ACCESSING THE 3D STRUCTURE OF NUCLEONS THROUGH TMD PHENOMENOLOGY

The three-dimensional structure of nucleons and transverse polarisation phenomena at high energy are subjects of very intensive researches. While collinear QCD at leading twist is not able to describe such kind of events, more extended approaches including intrinsic transverse momentum effects have turned out to be more suitable in this context. In this thesis, phenomenological studies on transverse momentum dependent (TMD) functions are presented. By adopting a TMD approach, we enlight the current knowledge on some of the leading twist polarised TMD functions, such as the (quark and gluon) Sivers distributions, the transversity and the Collins functions. The role of the underlying choices in phenomenological analyses, together with the assessment of TMD signals in the data, is particularly emphasised. Indeed, some of the assumptions made on the phenomenological model can influence the outcome of an analysis. In the case of the quark Sivers function, suitable choices on the set of parameters result in a more accurate uncertainty evaluation, while for the TMD transversity relaxing some initial hypothesis eases the tension between phenomenology and lattice QCD estimates of the isovector tensor charge. The poorly known gluon Sivers function (GSF) are also analysed, both in the framework of the Generalised Parton Model (GPM) and its colour gauge invariant extension (CGI-GPM), where two independent GSFs are defined. A first constraint on the size and the sign of these functions, as well as a new GPM extraction, are showed. The extracted TMDs are also adopted to study the single-spin asymmetries observed in polarised proton-proton and lepton-proton processes. The latter reaction is analysed, highlighting the role of the quasireal photon exchange in the unpolarised cross sections and in the asymmetries. Theoretical predictions compared against existing data show a very good description of the measured asymmetries. By means of the estimated gluon Sivers functions, single-spin asymmetries for inclusive J/ψ and photon production in polarised proton-proton reactions are also estimated. At the moment, no clear discrimination between the GPM and the CGI-GPM is possible. A novel Horizon 2020 project, aiming to create an online platform for hadronic and nuclear physics is also presented. In future, it would represent a valuable tool,% in these contexts, and its potential extension to TMD physics could become beneficial also for the TMD community. New data from colliders and fixed target experiments will come in the future. In the quest for understanding polarisation phenomena, a more agnostic, transparent and thorough approach in phenomenological analyses would help in preparing the TMD community to be ready for the next generation of experiments.