Study of charm hadronisation in pp collisions at $\sqrt{s}$ = 13 TeV with the ALICE experiment

<!--HTML-->In high-energy hadron collisions, the production at parton level of heavy-flavour quarks (charm and bottom) is described by perturbative Quantum Chromo-dynamics (pQCD) calculations, given the hard scale set by the quark masses. However, in hadron-hadron collisions, the predictions of the heavy-flavour hadrons eventually produced entail the knowledge of the parton distribution functions, as well as an accurate description of the hadronisation process. The latter is taken into account via the fragmentation functions measured at e$^{+}$e$^{-}$ colliders or in ep collisions, but several observations in LHC Run 1 and Run 2 data challenged this picture.<br>In this dissertation, I studied the charm hadronisation in proton-proton collision at $\sqrt{s}$ = 13 TeV with the ALICE experiment at the LHC, making use of a large statistic data sample collected during LHC Run 2.<br> The production of heavy-flavour in this collision system will be discussed, also describing various hadronisation models implemented in commonly used event generators, which try to reproduce experimental data, taking into account the unexpected results at LHC regarding the enhanced production of charmed baryons. The role of multiple parton interaction (MPI) will also be presented and how it affects the total charm production as a function of multiplicity.<br>The ALICE apparatus will be described before moving to the experimental results, which are related to the measurement of relative production rates of the charm hadrons $\sum$$_{e}^{0,}$$^{+}$$^{+}$ and $\Lambda$$_{e}^{+}$, which allow us to study the hadronisation mechanisms of charm quarks and to give constraints to different hadronisation models. Furthermore, the analysis of D mesons (D$^{0}$, D$^{+}$ and D$^{*+}$) as a function of charged-particle multiplicity and spherocity will be shown, investigating the role of multi-parton interactions.<br>This research is relevant per se and in the wider context of the mission of the ALICE experiment at the LHC. ALICE is devoted to the study of Quark-Gluon Plasma, which is a state of matter predicted by the QCD theory where quarks and gluons are deconfined. This is done by reproducing energy densities and temperatures similar to the one existing few microseconds after the Big Bang, on a smaller scale via relativistic heavy-ion collisions. Charm and beauty quarks are a powerful probe of that state since they are produced by the hard scattering at the first stages of the collisions and they can interact with the medium during its whole lifetime. Therefore, studying heavy-ion collision events requires a precise baseline of the expected heavy-flavour hadron production, which is provided by proton–proton collisions.