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Measurement of Transverse Spin Effects at COMPASS

Measurement of Transverse Spin Effects at COMPASS. Anselm Vossen ALU Freiburg. On Behalf of the COMPASS Collaboration. Outline. Transversity Possible Measurements Transversity at COMPASS Theory predictions k T dependent Quark Distribution q(x,k T ) Sivers Effect

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Measurement of Transverse Spin Effects at COMPASS

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  1. Measurement of Transverse Spin Effects at COMPASS Anselm Vossen ALU Freiburg On Behalf of the COMPASS Collaboration

  2. Outline Transversity • Possible Measurements • Transversity at COMPASS • Theory predictions • kT dependent Quark Distribution q(x,kT) • Sivers Effect • Sivers Asymmetries at COMPASS • Theory predictions • Outlook

  3. Lz Lz-1 _ _ Structure of the Nucleon Collins effect m’ + * * ? m _ + q Helicity base: independent amplitudes: Hadronic Tensor N _ +

  4.    ↑ ↓ ↓ ↑ ↑ ↑ ↑ ↑ Measuring Transversity • Transversity base: • Probabilistic interpretation   _ _ + = _ + Difference in densities for ↑, ↓ quarks in ↑ nucleon HERMES, BELLE:

  5. http://cerncourier.com/main/article/44/8/19/1 Collins Fragmentation - Modell K. Bruhwel, TJNAF Collins Effekt in String Fragmentation (X. Artru)

  6. Collins Effect in SIDIS Ph transverse momentum of the hadron S’ transverse spin of the struck quark m’ m S’ • Fragmentation Function for a transversely polarised quark into unpolarised hadron g* FC Ph x • “Collins angle” C = h- s’ z spin dependent part : “Collins fragmentation function” Number of Events:

  7. Collins Asymmetries

  8. COMPASS 2002-2004 Collins: Comparison with Theory • A.V. Efremov, K. Goeke and P. Schweitzer, PRD73 (2006) 094025 • DTq from Chiral Quark Soliton Model • Collins FF from Fit to HERMES Data •  unfav FF ~ - fav FF •  DTu, DTd cancel in deuterium

  9. m’ S’ m g* F2H Ph x z Transversity signal in 2 Hadron Correlations • Azimuthal Asymmetry measurement similar to Collins Asymmetry extraction • (z2P1+z1P2)/(z1+z2)=Ph

  10. Results, z Binning

  11. Results, xBj binning

  12. Predictions 2H Asymmetries Bacchetta, Radici: Phys.Rev. D74 (2006) 114007 p+-p- production

  13. x Sivers Effect + } x M. Burkardt, PRD, 66, 114005(2002)

  14. Sivers Effect m’ “Sivers angle” S m g* FS Ph x z Modulation inFSof the hadron distribution Correlation between the nucleon spin and quark motion Sivers effect ≠0⇒ Quark angular momentum≠0 modulation inFSof the hadron distribution

  15. Sivers Asymmetries

  16. COMPASS 2002-2004 Sivers: Comparison with Theory • M. Anselmino et al., PRD72 (2005) 094007 • including transverse momenta, FF from literature • 13 parameter fit to HERMES and COMPASS 2002 data to extract the Sivers functions • comparison with COMPASS 2002-2004dataok

  17. Conclusion/Outlook • Measurement of Transversity and Sivers effect provides new insights in soft QCD effects • Experiments have shown unexpected effects • COMPASS data on deuteron is important to extract DTd and D0Td • COMPASS measures this year with proton target Polarization data has often been the graveyard of fashionable theories. If theorists had their way, they might just ban such measurements altogether out of self-protection. J.D. Bjorken St. Croix, 1987

  18. COMPASS 2004 Setup • high energy beam • Large angular acceptance • Broad kinematical range two stages spectrometer Large Angle Spectrometer (SM1), Small Angle Spectrometer (SM2) tracking, calorimetry, PID NEW TECHNOLOGIES MuonWall SM2 E/HCAL E/HCAL SM1 MuonWall RICH Polarised Target mbeam 2002-2004: 6LiD (Deuteron) dilution factor f = 0.38 polarization PT =50%

  19. The RICH detector 3 m 6 m 5 m C4F10 photon detectors threshold momenta: • pp = 2 GeV/c • pK = 9 GeV/c • pP =17 GeV/c fully efficient for transverse data in 2003 and 2004

  20. COMPASS Transversly Polarised Target two 60 cm long cells with opposite polarization (systematics) dN/dz 4000 2000 0 -1000 0 1000 zvtx (mm) 2002-2004: 6LiD (Deuteron) dilution factor f = 0.38 polarization PT = 50% during data taking with transverse polarization polarization reversal in the 2 cells after ~ 5 days

  21. COMPASS Experiment @ CERN COmmonMuon andProtonApparatusforStructureandSpectroscopy • fixed target experiment at the CERN SPS • 240 physicists from 28 Institutes, 11 Countries SPS

  22. hazimuthal angle of the hadron s’ azimuthal angle of the transverse spin of the struck quark s’ = p -s sazimuthal angle of the transverse spin of the initial quark • “Collins angle” C = h- s’ = h+ s - p

  23. Predictions for 2H Signal Radici @ QCD N’06, Bacchetta, Radici: hep-ph/0608037 ARS ARS

  24. Results, MInv Binning

  25. COMPASS 2002-2004 Predictions COMPASS 2002-2004 Predictions Transverse Spin @ COMPASS • Transverse Spin Measurements provide new insights into Spin Structure and Quark dynamics inside the nucleon • COMPASS provides key measurements Collins (Transversity) Sivers (kT dependent unpol. PDF) M. Anselmino et al., PRD72 (2005) 094007 Efremov, Goeke, Schweitzer, PRD73 (2006) 094025 xBj xBj

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