Spin and Hadron Physics Group
at the University of Turin

Mole Antonelliana

Spin is a natural and fundamental degree of freedom in any quantum field theory and a complete understanding of QCD and the Standard Model must include the spin sector. Our work focuses on understanding, within QCD, the 3-dimensional momentum and spin structure of nucleons in terms of the spins and motions of their elementary constituents, quarks and gluons.

We study polarized scattering processes to determine the Transverse Momentum Dependent parton distribution functions (TMF-PDFs), which are phenomenological objects describing the probability
of finding partons inside a (polarized) proton, with a specific spin, a specific fraction of the proton momentum and a specific transverse momentum.

Similarly, we  are interested in studying the Transverse Momentum Dependent fragmentation functions (TMD-FFs), which describe the probability of a (polarized) parton to fragment into a hadron with a specific spin, a specific fraction of the parton momentum and a specific transverse momentum.

Experiments dedicated to the study of TMDs have been or are being performed at JLab, CERN-COMPASS, DESY-HERMES, BNL-RHIC, KEK-Belle. A strong effort in planning a future electron-ion collider (EIC) is being pursued by the spin-hadron physics community. We are strongly involved, as theorists and phenomenologists, with all of them. A global analysis of all data should allow a full reconstruction of the TMDs, and their QCD study should lead to understand their evolution. 

We are presently part of the European Project HadronPhysics3 participating in the Joint Research Activity 3D-Mom, dedicated to the Three-dimensional momentum structure of hadrons. 

The simple description of a fast moving nucleon as a collection of collinear partons, which is successful in many respects, cannot explain an ever increasing number of observed phenomena, typically the spin dependent ones. The intrinsic transverse motion of partons and their correlation with spin and orbital angular momentum, cannot be neglected  for a true and full understanding of a large class of phenomena.

The collinear QCD factorization scheme, allows to describe hard scattering processes as a convolution of partonic distributions, fragmentation functions and elementary interactions. For some processes, it has been generalized to include intrinsic transverse motion: in these cases the measured cross sections are a convolution of TMD-PDFs, TMD-FFs and elementary, non collinear QCD or QED partonic interactions. Then, from data, one can extract information on the TMDs, which describe, in momentum space, the full nucleon 3-dimensional structure.