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Newest Results from the Experiment

Explore the structure of baryons and study QCD at low energy using lattice gauge calculations and chiral perturbation theory. Investigate gluon polarization and flavor-dependent quark helicity in the nucleon.

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Newest Results from the Experiment

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  1. Newest Results from the Experiment F.-H. Heinsius (Universität Freiburg/CERN) Introduction Gluon polarization in the nucleon Transverse spin distribution Graduiertenkolleg Freiburg, 24.5.2006

  2. Towards understanding nonperturbative QCD • Nucleon is fundamental in understanding QCD • confinement • asymptotic freedom • spontaneous chiral symmetry breaking • Mass around us mostly due to nucleons • quark mass accounts for only about 1% • mostly due to dynamics of gluons and quarks • Do we understand the structure of baryons? • measurements • unpolarized: ≈40% of momentum: gluons → DIS • polarized: ≈30% of helicity: quarks • cannot be calculated from QCD (yet) • lattice gauge calculation • Study of special bound states: • double charmed baryons: 2 heavy & 1 light quark • hybrids: qq & gluon • glueballs: gluon bound state • allowed by QCD, but do they exist? • Study QCD at the low energy end using chiral perturbation theory: • light meson sector: polarisability of p and K

  3. Deep Inelastic Scattering Q² = negative momentum transfer squared • Probing the content of the proton • 1968 Friedman, Kendall, Taylor: Nobel prize 1990 • DESY: highest Q² • no further substructure

  4. Structure Functions parton carries all momentum gluon contribution reduces momentum carried by quarks parton carries 1/3 of momentum including sea quarks x = momentum fraction of the nucleon carried by the parton

  5. 40% of the momentum carried by gluons Parton Density Distributions

  6. Towards understanding nonperturbative QCD • Nucleon is fundamental in understanding QCD • confinement • asymptotic freedom • spontaneous chiral symmetry breaking • Mass around us mostly due to nucleons • quark mass accounts for only about 1% • mostly due to dynamics of gluons and quarks • Do we understand the structure of baryons? • measurements • unpolarized: ≈40% of momentum: gluons • polarized: ≈30% of helicity: quarks • cannot be calculated from QCD (yet) • lattice gauge calculation • Study of special bound states: • double charmed baryons: 2 heavy & 1 light quark • hybrids: qq & gluon • glueballs: gluon bound state • allowed by QCD, but do they exist? • Study QCD at the low energy end using chiral perturbation theory: • light meson sector: polarisability of p and K

  7. COMPASS: A Facility to study QCD 230 physicists, 10 countries, 25 institutes LHC COMPASS SPS m, p, or p/K beam

  8. Experiments with muon beam Gluon polarization DG/G in the nucleon Flavor dependent quark helicity density distributions Dq Transverse quark spin distribution functions h1(x) Spin transfer in L-hyperon production Vector meson production Generalised parton distributions Experiments with hadron beams Pion and kaon polarizabilities Diffractive production of exotic states Search for glueballs Light meson spectroscopy Production of double charmed baryons COMPASS: A Facility to study QCD Application and test of • perturbative QCD • nonperturbative QCD • effective theories • chiral perturbation theory • lattice QCD

  9. The COMPASS Spectrometer Trigger-hodoscopes μ Filter ECal & HCal SciFi SM2 RICH MWPC SM1 GEMs 6LiD Target Straws Silicon Drift chambers 160 GeVμ Micromegas 50 m Two stage spectrometer Polarized beam andtarget ~80% ≥50% SAT, LAT, PID 10-5<x<0.5, 10-3<Q2<100 (GeV/c)2

  10. Many new technologies for tracking and PID MicroMegas GEM Trigger-System Straws RICH readout Scintillating fiber trackers Readout electronics

  11. The polarized 6LiD-Target 3He – 4He dilution- refrigerator (T~50mK) super conductive solenoid (2.5 T) dipol (0.5 T) 1 2 60 cm long target containers target material: 6LiD (or NH3) 3 4 4 possible spin combinations: reversed every 8 hrs or: reversed once a week Polarization: ~50%

  12. Probing the Spin Structure of the Nucleon m´ m Nukleon x = fraction of nucleon momentum carried by quark

  13. A1 of the deuteron COMPASS, Phys. Lett. B 612 (2005) 154

  14. g1 of the deuteron COMPASS, Phys. Lett. B 612 (2005) 154 COMPASS 2002/2003

  15. QCD analysis of g1 fit to world data: DS=0.22 ± 0.03 stat. Q²=3 (GeV/c)² incl. COMPASS: DS=0.25 ± 0.02 stat. Q²=3 (GeV/c)² DG=0.4±0.2 stat. • Systematic uncertainties: • fit parametrisation • extrapolation x→0

  16. What makes up the Nucleon‘s Spin? QCD: …additional contributions from Gluons … … and angular momentum ! 1 1 = D + D Σ G 2 2 = ? ΔG Naive quark model: valence quarks CERN, SLAC, DESY, JLAB: DS~ 0.30

  17. Polarization of Gluons DG/G in the Nucleon Photon gluon fusion q = c cross section difference in charmed meson production → cross section known to NLO → experiment challenging q = u,d,s cross section difference in 2+1 jet production. In COMPASS: events with 2 hadrons with high pT → experiment easy → background difficult use two complementary measurements: Q²>1 (GeV/c)² and LEPTO MC Q²<1 (GeV/c)² and PYTHIA MC N

  18. DG/G from Open Charm (D-mesons) Photon gluon fusion D N • 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

  19. DG/G from Open Charm (D-mesons) mD0 with D* tagging mD0 without D* tagging 2002 - 2004 m2≈13 (GeV/c)2

  20. DG/G from high-pT meson pairs Photon Gluon Fusion h1 h2 N Q2 > 1 (GeV/c)2 LEPTO Monte Carlo Q2 < 1 (GeV/c)2 PYTHIA Monte Carlo

  21. How to get ΔG/G Q2>1 (GeV/c)2 fractions of cross section determined by Monte Carlo Leading Order QCD-Compton Photon Gluon Fusion

  22. DG/G from high-pT meson pairs Photon Gluon Fusion h1 h2 N Q2 > 1 (GeV/c)2 LEPTO Monte Carlo ΔG/G = 0.06 ± 0.31stat. ± 0.06syst. at <xg> = 0.13 ± 0.08 2002/2003

  23. Background for Q2<1 (GeV/c)2 PYTHIA:

  24. Background for Q2<1 (GeV/c)2 PYTHIA: • Uncertainty due to the unknown spin distribution in the hadronic structure of the photon

  25. Monte Carlo Tuning • scrutinize systematic error: 15 independent simulations • map the parameter space, i.e. • for kT in nucleon and photon • Fragmentation functions • „parton shower“ on/off, • renormalization scale Nucleon Photon

  26. DG/G from high-pT meson pairs Q2 < 1 (GeV/c)2 PYTHIA Monte Carlo ΔG/G = 0.016 ± 0.058stat. ± 0.055syst. +0.071 - 0.035 at <xg> = 0.085 2002-2004 Photon Gluon Fusion h1 h2 N Q2 > 1 (GeV/c)2 LEPTO Monte Carlo ΔG/G = 0.06 ± 0.31stat. ± 0.06syst. at <xg> = 0.13 ± 0.08 2002/2003

  27. DG/G summary DG=2.5 DG=0.6 DG=0.2 NLO fits to g1 m2 = 3 Gev2 GRSV: Glück et al.,Phys. Rev. D63 (2001) 094005

  28. Transverse Spin Distributions 3 independent structure functions are necessary to describe the spin structure of the nucleon at leading order: All of equal importance! h1(x) decouples from leading twist DIS because helicity of quark must flip No mixture with Gluon

  29. Transverse Spin Physics • 3 possible quark polarimeters suggested: • Azimuthal distribution of hadrons • Azimuthal dependence of the plane containing 2 hadrons • Measure transverse polarization of L • COMPASS studies all of them fS’ = azimuthal angle of target spin vector after scattering fh = azimuthal angle of hadron fC = fh - fS’ ‚ ‚ h muon beam transverse target polarisation

  30. Transverse Spin Physics Azimuthal distribution of hadrons Collins: spin dependent fragmentation of transversely polarised quarks into hadrons Efremov, Goeke, Scheitzer, hep-ph/060354 (fit to BELLE & HERMES data)

  31. Transverse Spin Physics Collins fragmentation function extracted from HERMES / BELLE From: Efremov, Goeke, Schweitzer hep-ph/0603054 COMPASS 2002 data compatible with fit deuteron essential to determine h1d

  32. Transverse Spin Physics Azimuthal distribution of hadrons Sivers: intrinsic kT dependence of the quark distribution Efremov, Goeke, Scheitzer, hep-ph/060354 (fit to BELLE & HERMES data) Anselmino et. al hep-ph/0507181

  33. Transverse Spin Physics Azimuthal dependence of the plane containing 2 hadrons all +/- combination per event 1 +/- combination per event: pt ordering • precise measurement of few % • systematics seems well under control • also compatible with zero • interesting to see proton in 2006

  34. Transverse Spin Physics Measure transverse polarization of L

  35. Results not covered Ξ(1530)0 Ξ(1530)0 • semi-inclusive asymmetries, r • single hadron high-pT • r spin-density matrix elements • L polarisation • pentaquark search • diffractive processes • J/Y production

  36. COMPASS upgrades for this year • New solenoid magnet • Larger acceptance • 70 mrad  180 mrad • RICH upgrade • Central region: MAPMT system • Outer region: new faster electronics • Other upgrades: • Large Drift Chamber • ECAL1 • …

  37. Summary / Outlook • COMPASS results (2002-2004) • polarisation of gluons in the nucleon • transversity: small asymmetries on deuteron • Many more results not covered • Major upgrade of spectrometer for this year • Next year: start spectroscopy • hybrids, glueballs, … • Long term future (2010+): generalised parton distributions (DVCS,…)

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