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Stephane Platchkov DAPNIA-laboratory, CEA-Saclay, France For the COMPASS collaboration

This talk presents recent results on the nucleon spin structure, including the contributions of gluons and quarks, as well as the flavor decomposition of spin distribution functions.

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Stephane Platchkov DAPNIA-laboratory, CEA-Saclay, France For the COMPASS collaboration

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  1. Stephane Platchkov DAPNIA-laboratory, CEA-Saclay, France For the COMPASS collaboration MENU-2007, Forschungszentrum Jülich, September 10-14, 2007 RECENT RESULTS FROM COMPASS S. Platchkov, MENU07

  2. Muon beam Gluon spin contribution Quark spin contribution Flavor decomposition of spin distribution functions Transverse spin distribution functions Transversity – two hadron correlations Vector meson production Spin transfer in L-hyperon production Search for pentaquarks OVERVIEW: PHYSICS OF COMPASS • Hadron beams • Pion and Kaon polarizabilities • Diffractive and Central production of exotic states • Search for glueballs • Light meson spectroscopy • Data taking • 2002, 2003, 2004, 2006,2007 • 2005: CERN SPS stop 29, COMPASS objectives: Study of QCD First priority until now: spin structure of the nucleon S. Platchkov, MENU07

  3. This talk: results on nucleon spin structure • Two COMPASS results • How large is the gluon contribution to the nucleon spin? • QCD analysis of the inclusive g1(x, Q2) data (Q2-evolution) • Possible now, thanks to the high accuracy of the data • What is the size of the polarized valence quark distribution? • Determined from Semi-Inclusive Deep Inelastic Scattering • Infer also the light quark sea polarization • Future prospects • COMPASS program in the next years S. Platchkov, MENU07

  4. Trigger-hodoscopes μ Filter 160 GeVμ ECal & HCal SciFi SM2 RICH MWPC SM1 6LiD Target Gems Straws Silicon Drift chambers Micromegas The COMPASS Spectrometer 50 m Polarized beam andtarget ~75% ≥50% SAT, LAT, PID 0.003<x<0.5, 10-3<Q2<10 GeV2 S. Platchkov, MENU07

  5. How large is the gluon contribution to the nucleon spin? (using a QCD fit to world data of g1(x)) S. Platchkov, MENU07

  6. DIS asymmetries – World results on A1d Most recent data from COMPASS: 2002 + 2003 + 2004 - PLB 647(2000)8 •  Good agreement with previous experiments • Much improved statistics at low x • A1 compatible with 0 at low x S. Platchkov, MENU07

  7. A1 was also measured for Q2<1 COMPASS acceptance Asymmetry compatible with 0 down to x=0.0005 S. Platchkov, MENU07

  8. Polarized g1(x) structure function R F2 S. Platchkov, MENU07

  9. From DGLAP evolution equations A common fit to the world data allows a separation of all flavors: a3, a8, DS, Dg ! Two different fit programs (Next to Leading Order) World data (except HERMES 2006) from p, d, 3He (230 points) Parton parameterizations at Q2=3 (GeV/c)2 Fit in scheme QCD analysis of world g1(x) data • All Pij and Ck are known to NLO: • Mertig, van Neerven, 1996 • Vogelsang, 1996 Polarized g1(x) structure functions S. Platchkov, MENU07

  10. QCD FIT RESULTS The fit provides two solutions: one with DG > 0 and one with DG < 0, All other parton densities are nearly identical S. Platchkov, MENU07

  11. QCD fit results DG > 0 DG < 0 Quark contribution to the nucleon spin: 0.30 ±0.02 Total gluon polarization ~│0.2-0.3│: DG< 0 or DG > 0 ! S. Platchkov, MENU07

  12. Strange quark distribution DG > 0 DG < 0 Fit result: In agreement with a determination from integral of g1(x) Strange sea is polarized and negative In agreement with Hermes (talk of P. Kravchenko) S. Platchkov, MENU07

  13. The two QCD fit results for DG We also have 3 direct measurements of DG (Photon-Gluon Fusion): - High-pt hadron pairs with Q2>0 - High pt hadron pairs with Q2<0 - Open charm events (D° -- Kp)  Talk by Roland Kuhn, Session 18 S. Platchkov, MENU07

  14. Gluon polarization measurements The direct measurements of DG(x) in agreement with the QCD fit S. Platchkov, MENU07

  15. What about the proton spin? (in a very naïve model) Everything started from here… Assume DG positive, +0.25 Assume DG negative, -0.25 A provocative digression W. Pauli N. Bohr Pauli and Bohr puzzled by the spin physics in the 30s S. Platchkov, MENU07

  16. What is the size/shape of the polarized valence quark distribution? Is the light quarks sea flavor symmetric? S. Platchkov, MENU07

  17. Access to the valence spin distribution • Use hadron asymmetries in semi-inclusive DI scattering: • Combine h+ and h- in “difference” asymmetry Q2 range 1 – 70 GeV • Advantages (LO): • Fragmentation functions cancel • No Particle Identification • Better statistical errors S. Platchkov, MENU07

  18. Valence quark spin distribution De Florian, Navarro, Sassot, 2005 Unpolarized distributions from MRST04 (Martin, Stirling and Thorne, 2004 At large x, use inclusive asymmetry (sea very small) S. Platchkov, MENU07

  19. Comparison with previous experiments Reasonable agreement, improved statistical errors S. Platchkov, MENU07

  20. Is the light quark sea symmetric? • Incluisve DISmeasures valence+sea contribution • Semi-inclusive DIS measures valence contribution • Take the difference through the first moments Two expectations, depending on the flavor symmetry of the sea S. Platchkov, MENU07

  21. First moment (integral) of Duv+Dvv Unmeasured contribution (0.7 < x < 1.0): 0.004 (est). HERMES PR D71(2005)012003 Gv = 0.41±0.07±0.05 Our result favors a flavor-asymmetric sea (within 2.5s) “This analysis of the HERMES data therefore favors a symmetric polarized light flavor sea” S. Platchkov, MENU07

  22. Flavor separation with 2006 and 2007 data Improved statistics, much lower x values S. Platchkov, MENU07

  23. Outlook and future prospects • OUTLOOK • Gluon contribution to the nucleon spin was measured • likely to be small or zero • Valence spin distribution was determined • Light quarks flavor-asymmetric polarized sea is favored • (Near-) future prospects • 2006: Data on the deuteron being analyzed – more statistics to come • 2007: Polarized data on the proton(L + T) is being collected – better insight into DIS, SIDIS, transversity • 2008+: Program with hadron beams : pion polarizability (talk by M.-L. Collantioni, Thursday, session 28), search for exotics, light-meson spectroscopy… S. Platchkov, MENU07

  24. Spare slides S. Platchkov, MENU07

  25. Shape of parton distributions S. Platchkov, MENU07

  26. g1(x) for Q2<1 S. Platchkov, MENU07

  27. Fit results and low x behavior Leader, Sidorov, Stamenov, Phys.Rev. D73(2006)034023. Gluck et al., Phys.Rev. D63(2001)094005. Blümlein Böttcher, Nucl.Phys.B636(2002)225. - Evolution shows little sensitivity to various fits - Fits of LSS-05, GRSV, BB show similar trend at low x - Sign of DG is mainly determined by the low x behavior of g1(x) S. Platchkov, MENU07

  28. Fit results on p, d, n S. Platchkov, MENU07

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