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Measurements of ΔG/G at COMPASS

This workshop presentation discusses the measurements of ΔG/G (gluon polarization) at COMPASS, focusing on the gluon contribution to the nucleon spin. The presentation covers topics such as asymmetries for photon-gluon fusion events, open charm analysis, high-pt analysis, and results on quarks and gluons. The results indicate a small or negative ΔG and provide insights into the gluon polarization in the nucleon.

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Measurements of ΔG/G at COMPASS

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  1. Measurements of ΔG/Gat COMPASS Sonja Hedicke, Universität Freiburg on behalf of the COMPASS Collaboration Workshop on Hadron Structure and Hadron Spectroscopy Prague, August, 1.-3. 2005 • Gluon Contribution to the Nucleon Spin • Asymmetries for Photon-Gluon Fusion events • Open Charm Analysis • High-pt Analysis • Results

  2. Quarks: Gluons: Orbital angular momentum: Lq + LG Expectation (SU(3), Δs=0, Ellis/Jaffe): DS = 0.6 Experiment: DS ≈ 0.2 The Nucleon Spin

  3. m q r P Photon Gluon Fusion Direct measurement of ΔG/G • Two approaches to tag PGFin COMPASS • q = c: • Open CharmD0, D* decay • q = u, d, s: • High-pt Hadron Pairs r PhotonGluonFusion (PGF)

  4. ΔG/G and counting rate asymmetry • Asymmetrytarget polarisation PT≈ 0.50 dilution factor f ≈ 0.40beam polarisation Pb≈ 0.76depolarisation factor D • ΔG/G • Different background contributions for the different channels

  5. Open Charm Selection MKpp - (MKp + mp) [MeV/c2] • zD > 0.2 (0.25 for D0)|cosq*| < 0.85 (0.5 for D0) • RICH identification for K± 9 GeV/c < p (K± ) < 50 GeV/c • 3.1 MeV/c2 < MKpp – (MKp+ mp) < 9.1 MeV/c2 MKp - MD0(PDG) [MeV/c2]

  6. D* (2002) MKpp - (MKp + Mp) (MeV) MKp - MD0(PDG) [MeV/c2] MKpp - (MKp + Mp) [MeV/c2] Open Charm Selection MKpp - (MKp + Mp) [MeV/c2]

  7. D0 D* (2002) (2002) MKpp - (MKp + Mp) (MeV) MKp - MD0(PDG) [MeV/c2] MKpp - (MKp + Mp) [MeV/c2] Open Charm Selection MKpp - (MKp + Mp) [MeV/c2] MKp - MD0(PDG) [MeV/c2]

  8. Open Charm Selection D0 (2002) 400 events D0 (2003) 1100 events COMPASS preliminary COMPASS preliminary D0 (untagged, 2003)4200 events D0 (untagged, 2002) 1400 events COMPASS preliminary COMPASS preliminary

  9. Open Charm Asymmetry • no asymmetry contribution from background • s PGF/s tot = S/(S+B) determined from the fit • aLL ( m (gg), y, Q2,…) parametrised for data by pt, zD, ywith Monte Carlo (AROMA with POLDIS) POLDIS: A.Bravar et al., Comput. Phys. Commun. 105 (1997) 42

  10. Open Charm Asymmetry aLL parametrised by pt, zD, y comparison of background-subtracted data and AROMA Monte Carlo

  11. ΔG/G from Open Charm First measurement of ΔG/G with Open Charm tagging Statistics will be doubled with data from 2004

  12. High-pt Hadron Pairs • Enrich photon-gluon fusion events with high-pt cutspt > 0.7 GeV/cpt12 + pt22 > 2.5 (GeV/c)2xF, z > 0.1m(h1,h2) > 1.5 GeV/c2 • Asymmetry contributions from background eventsaLL and si/stot from Monte Carlo

  13. QCD- Compton PGF LO DIS High-pt Hadrons, Q2 > 1 (GeV/c)2 PGF and background

  14. High-pt Hadrons, Q2 > 1 (GeV/c)2 • xB < 0.05 → in this region A1 ≈ 0 and LO DIS & QCD-Compton contributions can be neglected

  15. High-pt Hadrons, Q2 > 1 (GeV/c)2 • xB < 0.05 → in this region A1 ≈ 0 and LO DIS & QCD-Compton contributions can be neglected • Asymmetry: ALL/D = -0.015 ± 0.080 stat. ± 0.013 syst.(2002 + 2003) ≈ 0* *taken into account in the systematic error

  16. High-pt Hadrons, Q2 > 1 (GeV/c)2 Data – Monte Carlo (LEPTO) comparison Q2 leading hadron: p leading hadron: pt

  17. High-pt Hadrons, Q2 > 1 (GeV/c)2 • Ratio of PGF events: Results from Monte Carlo Gluon polarisation

  18. QCD- Compton PGF LO DIS High-pt Hadrons - Background all Q2

  19. QCD- Compton PGF LO DIS High-pt Hadrons - Background all Q2 low Q2 resolved photon

  20. direct resolv. g High-pt Hadrons, Q2 < 1 (GeV/c)2 0 0.1 0.2 0.3 Results from PYTHIA • Background as forQ2 > 1 (GeV/c)2 • Additional background from resolvedphoton events • Additional processes sensitive to gluonsin the nucleon PGF QCD-C LO DIS qq‘→ qq‘ qg→ qg gg→ gg low pt neglect LO DIS and low pt

  21. High-pt Hadrons, Q2 < 1 (GeV/c)2 Determination of ΔG/G

  22. High-pt Hadrons, Q2 < 1 (GeV/c)2

  23. High-pt Hadrons, Q2 < 1 (GeV/c)2

  24. High-pt Hadrons, Q2 < 1 (GeV/c)2

  25. High-pt Hadrons, Q2 < 1 (GeV/c)2

  26. High-pt Hadrons, Q2 < 1 (GeV/c)2 Contribution from resolved photons • unknown: polarised PDFs of the photon • known: unpolarised PDFs of the photon → use unpolarised PDFs to constrain polarised additional theoretical uncertainty Glück, Reya, Sieg, Eur. Phys. J. C20 (2001) 271

  27. High-pt Hadrons, Q2 < 1 (GeV/c)2 Determination of ΔG/G 2002 + 2003: ALL/D = 0.002 ± 0.019stat. ± 0.003syst

  28. 2 1 3 GRSV2000 with1 G = 2.5 2 G = 0.6 3 G = 0.2 High-pt Results Results favour small ΔG or ΔG crosses 0 around xg ≈ 0.1 Glück et al.,Phys. Rev. D63(2001) 094005

  29. Summary • Determination of the gluon polarisation ΔG/G at COMPASS with • Open Charm events • low statistics, theoretically cleanΔG/G = -1.08 ± 0.73 @ xg = 0.15 • High-pt events at Q2 > 1 (GeV/c)2 • higher statistics, but LO DIS and QCD-Compton background ΔG/G = +0.06 ± 0.31(stat.) ± 0.06 (syst.) @ xg = 0.13 • High-pt events at Q2< 1 (GeV/c)2 • very high statistics, additional resolved g backgroundΔG/G = +0.024 ± 0.089(stat.) ± 0.057 (syst.) @ xg = 0.095 • 2004 data analysis: statistics as for 2002 + 2003 → error √2 smaller • continue data taking in 2006

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