Correlating hard and soft particle production at the LHC via multiplicity-dependent prompt and non-prompt J/$\psi$ measurement

The hadronic production of the J/$\psi$ meson, a bound state of a charm and an anticharm quark, is a strong probe of the theory of QCD, which describes the strong interaction between colour charges. Due to their large mass, the heavy quarks are produced in a scattering with a large momentum transfer (hard scattering). In contrast, most charged particles are produced in scatterings with small momentum transfer (soft scatterings). Therefore, measurements of the correlation between J/$\psi$ production and the charged-particle multiplicity in hadronic collisions could provide more information about the interplay and differences between hard and soft particle production. Previous measurements of the multiplicity dependence of J/$\psi$ production in small hadronic systems have shown a stronger-than-linear increase of the relative yields with the event multiplicity. This trend shows that there seems to exist a fundamental difference between hard and soft particle production. A substantial part of the J/$\psi$ are produced through the decay of beauty hadrons. The analysis of the multiplicity dependence of these non-prompt J/$\psi$ could help to identify the difference in the multiplicity dependence between charm and beauty, and between open heavy-flavor hadrons (i.e. hadrons which have non-zero net heavy-flavor content, e.g. a heavy quark together with a light quark) and quarkonium. Several models explain the stronger-than-linear trend through either initial-state effects, such as a saturation of the gluon density distribution at small momentum fraction inside the initial proton, or final-state effects, where soft particle production could be saturated within a high-density medium similar to the QGP produced in heavy-ion collisions. In addition, studies with the PYTHIA8 generator have demonstrated that the autocorrelations (i.e. particles produced alongside the J/$\psi$ in the same production process) could also influence the trend. These autocorrelation effects could be disentangled by separating the multiplicity in several azimuthal regions with respect to the J/$\psi$ emission direction. Therefore, this thesis is dedicated to the measurement of the multiplicity dependence of prompt and non-prompt J/$\psi$ production at midrapidity, using data collected by the ALICE experiment at $\sqrt{s}=13$ TeV during the Run 2 of the LHC. The evolution of the prompt yield is found to be similar to the one of non-prompt yields, with only a slight hint of an increase of the non-prompt fraction with the multiplicity. Thus, the evolution of quarkonia yield with the multiplicity should not present significant differences from the open heavy-flavor ones. This also shows the similarity between charm and beauty evolution with multiplicity. The J/$\psi$ yields are further measured as a function of the multiplicity in three azimuthal regions. A strong increase as a function of the multiplicity in the toward region is measured, which indicates that the autocorrelations might play a role in the observed multiplicity dependence. In addition, a stronger increase when increasing the $p_{\text{T}}$ of the J/$\psi$ is present in the toward region, but not in the transverse region, showing that autocorrelations should be important in explaining the evolution with $p_{\text{T}}$. The increase in the transverse and away region is stronger compared to the expectations of a baseline of soft particle production, demonstrating that the increase of the J/$\psi$ yield is also a global effect of the different scales, and that autocorrelations are probably not the only effects at play.