Study of the Transverse-Momentum-Dependent structure of the nucleon in Semi-Inclusive DIS

The study of the internal structure of the nucleon is a hot topic in hadron physics. In recent years, a huge effort has been undertaken, both on the theoretical and on the experimental side, to provide a comprehensive description of the nucleon structure beyond the collinear Quantum Chromo Dynamics (QCD) approach, where all transverse degrees of freedom are assumed negligible. As a matter of fact, many experimental results can not be explained without considering the parton transverse momentum $k_{T}$, its transverse spin $s_{T}$ and the way how they correlate with the nucleon. In the recently developed QCD formalism, the nucleon structure is described by eight Transverse-Momentum-Dependent (TMD) Parton Distribution Functions (PDFs), which are generalizations of the three collinear ones (number density, helicity and transversity). To a large extent, our knowledge of these functions is still very poor. The aim of this Thesis is to contribute to the understanding of the nucleon structure through the study of two observables accessible in Semi-Inclusive Deep Inelastic Scattering (SIDIS) of high energy leptons off unpolarized protons: the transverse-momentum distributions and the amplitudes of the modulations in the azimuthal angle of the final state hadrons, the latter referred to as "azimuthal asymmetries". They give relevant information on the transverse momentum of the partons inside the nucleon and on the Boer-Mulders TMD PDF. This Thesis summarizes the work done in this direction during the Ph.D., which consisted in a complete analysis of part of the data collected in 2016 in COMPASS, a fixed target experiment at the CERN SPS using 160 GeV/$c$ $\mu^+$ and $\mu^-$ beams and a liquid hydrogen target. The data quality and the detector stability have been investigated, as well as the stability and the consistency of the reconstructed data. A significant effort has been put in the validation of the Monte Carlo simulations, necessary for the evaluation of the acceptance of the spectrometer and to estimate the contamination to the SIDIS sample by the hadrons produced in the decay of diffractively produced vector mesons (particularly $\rho^0$ and $\phi$), which give strong contributions both to the transverse-momentum distributions and to the azimuthal asymmetries of the inclusive hadrons. This diffractive process had also to be studied in detail and implemented in dedicated Monte Carlo simulations, reducing the systematic uncertainties affecting previous measurements. Other systematic effects have also been investigated, and the corresponding systematic uncertainties evaluated. A deep inspection of the various kinematic dependences has been performed. A phenomenological analysis of the new results is also presented, with a comparison of the current COMPASS findings with the previous ones obtained on a deuteron target. This Thesis is organized as follows. In Chapter 1 the transverse-momentum-dependent structure of the nucleon is introduced, with a focus on the SIDIS process and the related observables. The second Chapter is dedicated to the description of the COMPASS experiment with details on the 2016 setup. The third Chapter is devoted to the data analysis, with a description of the selection of the DIS events and of the hadron samples and of the acceptance corrections. The procedure for the extraction of the transverse-momentum distributions and of the azimuthal asymmetries, the estimation of their systematic uncertainties, the results and their interpretation are presented in Chapter 4 and 5 respectively. Chapter 6 hosts a study of the exclusive diffractive vector meson production, with a focus on the $\rho^0$ vector meson. The conclusions are given in Chapter 7.