Transverse Momentum Distributions: an experimental update

Transverse-momentum-dependent parton distribution functions (TMDs) have been recognized as crucial ingredients for a complete understanding of the nucleon structure and, more generally, of the dynamics of confined partons in the nonperturbative regime of quantum-chromodynamics. Describing correlations between the quark transverse momentum and the quark or the nucleon spin (i.e. spin-orbit correlations), they allow for a 3-dimensional description of the nucleon (nu- cleon tomography) in momentum space, and could provide insights into the yet unmeasured quark orbital angular momentum. At leading twist eight TMDs enter the semi-inclusive DIS (SIDIS) cross section in conjunction with a fragmentation function. In the last years, many transverse-spin and transverse-momentum effects have been measured in SIDIS (essentially by the HERMES, COMPASS and JLab experiments), in pp collisions (e.g. experiments at RHIC), in unpolarized Drell-Yan processes (experiments at Fermilab and CERN), and in e+ e− annihilation (BELLE and BABAR). An overview of the main experimental results concerning transverse phenomena in hard hadronic processes is presented, with particular emphasis on the SIDIS experiments.