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Measurements of the Spin-dependent Structure Function g 1 (x,Q 2 ) at COMPASS

Measurements of the Spin-dependent Structure Function g 1 (x,Q 2 ) at COMPASS. d. Helena Santos LIP - Lisboa On Behalf of the COMPASS Collaboration. The nucleon spin The COMPASS experiment Inclusive asymmetries The g 1 structure function.

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Measurements of the Spin-dependent Structure Function g 1 (x,Q 2 ) at COMPASS

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  1. Measurements of the Spin-dependent Structure Function g1(x,Q2) at COMPASS d Helena Santos LIP - Lisboa On Behalf of the COMPASS Collaboration • The nucleon spin • The COMPASS experiment • Inclusive asymmetries • The g1 structure function XXI Rencontres de Physique de La Vallée d’Aoste

  2. The Nucleon Spin naïve parton model:= u+d = 1 Gluons, sea and c quarks are important complete description: orbital angular momenta EMC (1988): = 0.12  0.09  0.14 s+s = –0.14  0.03 SN = ½ =½+ G +Lq+Lg(h=1) XXI Rencontres de Physique de La Vallée d’Aoste, March, 4th– 10th 2007 H. Santos (helena@lip.pt)

  3. Deep Inelastic Scattering ’(E’,p’) (E,p)  Q2 =–q2 = (p–p’)2  =E–E’ x = Q2/2M y= /E X N(M,P) XXI Rencontres de Physique de La Vallée d’Aoste, March, 4th– 10th 2007 H. Santos (helena@lip.pt)

  4. Polarised Deep Inelastic Scattering photon-nucleon asymmetry q(x) = q(x)+– q(x)– q(x) = q(x)++ q(x)– +: quark nucleon –: quark nucleon XXI Rencontres de Physique de La Vallée d’Aoste, March, 4th– 10th 2007 H. Santos (helena@lip.pt)

  5. The COMPASS Experiment at the CERN-SPS COmmonMuon andProton Apparatus forStructure and Spectroscopy LHC SPS M2 N Luminosity: ~5 . 1032 cm-2 s-1 Beam intensity: 2108 µ+/spill (4.8s/16.8s) Beam momentum: 160 GeV/c 230 physicists from 12 countries XXI Rencontres de Physique de La Vallée d’Aoste, March, 4th– 10th 2007 H. Santos (helena@lip.pt)

  6. Trackers Magnets RICH Electromagnetic Calorimeters Hadronic Calorimeters Absorbers Target 160 GeVμ+ SM2 SM1 polarisedbeamand target -80% 50% The COMPASS Spectrometer 50 m XXI Rencontres de Physique de La Vallée d’Aoste, March, 4th– 10th 2007

  7. Kinematic Domain 60—75% of inclusive triggers over the full COMPASS x range Semi-inclusive triggers dominant at low x and low Q2 Calorimetric triggers dominant for Q2 > 30 GeV2 H. Santos (helena@lip.pt)

  8. The Longitudinal Photon-Deuteron Asymmetry (d  deuteron)  is small in the COMPASS kinematic range, as well as A2 N  Number of detected events PB Beam polarisation PT  Target polarisation f  Dilution factor D  Depolarisation factor   1st cell (polarised anti-paralell to beam),   2nd cell (polarised parallel to beam) XXI Rencontres de Physique de La Vallée d’Aoste, March, 4th– 10th 2007 H. Santos (helena@lip.pt)

  9. CERN–PH–EP/2006–040 Inclusive Asymmetry, Q2 < 1 GeV2 • A1d asymmetry compatible with 0 at low x range (0.0005<x<0.02) • At low x A1d has been measured only by COMPASS and SMC • Systematic errors are mainly due to false asymmetries XXI Rencontres de Physique de La Vallée d’Aoste, March, 4th– 10th 2007 H. Santos (helena@lip.pt)

  10. The g1(x) Structure Function F2(x) XXI Rencontres de Physique de La Vallée d’Aoste, March, 4th– 10th 2007 H. Santos (helena@lip.pt)

  11. Inclusive DIS Asymmetry CERN–PH–EP/2006–29 COMPASS 2002, 03 & 04 data 89 x 106 events Q2 > 1 (GeV/c)2 d • A1 compatible with 0 for x < 0.05 • Large asymmetry at large x XXI Rencontres de Physique de La Vallée d’Aoste, March, 4th– 10th 2007 H. Santos (helena@lip.pt)

  12. Inclusive DIS Asymmetry Q2 > 1 (GeV/c)2 • Good agreement with previous experiments • Improved significantly statistics at low x • No tendency towards negative values at x < 0.03 XXI Rencontres de Physique de La Vallée d’Aoste, March, 4th– 10th 2007 H. Santos (helena@lip.pt)

  13. N The g1(x) Structure Function Q2 > 1 (GeV/c)2 XXI Rencontres de Physique de La Vallée d’Aoste, March, 4th– 10th 2007 H. Santos (helena@lip.pt)

  14. QCD Analyses  = u + d + s, q3 = u - d, q8 = u + d - 2s DGLAP equations: Input parameterisations (x–dependence at a fixed Q02): XXI Rencontres de Physique de La Vallée d’Aoste, March, 4th– 10th 2007 H. Santos (helena@lip.pt)

  15. Two different approaches have been used: • 1 - Numerical integration in (x,Q2) space (PRD58(1998) 112002) • 2 - Solution of DGLAP in space of moments (PRD70(2004) 074032) • Fits to world data  230 world data points, 43 from COMPASS • NLO analysis (MS scheme) QCD Fits Data well described by two solutions: G > 0 and G < 0 XXI Rencontres de Physique de La Vallée d’Aoste, March, 4th– 10th 2007 H. Santos (helena@lip.pt)

  16. World data and QCD fits at Q02 = 3 GeV2 Solutions with G < 0 Towards Structure Functions XXI Rencontres de Physique de La Vallée d’Aoste, March, 4th– 10th 2007 H. Santos (helena@lip.pt)

  17. World data and QCD fits at Q02 = 3 GeV2 Solutions with G > 0 Towards Structure Functions XXI Rencontres de Physique de La Vallée d’Aoste, March, 4th– 10th 2007 H. Santos (helena@lip.pt)

  18. Polarised Parton Distributions • Very small sensitivity of x(q+q) to xG XXI Rencontres de Physique de La Vallée d’Aoste, March, 4th– 10th 2007 H. Santos (helena@lip.pt)

  19. QCD Fits Results (world data) CERN–PH–EP/2006–29 Quark polarisation: (error  factor 2 larger without COMPASS data) Gluon polarisation (indirect determination via DGLAP): • Solutions with G > 0: • Solutions with G < 0: |G|  0.2 - 0.3 XXI Rencontres de Physique de La Vallée d’Aoste, March, 4th– 10th 2007 H. Santos (helena@lip.pt)

  20. Gluon Polarisation G/G Direct measurement of gluon polarisation with NLO QCD fits to g1 • Unpolarised G(x) from MRST • Bands correspond to statistical errors of G XXI Rencontres de Physique de La Vallée d’Aoste, March, 4th– 10th 2007 H. Santos (helena@lip.pt)

  21. First Moment of g1 (COMPASS data only) CERN–PH–EP/2006–29 • in literature (S.A. Larin et al., PLB404 (1997) 153): (from Y. Goto et al., PRD62 (2000) 034017: a8 = 0.585 ± 0.025) • extrapolating to Q2  â0 is interpreted as the fraction of the nucleon spin carried by the quarks,  = u + d + s XXI Rencontres de Physique de La Vallée d’Aoste, March, 4th– 10th 2007 H. Santos (helena@lip.pt)

  22. Conclusions & Outlook • Longitudinal A1 inclusive asymmetries measured using 2002, 2003 & 2004 COMPASS data • From the first moment of g1d, we extract the quark contribution to the nucleon spin (COMPASS data only): • Semi-inclusive analysis towards flavour spin distributions is inprogress • QCD fits to world data give for quark and gluon polarisation: |G|  0.2 — 0.3 XXI Rencontres de Physique de La Vallée d’Aoste, March, 4th– 10th 2007 H. Santos (helena@lip.pt)

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