Narrow Results

The study of transverse spin and transverse momentum effects is part of the scientific program of COMPASS, a fixed target experiment at the CERN SPS. For these studies, a 160 GeV/c momentum muon beam is scattered on a transversely polarized nucleon target, and the scattered muon and the forward going hadrons produced in DIS processes are reconstructed and identified in a magnetic spectrometer. The measurements have been performed on a deuteron target in 2002, 2003 and 2004, and on a proton target in 2007. The main results obtained measuring single spin asymmetries are reviewed, with particular emphasis on the most recent proton measurements. After two years of spectroscopy measurements with hadron beams, in 2008 and 2009, the Collaboration will resume measurements with the muon beam and a transversely polarized target in 2010.

We review what is currently known about the transverse spin structure of hadrons, in particular from observables that can be analyzed within a collinear framework. These effects have been around for 40 years and represent a critical test of perturbative QCD. We look at both proton–proton and lepton–nucleon collisions for various final states. While the main focus is on transverse single-spin asymmetries, we also discuss how longitudinal-transverse double-spin asymmetries offer a complimentary, yet equally important, source of information on the quark–gluon content of hadrons. We also summarize some recent progress in solidifying the theoretical formalism behind these observables and give an outlook on future directions of research.

A selection of recent key results obtained in semi-inclusive deeply inelastic scattering (SIDIS) experiments is presented. The observations strongly support the description of the nucleon structure in terms of transverse momentum dependent parton distribution functions, which represent the various correlations between the quarks spins, the quarks transverse momenta and the nucleon spin which give rise to specific spin-dependent azimuthal asymmetries.

We report preliminary results on a global fit of the Sivers effect in semi-inclusive deep-inelastic scattering (SIDIS), Collins effect in SIDIS, Collins effect in semi-inclusive annihilation (SIA), and A_N in proton-proton collisions. All of these transverse-spin observables are driven by the same quark-gluon-quark correlations, which allows for such a global analysis to be performed.

We report on the improved Collins-Soper-Sterman (iCSS) formalism presented in, in particular for the case of polarized observables, such as the Sivers effect in semi-inclusive deep-inelastic scattering. We also outline how this study can be extended beyond leading order to address the matching of collinear PDFs to transverse momentum integrated TMDs.

A variety of measurements performed utilizing transversely polarized proton-proton collisions at the Relativistic Heavy Ion Collider (RHIC) are now available. Recent results from the PHENIX and BRAHMS experiments are presented and discussed.