Speaker
Description
After fifty years of investigations, the nucleon structure is still far from being understood and continues to represent a unique test bench for QCD. Despite the enormous progresses achieved in five decades of deep-inelastic scattering (DIS) experiments, a number of crucial open questions are still on the carpet and subject of intense theoretical and experimental studies. In the last two decades, semi-inclusive DIS was established as a unique tool for the study of the non-collinear structure of nucleons, involving the parton transverse momentum pT as an additional degree of freedom. Requiring the detection of at least one final state hadron in coincidence with the scattered lepton, it opened the way not only to the measure of the chiral-odd transversity distribution, the last missing leading-twist collinear parton distribution function, but also to a variety of new pT-dependent PDFs, known as TMDs. Describing correlations between the quark transverse momentum and the quark or the nucleon spin (spin-orbit correlations), TMDs account for a number of intriguing effects observed in polarized and unpolarized reactions, and allow for a 3-dimensional description of the nucleon in momentum space (nucleon tomography). Furthermore, they could provide insights into the yet unmeasured quark orbital angular momentum. At leading-twist eight TMDs enter the SIDIS cross section in conjunction with a fragmentation function. In addition, going to the twist-3 level allows us to probe novel quark-gluon correlations. At HERMES, these distribution functions are probed through specific azimuthal modulations in the distribution of hadrons produced in semi-inclusive DIS of unpolarized or longitudinally polarized electrons and positrons off unpolarized and polarized nucleons. Amplitudes of some of these modulations sensitive to the beam polarization, recently extracted also in a three-dimensional kinematic space, will be presented in more detail.
