Multiplicity dependent J/ψ production in proton-proton collisions at the LHC

The discovery of the J/ψ particle in 1974 helped to fully establish the quark model of hadrons and sparked an increased interest in particle physics that led to rapid developments in the field in the following years. This period was later coined the November Revolution in particle physics. Due to the different energy scales at play in J/ψ production, both perturbative and nonperturbative aspects of Quantum ChromoDynamics (QCD) are relevant for an accurate description. Over time, increasingly sophisticated theoretical models have provided a more and more detailed picture of J/ψ production in hadronic interactions, albeit it is still defying a conclusive description of all aspects of its production. Recently, high particle multiplicities created in proton-proton collisions have been met with increased interest due to observations of apparent collective phenomena traditionally linked with the creation of a medium in heavy-ion collisions. The correlation between J/ψ production and the charged-particle multiplicity is an excellent observable addressing the interplay between hard and soft processes. It can help to shape a complete picture of proton-proton collisions, especially with regard to mechanisms like multiparton interactions and saturation effects in the particle production. ALICE is an experiment at the LHC, dedicated to the study of the hot and dense medium created in heavy-ion collisions. With its excellent tracking and particle identification capabilities in dense environments it is also well suited for the study of proton-proton collisions with high particle multiplicities. With the Run-2 data taking of the LHC, the highest collision energies ever created in the laboratory have become accessible. A special data taking campaign with the ALICE experiment focused on collecting collisions with high charged-particle multiplicities allows access to unprecedented multiplicity regimes. In this thesis, a measurement of the self-normalized inclusive J/ψ yield at mid-rapidity in proton-proton collisions at sqrt(s) = 13 TeV as a function of the self-normalized charged-particle multiplicity is presented. The analysis has been performed as a function of the multiplicity at mid-rapidity and at forward rapidity. Transverse momentum integrated J/ψ production and J/ψ production at low and high transverse momenta have been investigated. A stronger than linear increase of J/ψ production with particle multiplicity is observed, with no significant effect of a rapidity gap between the J/ψ and the region in which the particles are measured. The increase is strongest for J/ψ at high transverse momentum. The results of the analysis are compared to theoretical model predictions. Though differing in the assumptions on the physical processes, all of them agree at least qualitatively with the experimental results. With a comprehensive study of Monte Carlo simulations often overlooked autocorrelation effects between the J/ψ signal and the particle multiplicity are addressed. Based on this study, a refinement of the presented measurement via the exclusion of the regions most affected by autocorrelation effects from the particle multiplicity measurement is proposed.