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Experimental Study of Nucleon Structure and QCD

This workshop discusses the experimental results and ongoing research on the structure of nucleons and Quantum Chromodynamics (QCD). Topics include spin distributions, quark-hadron duality, moments of spin structure functions, and planned experiments with JLab 12 GeV. The importance of QCD in discovering new physics beyond the Standard Model and the challenges of understanding confinement are also emphasized.

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Experimental Study of Nucleon Structure and QCD

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  1. Experimental Study of Nucleon Structure and QCD J. P. Chen, Jefferson Lab Workshop on Confinement Physics, March 12, 2012 • Introduction • Selected JLab 6 GeV Experimental Results • Spin Distributions in the High-x (Valence Quark) Region • and Quark-Hadron Duality • Moments of Spin Structure Functions: • Spin Sum Rules and Polarizabilities • Transverse Spin, TMDs • Planned Experiments with JLab 12 GeV

  2. QCD: still unsolved in non-perturbative region • 2004 Nobel prize for ``asymptotic freedom’’ • non-perturbative regime QCD ? • Confinement: one of the top 10 challenges for physics! • QCD: Important for discovering new physics beyond SM • Nucleon structure is one of the most active areas

  3. Introduction • Quarks/Gulons are confined in hadron • To study/understand confinement: both static (spectroscopy) and dynamics • Nucleon: an ideal laboratory to study strong interaction (QCD) • Nucleon = valence quarks (uudor udd) + sea + gluons • Mass, charge, magnetic moment, spin, axial charge, tensor charge • Decomposition of each of the fundamental quantities Mass: ~1 GeV, but u/d quark mass only a few MeV each! Momentum: quarks carry ~ 50% Spin: ½, quarks contribute ~30% Spin Sum Rule Orbital Angular Momentum Relations to TMDs and GPDs Tensor charge Lattice QCD • Quarks and gluon field are in-separable • Multi-parton correlations are important • Transverse dimension is crucial for understanding nucleon structure and QCD, help understanding confinement • Elastic (Form Factors), Resonances, DIS, Spin, Transverse Spin, TMDs, GPDs

  4. Three Decades of Spin Structure Study • 1980s: EMC (CERN) + early SLAC quark contribution to proton spin is very small DS = (12+-9+-14)% !‘spin crisis’ (Ellis-Jaffe sum rule violated) • 1990s: SLAC, SMC (CERN), HERMES (DESY) DS = 20-30% the rest: gluon and quark orbital angular momentum A+=0 (light-cone) gauge(½)DS + Lq+ DG + Lg=1/2 (Jaffe) gauge invariant (½)DS + Lq + JG =1/2 (Ji) New decomposition (X. Chen, et. Al, Wakamatsu, …) What observable directly corresponds to Lz~ bx X py ? Bjorken Sum Rule verified to <10% level • 2000s: COMPASS (CERN), HERMES, RHIC-Spin, JLab, … : • DS ~ 30%; DGprobably small,orbital angular momentum probably significant • Valence Quark Spin Distributions • Sum Rules at low Q2, Higher-Twists • Transversity, Transverse-Momentum Dependent Distributions

  5. JLab Spin Experiments • Results: • Spin in the valence (high-x) region • Spin (g1/g2) Moments: Spin Sum Rules, Spin Polarizabilities • SSA in SIDIS: Transversity, TMDs • On-going • g2pat low Q2 • Future: 12 GeV • Inclusive: A1/d2, • Semi-Inclusive: Transversity, TMDs, Flavor-decomposition • Reviews: S. Kuhn, J. P. Chen, E. Leader, Prog. Part. Nucl. Phys. 63, 1 (2009)

  6. Valence Quark Spin Structure A1 at high x and flavor decomposition

  7. Why Are PDFs at High x Important? • Valence quark dominance: simpler picture -- direct comparison with nucleon structure models SU(6) symmetry, broken SU(6), diquark • x 1 region amenable to pQCD analysis -- hadron helicity conservation? role of quark orbit angular momentum? • Clean connection with QCD, via lattice moments (d2) • Input for search for new physics at high energy collider -- evolution: high x at low Q2 low x at high Q2 -- small uncertainties amplified -- example: HERA ‘anomaly’ (1998)

  8. World data for A1 Proton Neutron

  9. JLab E99-117Precision Measurement of A1nat Large x Spokespersons: J. P. Chen, Z. Meziani, P. Souder; PhD Student: X. Zheng • First precision A1n data at high x • Extracting valence quark spin distributions • Test our fundamental understanding of valence quark picture • SU(6) symmetry • Valence quark models • pQCD (with HHC) predictions • Quark orbital angular momentum • Crucial input for pQCD fit to PDF • PRL 92, 012004 (2004) • PRC 70, 065207 (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 and pQCD PDF fits without HHC constraint

  11. pQCD with Quark Orbital Angular Momentum H. Avakian, S. Brodsky, A. Deur, and F. Yuan, PRL 99, 082001 (2007) Inclusive Hall A and B and Semi-Inclusive Hermes BBS BBS+OAM

  12. Spin-Structure in Resonance Region: E01-012Study Quark-Hadorn Duality Spokesperson: N. Liyanage, J. P. Chen, S. Choi;PhD Student: P. Solvignon PRL 101, 1825 02 (2008) A13He (resonance vs DIS) G1 resonance vs. pdfs x Q2 x

  13. Projections for JLab at 11 GeV A1p at 11 GeV (CLAS12) A1n at 11 GeV (Hall C/A)

  14. Moments of Spin Structure Functions Sum Rules, Polarizabilities

  15. First Moment of g1p:G1p Total Quark Contribution to Proton Spin (at high Q2) Twist expansion at intermediate Q2, LQCD, ChPT at low Q2 G1p EG1b, arXiv:0802.2232 EG1a, PRL 91, 222002 (2003) Spokespersons: V. Burkert, D. Crabb, G. Dodge,

  16. First Moment of g1n:G1n G1n E94-010, PRL 92 (2004) 022301 E97-110, preliminary EG1a, from d-p

  17. G1 of p-n EG1b, PRD 78, 032001 (2008) E94-010 + EG1a: PRL 93 (2004) 212001

  18. Effective Coupling Extracted from Bjorken Sum A. Deur, V. Burkert, J. P. Chen and W. Korsch PLB 650, 244 (2007) and PLB 665, 349 (2008) as/p

  19. Second Spin Structure Function g2 Burkhardt - Cottingham Sum Rule Spin Polarizabilities

  20. Precision Measurement of g2n(x,Q2): Search for Higher Twist Effects • Measure higher twist quark-gluon correlations. • Hall A Collaboration, K. Kramer et al., PRL 95, 142002 (2005)

  21. Preliminary results on neutron from E01-012Spokespersons: J. P. Chen, S. Choi, N. Liyanage, plots by P. Solvignon

  22. Burkhardt - Cottingham Sum Rule 0<X<1 :Total Integral P Brawn: SLAC E155x Red: Hall C RSS Black: Hall A E94-010 Green: Hall A E97-110(preliminary) Blue: Hall A E01-012 (spokespersons: N. Liyanage, former student, JPC) (preliminary) N BC = Meas+low_x+Elastic “Meas”: Measured x-range 3He • “low-x”: refers to unmeasured low x part • of the integral. • Assume Leading Twist Behaviour very prelim Elastic: From well know FFs (<5%)

  23. BC Sum Rule P BC satisfied w/in errors for JLab Proton 2.8 violation seen in SLAC data N BC satisfied w/in errors for Neutron (But just barely in vicinity of Q2=1!) 3He very prelim BC satisfied w/in errors for 3He

  24. Neutron Spin Polarizabilities • dLT insensitive to D resonance • RB ChPT calculation with resonance for g0 agree with data at Q2=0.1 GeV2 • Significant disagreement between data and both ChPT calculations for dLT • Good agreement with MAID model predictions g0dLT E94-010, PRL 93 (2004) 152301 Q2 Q2

  25. Spin Polarizabilities Preliminary E97-110 (and Published E94-010)Spokesperson: J. P. Chen, A. Deur, F. Garibaldi, plots by V. Sulkosky • Significant disagreement between data and both ChPT calculations for dLT • Good agreement with MAID model predictions g0dLT Q2 Q2

  26. Axial Anomaly and the LTPuzzle N. Kochelev and Y. Oh; arXiv:1103.4891v1

  27. E08-027 : Proton g2 Structure Function Fundamental spin observable has never been measured at low or moderate Q2 Spokespersons: Camsonne, Chen, Crabb, Slifer(contact), 6 PhD students, 3 postdocs • BC Sum Rule : violation suggested for proton at large Q2,but found satisfied for the neutron & 3He. • Spin Polarizability: Major failure (>8s) of PT for neutron dLT. Need g2 isospinseparation to solve. • Hydrogen HyperFine Splitting : Lack of knowledge of g2 at low Q2 is one of the leading uncertainties. • Proton Charge Radius : also one of the leading uncertainties in extraction of <Rp> from m-H Lamb shift. Running until 5/2012 BC Sum Rule Spin Polarizability LT

  28. Single Target-Spin Asymmetries in SIDIS Transversity/Tensor Charge

  29. Transversity • Three twist-2 quark distributions: • Momentum distributions: q(x,Q2) = q↑(x) + q↓(x) • Longitudinal spin distributions: Δq(x,Q2) = q↑(x) - q↓(x) • Transversity distributions: δq(x,Q2) = q┴(x) - q┬(x) • It takes two chiral-odd objects to measure transversity • Semi-inclusive DIS Chiral-odd distributions function(transversity) Chiral-odd fragmentation function(Collins function) • TMDs: (without integrating over PT) • Distribution functions depends on x, k┴ and Q2 : δq, f1T┴ (x,k┴,Q2), … • Fragmentation functions depends on z, p┴ and Q2 : D, H1(x,p┴,Q2) • Measured asymmetries depends on x, z, P┴ and Q2 : Collins, Sivers, … (k┴, p┴ and P┴ are related)

  30. Nucleon Spin Quark Spin Leading-Twist TMD PDFs h1= Boer-Mulders f1 = h1L= Worm Gear (Longi-Tranversity) Helicity g1 = h1= Transversity f1T= g1T= h1T= Sivers Worm Gear Trans-Helicity Pretzelosity : Survive trans. Momentum integration

  31. Unified View of Nucleon Structure d2kT drz d3r TMDs f1u(x,kT), .. h1u(x,kT)‏ GPDs 6D Dist. Wpu(x,kT,r ) Wigner distributions 3D imaging dx & Fourier Transformation d2kT d2rT Form Factors GE(Q2), GM(Q2)‏ PDFs f1u(x), .. h1u(x)‏ 1D

  32. Separation of Collins, Sivers and pretzelocity effects through angular dependence

  33. COMPASS Sivers asymmetry 2010 data x > 0.032 region - comparison with HERMES results NEW NEW

  34. Status of Transverse Spin Study Large single spin asymmetry in pp->pX Collins Asymmetries - sizable for the proton (HERMES and COMPASS) large at high x,p- and p+has opposite sign unfavored Collins fragmentation as large as favored (opposite sign)? - consistent with 0 for the deuteron (COMPASS) Sivers Asymmetries - non-zero for p+ from proton (HERMES), new COMPASS data - consistent with zero for p- from proton and for all channels from deuteron - large for K+? Collins Fragmentation from Belle Global Fits/models: Anselmino, Prokudin et al., Vogelsang/Yuan et al., Pasquini et al., Ma et al., … Very active theoretical and experimental efforts RHIC-spin, JLab (6 GeV and 12 GeV), Belle, FAIR, J-PARC, EIC, … First neutron measurement from Hall A 6 GeV (E06-010) Solenoid with polarized 3He at JLab 12 GeV Unprecedented precision with high luminosity and large acceptance

  35. E06-010 3He Target Single-Spin Asymmetry in SIDISSpokespersons: J. P. Chen, E. Cisbani, H. Gao, X. Jiang, J-C. Peng, 7 PhD students X. Qian, et al. PRL (2011) 107:072003 (2011) 3He Collins SSA small Non-zero at highest x for p+ 3He Sivers SSA: negative for π+, Blue band: model (fitting) uncertainties Red band: other systematic uncertainties

  36. Results on Neutron Collins asymmetries are not large, except at x=0.34 Sivers negative Blue band: model (fitting) uncertainties Red band: other systematic uncertainties

  37. Asymmetry ALT Result • 3He ALT Positive for p- Preliminary To leading twist:

  38. Asymmetry ALT Result J. Huang et al., PRL • 3He ALT : Positive for p- Preliminary To leading twist:

  39. Neutron ALT Extraction • Corrected for proton dilution, fp • Predicted proton asymmetry contribution < 1.5% (π+), 0.6% (π-) • Dominated by L=0 (S) and L=1 (P) interference • Consist w/ model in signs, suggest larger asymmetry Trans-helictiy Preliminary

  40. JLab 12 GeV Era: Precision Study of TMDs From exploration to precision study with 12 GeV JLab Transversity: fundamental PDFs, tensor charge TMDs: 3-d momentum structure of the nucleon  Quark orbital angular momentum Multi-dimensional mapping of TMDs 4-d (x,z,P┴,Q2) Multi-facilities, global effort Precision  high statistics high luminosity and large acceptance

  41. (study done with CDF magnet, 1.5T) GEMs

  42. 12 GeV: Mapping of Collins/Siver Asymmetries with SoLID E12-10-006 3He(n), Spokespersons: J. P. Chen, H. Gao, X. Jiang, J-C. Peng, X. QianE12-11-007(p) , Spokespersons: K. Allda, J. P. Chen, H. Gao, X. Li, Z-E. Mezinai • Both p+ and p- • For one z bin (0.4-0.45) • Will obtain many z bins (0.3-0.7) • Tensor charge

  43. Map Collins and Sivers asymmetries in 4-D (x, z, Q2, PT)

  44. Expected Improvement: Sivers Function f1T= • Significant Improvement in the valence quark (high-x) region • Illustrated in a model fit (from A. Prokudin)

  45. E12-11-107: Worm-gear functions (“A’ rating: ) Spokespersons: Chen/Huang/Qiang/Yan • Dominated by real part ofinterference between L=0 (S) and L=1 (P) states • No GPD correspondence • Lattice QCD -> Dipole Shift in mom. space. • Model Calculations -> h1L =? -g1T. h1L= Longi-transversity Trans-helicity Center of points: g1T=

  46. Discussion Unprecedented precision 4-d mapping of SSA Collins and Sivers p+, p- and K+, K- New proposal polarized proton with SoLID Study factorization with x and z-dependences Study PT dependence Combining with the world data extract transversity and fragmentation functions for both u and d quarks determine tensor charge study TMDs for both valence and sea quarks study quark orbital angular momentum study Q2 evolution Global efforts (experimentalists and theorists), global analysis much better understanding of multi-d nucleon structure and QCD Longer-term future: EIC to map sea and gluon SSAs

  47. Summary • Nucleon (spin) Structure provides valuable inf on QCD dynamics • A decade of experiments from JLab: exciting results • valence spin structure , duality • spin sum rules and polarizabilities • precision measurements of g2: high-twist • first neutron transverse spin results: Collins/Sivers/ALT • Bright future • 12 GeV Upgrade will greatly enhance our capability • Precision determination of the valence quark spin structure flavor separation • Precision extraction of transversity/tensor charge/ TMDs

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