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NLO QCD fits to polarized semi-inclusive DIS data

NLO QCD fits to polarized semi-inclusive DIS data. Rodolfo Sassot Universidad de Buenos Aires. JLAB MAY 5, 2005. 30 years studying the short distance structure of the proton:. from hints on point-like structure, to parton densities. QCD + DIS high precision PDFs.

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NLO QCD fits to polarized semi-inclusive DIS data

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  1. NLO QCD fits to polarized semi-inclusive DIS data Rodolfo Sassot Universidad de Buenos Aires JLAB MAY 5, 2005

  2. 30 years studying the short distance structure of the proton: from hints on point-like structure, to parton densities. QCD + DIS high precision PDFs predictions and tests of ‘new physics’ R. Sassot JLAB, May 5, 2005

  3. why spin? • inessential complication? indispensable tool for unveiling hadron structure, search for the unexpected. why polarized DIS? • best benchmark for hadron structure, • top cited spin crisis, • 90’s experimental programs. why polarized SIDIS? • flavor and quark-antiquark discriminatrion, • increasing statistics, • universality and precision of fragmentation functions (FF). R. Sassot JLAB, May 5, 2005

  4. Outline: • QCD improved parton model description of polarized DIS and SIDIS • global QCD fits to polarized data. • uncertainties in pPDFs • uncertainties in future/ongoing measurements R. Sassot JLAB, May 5, 2005

  5. Polarized DIS (pDIS) ‘structure function’ ‘parton distribution’ (pPDF) R. Sassot JLAB, May 5, 2005

  6. pDIS flavor decomposition R. Sassot JLAB, May 5, 2005

  7. QCD improved parton model: LO and NLO ‘naïve’ ‘LO’ W R. Mertig, W. L. van Neerven, Z.Phys.C70 637 (1996) W. Vogelsang, Phys.Rev.D54 20023 (1996) ‘NLO’ ‘add a gluon’ R. Sassot JLAB, May 5, 2005

  8. Polarized SIDIS (pSIDIS) D. de Florian, C. Garcia Canal, R.S. Nuc.Phys.B470 195 (1996) ‘fragmentation function’ (FF) R. Sassot JLAB, May 5, 2005

  9. Flavor decompositon in pSIDIS R. Sassot JLAB, May 5, 2005

  10. NLO combined analysis: • and non perturbative quantities to be measured, • QCD predicts Q -dependence, not boundary conditions 2 2 • data obtained for different values of x and Q 1. make ansatz about the x-dependence at some scale 2. evolve from to and compute 3. compare with data 4. fit to minimize R. Sassot JLAB, May 5, 2005

  11. R. Sassot JLAB, May 5, 2005

  12. borrowed from W. Vogelsang R. Sassot JLAB, May 5, 2005

  13. NLO combined analysis: parameterizations pDIS can probe: pSIDIS give access to: 20 parameters positivity relative to MRST02: A.D. Maritn et al. Eur.Phys.J.C28 (2002) 455 R. Sassot JLAB, May 5, 2005

  14. NLO combined analysis: data R. Sassot JLAB, May 5, 2005

  15. Fragmentation functions input: KRE: S. Phys.Rev.D62 054001 (Kretzer 2000) KKP: B. A. Kniehl, G. Kramer, B. Potter, Nucl.Phys.B582 514 (2000) flavorseparation: R. Sassot JLAB, May 5, 2005

  16. Results: 478-20=458 d.o.f 313 pDIS 165 pSIDIS Interplay between pDIS and pSIDIS R. Sassot JLAB, May 5, 2005

  17. Uncertainties: J. Pumplin et al. Phys.Rev.D65 014011 (2002) R. Sassot JLAB, May 5, 2005

  18. R. Sassot JLAB, May 5, 2005

  19. Future/ongoing measurements: R. Sassot JLAB, May 5, 2005

  20. Future/ongoing measurements: R. Sassot JLAB, May 5, 2005

  21. Conclusions: successfull experimental programs • pPDFs have evolved dramatically: tools for the analysis (NLO) from ‘scenarios’ to ‘constraints’ • pSIDIS opens a window to sea quarks and helps to constrain other densities perfect consistency pDIS/pSIDIS • overall picture combined NLO global fits: improved constraints on pPDFs best fits favor SU(3) in the sea • dependence on FFs as a caveat & opportunity: tackle both problems together? PHENIX: • future prospects: encouraging COMPASS & CLAS, E04-113 and FFs R. Sassot JLAB, May 5, 2005

  22. Acknowledgments: Xiaodong Jiang, fruitfull collaboration, JLAB, hospitality and support. R. Sassot JLAB, May 5, 2005

  23. R. Sassot JLAB, May 5, 2005

  24. Role of pDIS and pSIDIS in the extraction of pPDFs: • pDIS as the main source of information on pPDFs. • hadronic decays data & assumptions: flavor symmetry • e.m. interactions can not discriminate quarks from antiquarks: large uncertainties in sea quark and gluon pPDFs • pSIDIS as a way out: SMC @ CERN: first steps; encouraging HERMES @ DESY: from consistency checks to constraints COMPASS @ CERN: ….. E04-113 @ JLAB: ….. R. Sassot JLAB, May 5, 2005

  25. Uncertainties: R. Sassot JLAB, May 5, 2005

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