COMPASS results on transverse spin and transverse momentum effects

COMPASS is a fixed target experiment at the CERN SPS with a broad physics programme. An important part of this programme consists in the study of the spin structure of the nucleons by measuring semi-inclusive deep inelastic scattering (SIDIS) of a high energy muon beam off polarised deuteron and proton targets. These measurements allow for a first complete investigation of transverse spin and intrinsic transverse momentum effects. From the data collected in the years 2002, 2003 and 2004 using a transversely polarised $^{6}$LiD (polarised deuteron) target, the COMPASS Collaboration has measured the Collins and the Sivers asymmetries, which allow to access the transversity and the Sivers distribution functions, both particulary interesting in the present theoretical description of the nucleon structure. Using the same data, the other six azimuthal single spin asymmetries expected in SIDIS off transversely polarised targets have been measured. New preliminary results for azimuthal spin asymmetries from the data collected in 2004 with the longitudinally polarised $^{6}$LiD target are presented in a different talk at this Workshop. Also, preliminary results for the spin independent azimuthal asymmetries on deuteron have been produced and already shown at several conferences. This talk only addresses the Collins and the Sivers asymmetries. The new results from the data collected in 2007 using a transversely polarised NH$_{3}$ (polarised proton) target are presented, and the future plans of the COMPASS Collaboration related to the study of transverse spin effects are briefly described.