Spin Physics and Transverse Structure

Spin is a welcome complication in the study of partonic structure that has led to new insights, even if theoretically and experimentally not all dust has settled, in particular on quark flavor dependence and gluon spin. At the same time it opened new questions on angular momentum and effects of transverse structure. In this talk the focus is on the role of the transverse momenta of partons. Like for collinear parton distribution functions (PDFs), we are also in the case of transverse momentum dependent (TMD) PDFs, talking about forward matrix elements. TMD PDFs (or in short TMDs) extend collinear PDFs with only spin-spin correlations to PDFs that include spin-momentum correlations, including also time-reversal-odd (T-odd) correlations, relevant for the description of single spin asymmetries. In this way TMDs open up new ways of studying the spin structure. Their operator structure within QCD, however, is more complex leading to various ways of breaking of universality.Comment: 8 pages, to appear in proceedings of DIS2015 (XXIII International Workshop on Deep-Inelastic Scattering and Related Subjects, 27 April - 1 May, 2015, Dallas, USA

Current fragmentation in semiinclusive leptoproduction

Current fragmentation in semiinclusive deep inelastic leptoproduction offers, besides refinement of inclusive measurements such as flavor separation and access to the chiral-odd quark distribution functions $h_1^q(x) = \delta q(x)$, the possibility to investigate intrinsic transverse momentum of hadrons via azimuthal asymmetries.Comment: 14 pages, Latex using aipproc.sty and epsfig.sty; plenary talk given at the Second Workshop on Physics with an Electron Polarized light-Ion Collider (EPIC), Sept. 14-16, 2000, MIT, Cambridge, US

Sivers Effect Asymmetries in Hadronic Collisions

We argue that weighted azimuthal single spin asymmetries in back-to-back jet or pion production in polarized proton-proton scattering can be written as convolutions of universal distribution and fragmentation functions with gluonic pole cross sections as hard functions. Gluonic pole cross sections are gauge-invariant weighted sums of Feynman diagrams. The weight factors are a direct consequence of the (diagram-dependence of) gauge links. The best known consequence of the gauge links is the generation of the Sivers effect that is a source for single-spin asymmetries. Moreover, due to the dependence of the gauge links on the color-flow of the hard diagram the Sivers effect in SIDIS enters with opposite sign as it does in Drell-Yan scattering. The weight factors in the gluonic pole cross sections are the appropriate generalizations to more complicated processes of this relative sign difference. Furthermore, it is argued that the gluon-Sivers effect appears in twofold.Comment: Contribution to the 17th International Spin Physics Symposium (SPIN2006), Kyoto, Japan, Oct. 2-7, 200

Perspectives in Polarized Leptoproduction

We discuss specific observables that can be measured in deep inelastic leptoproduction in the case of 1-particle inclusive measurements, namely azimuthal asymmetries and power-suppressed (higher twist) corrections. These quantities contain information on the intrinsic transverse momentum of partons, with close connection to the gluon dynamics in hadrons.Comment: 10 pages with figures, LaTex, uses espcrc1.sty and epsfig.sty. Invited talk at the workshop on The Structure of the Nucleon (NUCLEON99), Frascati, June 7-9, 199