Transverse Spin and Transverse Momentum Effects at COMPASS

Transverse Spin and Transverse Momentum Effects at COMPASS C. Schill (Universität Freiburg) on behalf of the COMPASS collaboration • Transversity: – single hadrons, hadron pairs, Λ hyperons • TMDs: – measured with transversely polarized and unpolarized nucleons • Future measurements: – Drell-Yan NAPP 10, October 4 th, Dubrovnik, Croatia

s n ai a r u J m o t n u CERN. gif La c. L COMPASS LHC ém an SPS N 2

COMPASS Beam: 160 Ge. V µ+, pol. 80% from /K decay longitudinally or transversely polarized fixed target

COMPASS target system 3 He – 4 He dilution refrigerator (T~50 m. K) 6 Li. D or NH 3 50/90% polarization 40/16% dilution factor μ solenoid 2. 5 T dipole magnet 0. 5 T acceptance ± 180 mrad

Transverse spin physics 3 distribution functions are necessary to describe the spin structure of the nucleon at LO: q(x) quark distribution in unpolarized DIS l N l' X Dq(x) helicity distribution in polarized DIS l N l' X DTq(x) = q↑↑(x) - q↑↓(x) transversity distribution in polarized SIDIS l N l' h X Collins FF l N l' h h X Interference FF l N l' Λ X FF of q Λ

Transversity ΔTq • Couple to chiral odd Collins FF Azimuthal cross-section asymmetry:

Transversity: Collins asymmetries • large asymmetry when scattering on a proton ~10% • deuteron result opposite sign of ΔTu and ΔTd proton h+ h− deuteron h+ h− PLB 692 (2010), 240 PLB 673 (2009), 127

New results for identified hadrons + K+-

Comparison COMPASS-HERMES Proton COMPASS HERMES • good agreement in common x range +

Transversity: global fit • Fit to Ad. COLL(x) from COMPASS and Ap. COLL(x) from HERMES and Collins fragmentation function from BELLE (e+e- scattering) • in good agreement with new proton data Anselmino et al.

Transversity: dihadron-interference

Transversity: dihadron interference Proton Deuteron

Comparison COMPASS-HERMES Proton HERMES values scaled with 1/D nn Global fit possible when interference fragmentation function measured (BELLE)

Transverse Λ polarization

Transverse Λ polarization Proton

TMDs Taking into account the transverse momentum k. T of the quarks: 8 transverse momentum dependent distribution functions (TMDs)

Sivers effect virtual Photon

Sivers asymmetries Proton pos. asymmetry for pos. hadrons Deuteron compatible with zero PLB 692 (2010), 240 PLB 673 (2009), 127

New results for identified hadrons + K+-

Sivers: Comparison with HERMES + + possible W dependence

Sivers distribution function Global fit using Ad. Siv( ±) and Ad. Siv(K±) from COMPASS and Ap. Siv( ±) and Ap. Siv(K±) from HERMES: M. Boglione in collaboration with M. Anselmino, U. D’Alesio, A. Kotzinian, S. Melis, F. Murgia, A. Prokudin, C. Turk

Azimuthal asymmetries in unpolarized DIS • Cahn effect: sensitive to intrinsic k. T of the quarks in the nucleon • Boer-Mulders effect: Boer-Mulders TMD

Cahn effect The unpolarized SIDIS cross section: The elementary cross-section: Taking into account the quark transverse momentum:

Boer-Mulders effect Side view Front view up A. Bacchetta quarks proton down Quark spin can be Collins unevenlyfunction distributed in transverse space Convoluted with h and cos 2 h moments AContributes distortion in to thecos distribution of quark spin in transverse space can give rise to a Boer-Mulders function Burkardt, hep-ph/0510408

Results: cos modulation comparison with theory

Results: cos 2 modulation

Other TMDs virt. Photon

Pretzelosity and Worm Gear

Outlook COMPASS future • Short term plans: 2010, 2011 • COMPASS II proposal: 2013 and beyond

Outlook one full year of transverse data taking has nearly finished COMPASS, PLB 692 (2010) 240 h+ h−

2011: Longitudinal, one year on a NH 3 target n Goal: improve statistics on proton deuteron proton E = 200 Ge. V I = 2 x 108 µ/spill COMPASS 2011 estimated

2013: 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: M. Anselmino et al. Proc. of Transversity 2008, Ferrara

Summary n COMPASS is a major player in the nucleon spin physics n PRESENT n n n n large azimuthal asymmetries of hadrons produced scattering off an unpolarized deuteron target Collins and Sivers asymmetries for and K scattering off a transversely polarized proton and a deuteron target two hadron interference asymmetries transverse Λ polarization small, compatible with 0 all 8 TMDs scattering off a proton and a deuteron target NEAR FUTURE (2010 and 2011) n Improved statistics of the asymmetries scattering off a proton target (transversely and longitudinally polarized) FUTURE (2013) n DRELL-YAN measurements

Spares

Dihadron-interference

Dihadron-interference

Dihadron-interference Proton Prediction by Bacchetta, Radici, hep-ph/0608037 (Interference Fragmentation function scaled down to fit HERMES data)