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Spin Structure at High x

This paper presents the results of the precision measurement of valence quark spin structure at high x, including A1n at high x, spin-flavor decomposition, other neutron results, A1p and A1d results, and the precision measurement of g2n and higher twist effects. It discusses the different models and predictions, as well as the importance of quark orbital angular momentum. The publication also includes the results of the A2n, g1n, and g2n measurements.

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Spin Structure at High x

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  1. Spin Structure at High x Jian-Ping Chen, Jefferson Lab For the JLab Hall A, E99-117, E97-103 and CLAS Collaborations DIS2005, Madison, April, 2005 • Introduction • Valence quark neutron spin structure: A1n at high x • Spin-flavor decomposition: Du/u and Dd/d • Other neutron results: A2n, g1n, g2n, and d2n • Preliminary A1p and A1d results • Precision measurement of g2n and higher twist effects • Summary

  2. Unpolarized Parton Distributions (CTEQ6) • After 40 years DIS experiments, unpolarized structure of the nucleon reasonably well understood. • High x  valence quark dominating

  3. Polarized Parton Distributions (BB)

  4. Valence Quark Spin Structure A1 at high x

  5. 1 1 1 ­ = ­ + ­ - ¯ p u ( ud ) u ( ud ) u ( ud ) = = = 0 1 1 S S S 3 2 18 1 2 - ­ - ¯ d ( uu ) d ( uu ) = = 1 1 S S 3 3 Predictions for large xBj Proton Wavefunction (Spin and Flavor Symmetric)

  6. World Data on A1n and Models • SU(6): A1n=0 • Valence quark models • pQCD assuming HHC (hadron helicity conservation) • PDF fits (LSS) • Statistical model • Chiral Soliton model • Local duality model • Cloudy bag model Need precision data at high x

  7. JLab E99-117Precision Measurement of A1n at Large xSpokespersons: J. P. Chen, Z. -E. Meziani, P. Souder, PhD Student: X. Zheng • Precision A1n data at high x 2.7GeV2 < Q2 < 4.8 GeV2, W > 2 GeV • Extracting valence quark spin distributions • Test our fundamental understanding of valence quark picture • SU(6) symmetry • Valence quark models • pQCD (with HHC) predictions • Other models: Statistical Model, Chiral Soliton Model, PDF fits, …. • Quark orbital angular momentum • Crucial input for pQCD fit to PDF

  8. E99-117 Setup at JLab Hall A 5.7 GeV, 80% polarized electron beam on 40% polarized 3He target Luminosity~1036

  9. E99-117 A1n Results • First precision A1n data at x > 0.3 • Comparison with model calculations • SU(6) symmetry • Valence quark models • pQCD (with HHC) predictions • Other models: Statistical model, Chiral Soliton model, PDF fits, … • Crucial input for pQCD fit to PDF • PRL 92, 012004 (2004)

  10. Polarized Quark Distributions • Combining A1n and A1p results • Valence quark dominating at high x • u quark spin as expected • d quark spin stays negative! • Disagree with pQCD model calculations assuming HHC (hadron helicity conservation) • Quark orbital angular momentum • Consistent with valence quark models, statistical model or pQCD PDF fits

  11. Comparison with models • Chiral Soliton Models • G.A. Miller’s model with quark orbital angular momentum, which gives an explanation to the proton GEp/GMp results. • pQCD: factorization at large x, NLO, Resummation, …?

  12. A2n results • By-product • Precision better than the world best results • Also g1n and g2n results • Improved d2n precision by a factor of 2: d2n=0.0062 ± 0.0028 • PRC 70, 065207 (2004)

  13. Precision g1n and g2n results

  14. Discussion • Precision data of A1n and g1n at high x: A1n positive above x=0.5 • Extracted Du/u and Dd/d: Dd/d stays negative! • Disagree with leading order pQCD model (HHC) Quark orbital angular momentum important • g1n, g2n, A2n data • Extracted d2n, a factor of 2 improvement Results published in PRL 92, 012004 (2004), PRC 70, 065207(2004) and in the news: AIP Physics News Update, Physics Today Update, Science online (Science Now), Science News, DNP web featured article

  15. A1 with 12 GeV Upgrade One of the flagship experiments: complete a chapter on the valence quark structure

  16. A1p, A1d at high x Preliminary results from JLab CLAS (S. Kuhn, et al.)

  17. Preliminary CLAS A1p results: W>2

  18. Preliminary CLAS A1d results: W>2

  19. Precision measurement of g2n Higher twist effects: quark-gluon correlations

  20. Quark-Gluon Correlations • In simple partonic picture g2(x)=0 • Wandzura and Wilczek have shown that g2 can be written in two parts: • twist-2 contributions given by g1 • the other originating from quark-gluon correlations (twist-3)

  21. Jefferson Lab Hall A Experiment E97-103 Precision Measurement of the Neutron Spin Structure Function g2n(x,Q2): A Search for Higher Twist Effects T. Averett, W. Korsch (spokespersons) K. Kramer (Ph.D. student) • Precision g2n, 0.57 < Q2 < 1.34 GeV2,W > 2 GeV, at x ~ 0.2. • Direct comparison to twist-2 g2ww prediction using world g1n data. • Quantitative measurement of higher twist effects provides information on • nucleon structure beyond simple parton model (e.g. quark-gluon correlations).

  22. E97-103 Results: g2n vs. x Improved precision of g2n by an order of magnitude

  23. E97-103 results: g2n vs. Q2 • Measured g2n consistently higher than g2ww

  24. E97-103 results: g1n • Agree with NLO fit to world data, evolved to our Q2

  25. Summary • Precision spin structure data at high x from JLab • Valence quark neutron spin structure • A1n at high x, an order of magnitude improvement: • Crucial input to fundamental understanding of valence quark picture: first time clearly goes positive • Valence spin-flavor decomposition: Du/u, Dd/d • Disagreement with pQCD (HHC)  Quark OAM important • A2n, g1n, g2n • d2n:a factor of 2 improvement • Proton/deuteron results: A1p and A1d • Precision measurement of g2nat low Q2 • An order of magnitude improvement in precision • g2n consistently higher than g2 WW • Higher twist effects: quark-gluon correlations

  26. A1n and Du/u, Dd/d results in the news • Physics News Update, 12/18/2003 ‘Bringing the Nucleon into Sharper Focus’ • Science Now , 12/23/2003 ‘Quarks in a Surprising Spin’ • Science News, 1/3/2004 ‘Topsy Turvy’ • Physics Today Update, 2/2004 ‘Spinning the Nucleon into Sharper Focus’ • APS-DNP current research topic, 5/5/2004 ’The Spin Structure of the Nucleon in the Valence Quark Region’

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