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QCD Spin Physics: Some Recent Highlights

This presentation highlights recent developments in Quantum Chromodynamics (QCD) spin physics, focusing on the internal structure of nucleons and the role of gluons, partons, and orbital angular momentum. The discussion includes insights from recent experiments, emphasizing the proton spin crisis over the past 25 years and advancements in nucleon helicity parton distribution functions. The relevance of fragmentation functions and transverse-spin phenomena in understanding these complex interactions will also be explored, alongside implications for ongoing research and theoretical frameworks.

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QCD Spin Physics: Some Recent Highlights

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  1. QCD Spin Physics:Some Recent Highlights Werner Vogelsang Tübingen University IWHSS12, Lisboa, 04/16/2012

  2. Exploring the nucleon: a fundamental quest Know what we are made of ! Understand the strong force: “QCD” Use protons as tool for discovery (e.g. LHC )

  3. 25 years since the “proton spin crisis”: We have come a long way … •what role do gluons play for the proton spin ? •and strangeness ? •what orbital angular momenta do partons carry ? •what’s the 3D image of the nucleon in terms of quarks and gluons  spacially ( GPDs) ?  in momentum (TMDs) ? A far-developed theory framework to address this Plus, first-rate experimental facilities

  4. Today’s talk : Not a comprehensive review – a collection of recent results with emphasis on relevance for COMPASS  Nucleon helicity PDFs • status, latest results • theory efforts  Role of fragmentation functions  Transverse-spin phenomena

  5. Nucleon Helicity Parton Distributions

  6. •NLO (MS) “global analysis” de Florian, Sassot, Stratmann, WV Probes of nucleon helicity structure:

  7. Status ~ 2009 : de Florian, Sassot, Stratmann, WV small in measured regime ! best constrained driven by SIDIS SIDIS indications for DIS + SU(3) strangeness puzzle?

  8. Recent developments: new input from experiments

  9. •recent COMPASS semi-inclusive data: not covered by HERMES not available from HERMES DSSV+ (Stratmann,Sassot)

  10. •implications for ? Stratmann @ DIS2011 •tendency toward negative low-x also from SIDIS ? •heavily relies on kaon fragmentation Leader,Sidorov,Stamenov

  11. •still have a lot to learn about strangeness: HERMES ATLAS

  12. New devlopments on : gives gluon with

  13. DSSV

  14.  Lepton scattering: (yet to be included in global analysis)

  15. COMPASS cross section for NLO: Jäger, Stratmann, WV thesis C. Hoeppner / A. Morreale

  16. Fixed-target regime: Large logarithmic terms only soft/collinear gluons allowed  all-order resummation relevant

  17. LO NLO NNLO · · · · · · LL NLL Fixed order Resummation

  18. All-order resummation: Laenen, Oderda, Sterman; Catani et al.; Kidonakis, Sterman; Bonciani et al.; de Florian, WV; Almeida, Sterman, WV • soft-gluon effects exponentiate : Mellin moment in Leading logarithms:

  19. Beyond leading logs: h large-angle soft gluons (NLL) soft & coll. gluons (LL)

  20. de Florian, Pfeuffer, Schäfer, WV (prel.)

  21. News on fragmentation functions

  22. Fragmentation functions Dqh(z): •crucial for pQCD description of hadron-production data: •encode hadronization in hard-scattering reactions •universality / factorization •global analyses with uncertainty estimates: de Florian, Stratmann, Sassot (DSS) e+e-, ep, pp Albino, Kniehl, Kramer (AKK) e+e-, pp Hirai, Kumano, Nagai. Sudoh (HKNS) e+e-

  23. corrected mistake in NLO calculation ! direct only thesis C. Hoeppner

  24. (M. Stratmann at INT workshop 02/2012) LEP data mostly fix singlet !

  25. BELLE (M. Leitgab at DIS 2012) DSS (LO) •marks new era of precision fragmentation functions. Evolution  e+e- vs. RHIC(pp) •beware of large-z effects / power corrections !

  26. courtesy de Florian,Pinto,Sassot,Stratmann (w/o HERMES data)

  27. courtesy de Florian,Pinto,Sassot,Stratmann (w/o HERMES data)

  28. Transverse-spin phenomena

  29. STAR, BRAHMS, PHENIX E704 HERMES, COMPASS, JLab

  30. • single-inclusive processes, e.g. • single large scale p • power-suppressed ~1/p in QCD • collinear factorization • probe qqg correlations, e.g. • two-scale processes: small & measured q << Q • SIDIS at HERMES,COMPASS • TMD factorization for simplest observables, e.g. Sivers • crucial role of gauge links / non-universality • spin-orbit correlations, “lensing”, 3D imaging

  31. TMDs and twist-3 functions are closely related: Boer,Mulders,Pijlman, Ji, Qiu,WV,Yuan; Koike,WV,Yuan Zhou,Yuan,Liang Bacchetta,Boer,Diehl,Mulders • at operator level: • it means we can confront SIDIS and RHIC data  a sign puzzle Kang,Qiu,Yuan,WV

  32. from • Collins-type effect dominant in ? Kang,Qiu,Yuan,WV from pp What to conclude ? inconsistency in QCD formalism for single-spin ? more mundane : can one get away with nodes in x / kT?

  33. Joint fit to SIDIS and pp data: Kang,Prokudin SIDIS pp (STAR) Works (reasonably) well for SIDIS and STAR, but fails For BRAHMS!

  34. Has ramifications for DY spin asymmetry: Kang,Prokudin w/o node node Strengthens case for study of DY “sign change” ! AnDY, COMPASS, E906,W bosons at RHIC

  35. d/dq q ~ Q coll.fact. q QCD Collins,Aybat,Rogers,Qiu Kang,Xiao,Yuan Aybat,Prokudin,Rogers Anselmino,Boglione,Melis QCD evolution of TMDs well-known feature: emergence of Sudakov logarithms

  36. • can be resummed to all orders in strong coupling Collins,Soper,Sterman;… Koike, Nagashima,WV Kang,Xiao,Yuan (e.g. Drell-Yan, simplified) Sudakov exponent • can be formulated to give evolution of TMDs in terms of Mert Aybat,Rogers, Collins,Qiu Kang,Xiao,Yuan

  37. • for Sivers function obtain (~ Sudakov) where non-pert. piece

  38. • initial phenomenology very encouraging Aybat,Prokudin,Rogers Anselmino, Boglione, Melis

  39. Takes this field to new level ! However, a number of caveats: • various simplifying assumptions • results will depend on large-b prescription: b* only one possible choice choice of parameters such as g2, bmax • matching to large-kT tail (“Y-term” / TF) • so far “just” leading log

  40. Enjoy IWHSS12 !

  41. …well described by NLO at RHIC …but data much higher than NLO at fixed-target energies !

  42. • spin sum rule / orbital angular momentum: a long-standing debate News Recently: Wakamatsu Chen et al. Leader

  43. •other “contemporary” analyses of polarized (SI)DIS data: Leader,Sidorov,Stamenov Forte et al. (NNPDF) Sissakian, Shevchenko, Ivanov

  44. There are many observables that are sensitive to OAM. Question is connection to spin sum rule News Sivers <-> OAM in a quantitative way ? Bacchetta,Radici • combines:  Ji’s expression for in terms of GPDs  connection between moment of and GPD (Burkardt’s lensing idea)  joint fit of SIDIS Sivers asymmetries and magnetic moments • OAM from Wigner distributions ? Lorce,Pasquini

  45. A far-developed theory framework to address this: (F. Yuan at DIS 2012) plus, first-rate experimental facilities

  46. •connection to hyperon β-decays, SU(3) (axial charges) Bjorken; Ellis, Jaffe; Sehgal; Karliner, Lipkin; Ratcliffe;… •strangeness? ?

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