The Transverse Spin

Contents : 1. Pre-history 2. Transversity versus helicity 3. The massless limit. "Cardan" and "see-saw" transformations 4. Transversity distribution delta q(x). The diquark spectator model 5. Soffer inequality 6. Tensor charge sum rule 7. t-channel analysis 8. Selection rules for delta q(x) measurements 9. Evolution with Q squared 10. Quark polarimetry. The sheared-jet (Collins) effect 11. Single-spin asymmetries in inclusive experiments 12. Quark distribution dependent on both spin and transverse momentum 13. First evidence of quark transversityComment: 20 pages, 7 figures. Lecture given at Xeme Seminaire Rhodanien de Physique "Le Spin en Physique", 3-8 March, Torino, Italy. To appear in the proceeding

The magnetic and electric transverse spin density of spatially confined light

When a beam of light is laterally confined, its field distribution can exhibit points where the local magnetic and electric field vectors spin in a plane containing the propagation direction of the electromagnetic wave. The phenomenon indicates the presence of a non-zero transverse spin density. Here, we experimentally investigate this transverse spin density of both magnetic and electric fields, occurring in highly-confined structured fields of light. Our scheme relies on the utilization of a high-refractive-index nano-particle as local field probe, exhibiting magnetic and electric dipole resonances in the visible spectral range. Because of the directional emission of dipole moments which spin around an axis parallel to a nearby dielectric interface, such a probe particle is capable of locally sensing the magnetic and electric transverse spin density of a tightly focused beam impinging under normal incidence with respect to said interface. We exploit the achieved experimental results to emphasize the difference between magnetic and electric transverse spin densities.Comment: 7 pages, 4 figure

Transverse Spin Physics at COMPASS

The study of transverse spin effects is part of the scientific program of COMPASS, a fixed target experiment at the CERN SPS. COMPASS investigates the transversity PDFs in semi-inclusive DIS, using a longitudinally polarized muon beam of 160 GeV/c impinging on a transversely polarized target. From 2002 to 2004, data have been collected using a $^6$LiD target transversely polarized. Transversity has been measured using different quark polarimeters: the azimuthal distribution of single hadrons, the azimuthal dependence of the plane containing hadron pairs, and the measurement of the transverse polarization of baryons ($\Lambda$ hyperons). All the asymmetries have been found to be small, and compatible with zero, a result which has been interpreted as a cancellation between the u and d-quark contributions. In 2007 COMPASS has taken data using a NH$_3$ polarized proton target which will give complementary information on transverse spin effects.Comment: 4 pages, 2 figures, proceedings of 16th International Workshop on Deep Inelastic Scattering and Related Subjects (DIS 2008), London, England, 7-11 Apr 200

Transverse Spin Physics at COMPASS

The investigation of transverse spin and transverse momentum effects in deep inelastic scattering is one of the key physics programs of the COMPASS collaboration. Three channels have been analyzed at COMPASS to access the transversity distribution function: The azimuthal distribution of single hadrons, involving the Collins fragmentation function, the azimuthal dependence of the plane containing hadron pairs, involving the two-hadron interference fragmentation function, and the measurement of the transverse polarization of Lambda hyperons in the final state. Azimuthal asymmetries in unpolarized semi-inclusive deep-inelastic scattering give important information on the inner structure of the nucleon as well, and can be used to estimate both the quark transverse momentum k_T in an unpolarized nucleon and to access the so-far unmeasured Boer-Mulders function. COMPASS has measured these asymmetries using spin-averaged 6LiD data.Comment: Proceedings of QCD10 conference, Montpellie