COMPASS, a new fixed target experiment at CERN, aimed at the study of nucleon spin structure and hadron spectroscopy, has started to collect physics data in Autumn 2001. This paper describes the COMPASS apparatus and the measurement of the gluon polarisation ∆G/G. The apparatus consists in a solid state polarised target and a two stage spectrometer with high resolution tracking, particle identi...

The 1/Nc–expansion of QCD suggests large flavor asymmetries of the polarized antiquark distributions in the nucleon. This is confirmed by model calculations in the large–Nc limit (chiral quark–soliton model), which give sizable results for ∆ū(x)−∆d̄(x) and ∆ū(x)+∆d̄(x)−2∆s̄(x). We compute the contributions of these flavor asymmetries to the spin asymmetries in hadron production in semi-inclusive d...

Transversity and transverse momentum-dependent parton distribution functions (TMDs) are been measured in semi-inclusive deep inelastic scattering (SIDIS) by using a transversely polarized target at the COMPASS experiment. COMPASS is a fixed target experiment at the CERN M2 beamline, which provides a 160 GeV/c polarized μ+ beam. In the years 2002-2004 COMPASS has collected data with a transverse...

The class of interference fragmentation functions, arising from interference among different hadron production channels, is reconsidered. Their symmetry properties with respect to chiral transformations allow building spin asymmetries where the quark transversity distribution can be factorized out at leading twist. For the case of two leading spinless hadrons inside the same current jet, the pa...

The impact-parameter representation of the spin-flip amplitude of hadron elastic scattering is examined in different unitarisation schemes, taking the Born term of the spin-flip amplitude from the Dubna Dynamical Model (DDM). It is shown that the basic properties of the unitarisation schemes are independent of the functional form used for unitarisation but heavily depend on the asymptotic value...

We study the twist-2 double-spin asymmetries for the 2-jets and large-pT hadron (π, Λ etc) production in semi-inclusive deep inelastic scattering to O(αs) in perturbative QCD. After deriving the complete set of the polarized cross section which is differential with respect to the transverse momentum, we discuss characteristic features of the azymuthal spin asymmetries, using existing parton den...

Polarized proton-deuteron (pd) Drell-Yan processes are investigated for studying new spin structure of a spin-1 hadron. Our formalism indicates that there exist many structure functions: 108 in general and 22 in the QT-integrated case. A naive parton model suggests that at least one of them should be related to the tensor polarized distributions, which have not been measured at all. There are s...

We discuss Wγ production in polarized p p collisions at RHIC energy. We point out that the RHIC collider has two advantages over other hadron colliders to measure the characteristic feature of Wγ production: (1) the RHIC energy is not so high and (2) the polarized beams are available. We calculate the tree level cross section for Wγ production using a generic spin basis for W and discuss both t...