CERN Accelerating science

Through ultrarelativistic particle collisions at the LHC, it is possible to deconfine quarks and gluons. This deconfinement gives rise to a strongly interacting medium, referred to as the Quark-Gluon Plasma (QGP). One of the earliest proposed and observed signatures of the QGP was the enhanced production of strange hadrons since the medium can thermally produce strange quarks. However, recent studies in small systems, such as proton--proton (pp) and proton-lead (pPb) collisions, have exhibited similar features. These findings are quite puzzling, as the formation of a QGP in these small collision systems challenges current theoretical frameworks. In this Thesis, I present two different studies on the production of $\phi$ mesons, in relation to $\Xi$ baryons, in pp collisions at \s = 13 TeV, measured with the ALICE apparatus. Both of these studies aim to investigate the origin of strange hadron enhancement in high-multiplicity pp collisions. First, I report measurements of $\phi$ production as a function of the event-shape observable Unweighted Transverse Spherocity, $S_{\rm O}^{p_{\rm T}=1}$. With $S_{\rm O}^{p_{\rm T}=1}$, it is possible to categorize events by their azimuthal topology. I utilize $S_{\rm O}^{p_{\rm T}=1}$ to contrast particle production in collisions dominated by many soft initial interactions, with collisions dominated by a single hard scattering. I find that strangeness enhancement is prominent in soft, isotropic topologies, whereas events with di-jet topologies showcase a clear suppression of strange particles. The second study presents the production of $\phi$ mesons and $\Xi$ hadrons as a function of the Relative Transverse Activity $R_{\rm T}$. With $R_{\rm T}$, one can control the size of the Underlying Event (UE). By varying $R_{\rm T}$, it is therefore possible to study the interplay between particle production from hard fragmentation in jets, and soft particles produced by the UE. The reported results suggest that strange particle production is mainly a feature of the UE. When put together, the two studies suggest that high-multiplicity pp collisions are in general dominated by soft physics, which is also responsible for the strangeness enhancement, while high-multiplicity events dominated by hard physics are rare outliers.