CERN Accelerating science

The ALICE experiment aims to explore the properties of strongly interacting matter under extreme conditions, particularly the transition from nuclear matter to Quark-Gluon Plasma (QGP). Heavy-flavour quarks, being produced in hard-parton scattering in the early stages of the ultra-relativistic hadronic collisions, experience the full evolution of this hot dense medium, therefore representing ideal probes for its characterisation. The products of the HF quark fragmentation are sensitive to the modification of the parent quark properties providing an insight into heavy-quark interactions with the medium constituents. In this regard, the study of heavy-flavour hadron observables in small collision systems, as proton--proton, serve as a baseline for disentangling the effects induced by this deconfined state-of-matter. In addition, measurements in small systems are fundamental for testing the current understanding of Quantum Chromodynamics and for validating Monte Carlo generators trying to describe the heavy-flavour quark production and fragmentation in the measured final-state particle. This thesis is devoted to the study of azimuthal correlations between charm hadrons and charged particles in proton--proton collisions at the energy of $\sqrt{s}=13$ TeV in the centre-of-mass, using data collected during Run 2 of LHC data taking (2015-2018). Such measurements provide an insight into the charm-quark fragmentation and a detailed description of the charm-jet shape and particle multiplicity as a function of the transverse momentum of both trigger and associated particles. In addition, the comparison between charm meson and baryon measurements can shed light on possible modifications in the charm-jet properties induced by different hadronisation mechanisms. The measurements are performed by reconstructing D mesons and $\Lambda_\mathrm{c}^{+}$ baryons and evaluating the angular distribution in the transverse plane of other charged primary particles with respect to the direction of the charm hadron. D-meson results are in good agreement with previous measurements at smaller centre-of-mass energy, showing no dependence of the charm fragmentation processes on the collision energy. The comparison with several state-of-the-art Monte Carlo generators and models suggests a satisfactory, but not perfect, description of the charm-quark fragmentation processes by the PYTHIA generator in the presence of D mesons. A noticeable difference is instead measured between $\Lambda_\mathrm{c}^{+}$-triggered measurements and D meson results, manifesting in a more abundant charm-jet associated yield for small trigger transverse momentum. This potentially suggests a softer fragmentation of the charm-quark in baryons than in mesons, as hinted by the $\Lambda_\mathrm{c}^{+}$-tagged jets measurements, or effects induced by different hadronisation mechanisms. PYTHIA 8 predictions including colour-reconnection mechanisms beyond leading colour approximation as hadronisation mechanisms, do not reproduce this enhancement, despite correctly predicting the $\Lambda_\mathrm{c}^{+}$/D$^0$ production yield ratio measurement. It is also demonstrated that the effect can not be justified by the presence of additional yet unobserved heavier charm-baryon states decaying in a $\Lambda_\mathrm{c}^{+}$ as predicted by the SHM+RQM model.