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 3dimensional 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 (TMFPDFs), 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 (TMDFFs),
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, CERNCOMPASS, DESYHERMES,
BNLRHIC, KEKBelle.
A strong effort in planning a future electronion collider (EIC) is
being pursued by the spinhadron 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 3DMom, dedicated to the Threedimensional 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
TMDPDFs, TMDFFs 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 3dimensional
structure.

