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Spin Physics at COMPASS

Spin Physics at COMPASS. Christian Schill Universität Freiburg. on behalf of the COMPASS collaboration. Selection of Results Polarized DIS and polarized distribution functions Direct measurements of gluon polarization D g/g Transversity measurements.

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Spin Physics at COMPASS

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  1. Spin Physics at COMPASS Christian Schill Universität Freiburg on behalf of the COMPASS collaboration • Selection of Results • Polarized DIS and polarized distribution functions • Direct measurements of gluon polarization Dg/g • Transversity measurements Rutherford Centennial Conference on Nuclear Physics 2011 Manchester, August 8, 2011.

  2. COMPASS polarized target system solenoid 2.5T dipole magnet 0.5T acceptance ± 180 mrad 3He – 4He dilution refrigerator (T~50mK) 6LiD or NH3 50/90% polarization 40/16% dilution factor Longitudinally or transversely polarized μ 160 GeV long. polarized muon beam

  3. Longitudinal spin asymmetries in DIS • Inclusive scattering • Semi-inclusive scattering

  4. Proton A1 asymmetries incl. & semi-incl. asymmetries for identified π’s and K’s incl. π+ K + π– K− PLB 693 (2010) 227

  5. Deuteron A1 asymmetries Inclusive & semi-inclusive asymmetries for identified π’s and K’s incl. π+ K + π– K− PLB 680 (2009) 217–224

  6. Proton & deuteron g1(x,Q2) new p d CLAS HERMES From first moment of g1d: • unique kinematical domain • important for global QCD analyses ∆Σ=a0=0.33±0.03±0.05 (evol. Q2=∞)

  7. Flavour distributions 5-flavour fit, assuming Δs = Δs LO analysis of 5p+5d asymmetries, DSS FF Line: NLO DSSV not including these data PLB693 (2010) 227 PRD80 (2009)034030 D. de Florian, R. Sassot, M. Stratmann, and W. Vogelsang Phys.Rev.Lett.101:072001,2008

  8. Flavour asymmetry? PLB693 (2010) 227 • Rather small effect, Δu > Δd

  9. Flavour distributions: Ds = Ds ? LO analysis of 5p+5d asymmetries, DSS FF 6 flavours: u, d, s, u, d, s No significant difference! Go on assuming Ds = Ds PLB693 (2010) 227 Varenna, July 2011

  10. Gluon polarization g quarks gluons orbital unknown small ~0.15 poorly known How to measure g?

  11. Gluon polarization is measurable in PGF Photon–gluon fusion (PGF) • Measure A|| • calculate and background by Monte Carlo N

  12. Hadron production Analysis of hadron-pair asymmetries: open charm: single D meson AROMA cleanest process wrt physics background high-pThadron pairs with Q2 > 1 GeV2 LEPTO high-pThadron pairs with Q2 < 1 GeV2 PYTHIA D0 → K π D0 → K ππ0

  13. Results for Δg • All results compatible with zero! • Confirmed by RHIC results in pp

  14. Transverse Spin Effects

  15. Transversity PDFs • TransversityDTq(x) • Transverse quark spin distribution in transversely polarized nucleon • Measurement • Semi-inclusive DIS • transversely polarized target • Collins azimuthal asymmetry q(x) Dq(x) DTq(x)

  16. Collins asymmetry π positive negative x z PT K

  17. Collins Asymmetry: Global Fit • Fit to COMPASS d, HERMES, BELLE (FF, e+e-) • Compared to COMPASS proton data

  18. Proton Sivers Asymmetry • non-zero asymmetry for pos. hadrons on the proton h+ h−

  19. Summary • COMPASS is a major player in the nucleon spin physics • PRESENT • Many important results with µ+ beam • Lots of data are being analyzed, additional results to come • FUTURE ( >2012): COMPASS phase II • GPD measurements (µ+ and µ- beams) • DRELL-YAN measurements

  20. Proposal for 2012 and beyond • Study of the Generalized Parton Distributions (GPDs) • Natural extension of: Form factors PDFs (spatial distributions) (momentum distributions) • - Deeply Virtual Compton Scattering (DVCS) • - Hard Exclusive Meson Production (HEMP): r, w, f, p, h Access through:

  21. Measurements of TMDs using Drell-Yan Single Target Spin Asymmetry AUT Drell-Yan (pp ) gives direct access to chiral odd PDFs like the Sivers function COMPASS Projected Models for Sivers function

  22. Spares

  23. F2(x,Q2) g1(x,Q2) p

  24. Proton & Deuteron g1(x) world data Proton data - world Deuteron data - world Subm. Phys. Lett. B (hep-ex/1001.4654) Phys. Lett. B 647 (2007) 8 COMPASS data: 2007 COMPASS data: 2002 – 2006 From first moment of g1d: ∆Σ=a0= 0.33±0.03±0.05 (evol. to Q2=∞)

  25. Test of the Bjorken sum rule 3 Par. NLO Fit |gA/gV| = 1.2694 ± 0.0028 from neutron beta decay (PDG)

  26. Results for Δg – next to leading order

  27. Collins Asymmetries proton new h+ h− h+ deuteron h−

  28. Transversity ∆Tq(x) from hadron pairs • Alternative: couple ∆Tq(x) to chiral odd 2-hadron interference FF cross-section asymmetry:

  29. Two-hadron asymmetry • large asymmetries • interference FF and transversity sizable

  30. Comparison to a recent Fit • Recent fit (dominated by HERMES, COMPASS p not yet in)

  31. GPDs at COMPASS – kinematical coverage • Unique features • Both µ+ and µ- polarized beams ZEUS H1 Luminosity limit COMPASS HERMES JLab L=1032 with a 2.5m LH2 target • Complementary to HERA and HERMES/JLAB Prediction for 1 year of data taking

  32. Cross section Experimental apparatus must detect Incident and scattered lepton Outgoing real photon Recoiling soft proton Generalized Parton Distributions and DVCS known ds Deep VCS + Bethe-Heitler2 g g* x+x x-x GPDs P2 P1 t

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