Measurement of Transverse Spin Effects at COMPASS Anselm Vossen ALU Freiburg On Behalf of the COMPASS Collaboration
Outline Transversity kT dependent Quark Distribution q(x,kT) Outlook Possible Measurements Transversity at COMPASS Theory predictions kT dependent Quark Distribution q(x,kT) Sivers Effect Sivers Asymmetries at COMPASS Outlook
Structure of the Nucleon Collins effect m’ _ Lz-1 Lz _ + * * ? m _ + q Helicity base: independent amplitudes: Hadronic Tensor N _ +
Measuring Transversity Transversity base: Probabilistic interpretation ↑ ↓ ↑ _ _ + = _ + Difference in densities for ↑, ↓ quarks in ↑ nucleon HERMES, BELLE:
Collins Fragmentation - Modell http://cerncourier.com/main/article/44/8/19/1 K. Bruhwel, TJNAF Collins Effekt in String Fragmentation (X. Artru)
Collins Effect in SIDIS Ph transverse momentum of the hadron S’ transverse spin of the struck quark m’ m S’ g* FC Ph x “Collins angle” C = h - s’ z Fragmentation Function for a transversely polarised quark into unpolarised hadron spin dependent part : “Collins fragmentation function” Number of Events:
Collins Asymmetries
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 COMPASS 2002-2004
Transversity signal in 2 Hadron Correlations Azimuthal Asymmetry measurement similar to Collins Asymmetry extraction (z2P1+z1P2)/(z1+z2) =Ph S’ Ph F2H m m’ z x g*
Results, z Binning
Results, xBj binning
Predictions 2H Asymmetries Bacchetta, Radici: Phys.Rev. D74 (2006) 114007 p+-p- production
Sivers Effect + } x x M. Burkardt, PRD, 66, 114005(2002)
Sivers Effect m’ “Sivers angle” m S g* FS Ph x Modulation in FS of the hadron distribution Correlation between the nucleon spin and quark motion Sivers effect ≠0 ⇒ Quark angular momentum ≠ 0 z modulation in FS of the hadron distribution
Sivers Asymmetries
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 2002-2004 data ok COMPASS 2002-2004
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
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 m beam Polarised Target 2002-2004: 6LiD (Deuteron) dilution factor f = 0.38 polarization PT =50%
The RICH detector 3 m 6 m 5 m threshold momenta: pp = 2 GeV/c 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
COMPASS Transversly Polarised Target two 60 cm long cells with opposite polarization (systematics) dN/dz 4000 2000 -1000 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
COMPASS Experiment @ CERN COmmon Muon and Proton Apparatus for Structure and Spectroscopy SPS fixed target experiment at the CERN SPS 240 physicists from 28 Institutes, 11 Countries
h azimuthal angle of the hadron s’ azimuthal angle of the transverse spin of the struck quark s’ = p - s s azimuthal angle of the transverse spin of the initial quark “Collins angle” C = h - s’ = h + s - p
Predictions for 2H Signal Radici @ QCD N’06, Bacchetta, Radici: hep-ph/0608037 ARS ARS
Results, MInv Binning
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 COMPASS 2002-2004 Predictions COMPASS 2002-2004 Predictions xBj xBj