Measurement of Transverse Spin Effects at COMPASS Anselm

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 – Sivers Asymmetries at COMPASS – Theory predictions • Outlook

Structure of the Nucleon Collins effect m’ m Lz-1 Lz + * Helicity base: independent amplitudes: + ? q __ * _ 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 “Collins angle” C = h - s’ m m’ S’ * F C Ph x 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, PRD 73 (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 • (z 2 P 1+z 1 P 2)/(z 1+z 2) =Ph m’ m * S’ F 2 H Ph x z

Results, z Binning

Results, x. Bj binning

Predictions 2 H Asymmetries Bacchetta, Radici: Phys. Rev. D 74 (2006) 114007 p+-p- production

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

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

Sivers Asymmetries

Sivers: Comparison with Theory M. Anselmino et al. , PRD 72 (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 D 0 Td J. D. Bjorken St. Croix, 1987 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. • COMPASS measures this year with proton targe

• • • COMPASS 2004 Setup high energy beam Large angular acceptance Broad kinematical range two stages spectrometer Large Angle Spectrometer (SM 1), Small Angle Spectrometer (SM 2) tracking, calorimetry, PID SM 2 NEW TECHNOLOGIES E/HCAL Muon. Wall E/HCAL SM 1 Muon. Wall RICH m beam Polarised Target 2002 -2004: 6 Li. D (Deuteron) dilution factor f = 0. 38 polarization PT =50%

The RICH detector 3 m 6 m 5 m photon detectors C 4 F 10 threshold momenta: ● p = 2 Ge. V/c p ● p = 9 Ge. V/c K ● p =17 Ge. V/c P fully efficient for transverse data in 2003 and 2004

COMPASS Transversly Polarised Target two 60 cm long cells with opposite polarization (systematics) d. N/dz 4000 2000 0 -1000 0 1000 zvtx (mm) 2002 -2004: 6 Li. D (Deuteron) dilution factor f = 0. 38 polarization PT = 50% during data taking with transverse 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 2 H Signal Radici @ QCD N’ 06, Bacchetta, Radici: hep-ph/0608037 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 Sivers (k. T dependent unpol. PDF) COMPASS 2002 -2004 Predictions M. Anselmino et al. , PRD 72 (2005) 094007 Efremov, Goeke, Schweitzer, PRD 73 (2006) 094025 Collins (Transversity) COMPASS 2002 -2004 Predictions x. Bj