes proton spin • r, w, K* able to carry spin • Include interference terms • Get reasonably good agreement with HERMES semi-inclusive data for sea distributions

  9. Spin Dependent Sea Dists. • Data from Hermes • Small symmetry breaking

  10. Spin Dependent Structure Functions • Dominated by valence distributions • NN, N most important fluctuations • At finite Q2 spin of cloud hadrons are not parallel with initial nucleon spin • Both longitudinal and transverse spin components of cloud contribute to observed structure functions

  11. Spin Dependent Structure Functions

  12. Spin Dependent Fluctuations • Long. Fluctuations require both N and D • s = 3/2 state important

  13. Spin Dependent Fluctuations • N is more important for transverse fluct. • n.b g  0.19

  14. ‘Bare’ Hadron SFs • Use bag model for N, D parton dists • Add Dg(x) ‘by hand’ • Hyperfine splitting between N and D • Use NLO evolution • Unpol. dists agree with DIS data • g2(x) from Wandzura-Wilczek • No higher twist component

  15. ‘Bare’ Hadron g1(x) • Data from Hermes

  16. ‘Bare’ Hadron g2(x) • Data from E155, Jlab E-99-117

  17. MCM Contributions to g1p

  18. MCM Contributions to g1n

  19. MCM Contributions to g2p

  20. MCM Contributions to g2n

  21. Summary • Longitudinal (g1) structure ftns of cloud hadrons affect observed transverse (g2) structure ftns • MCM contributions to g1 and g2 of proton are small • MCM contributions to g1, g2 of neutron are 10% • Similar size to higher twist in g2n • Theorists have to be careful!

  22. Thanks • Fu-Guang Cao (Massey) • François Bissey (Massey)

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