CERN Accelerating science

Heavy-ion collisions are a unique tool to study the quark-gluon plasma (QGP), a state of matter in which quarks and gluons are not bound within hadrons by the strong force. QGP is expected to form when high energy density and temperature conditions are reached, as in heavy-ion collisions. One proposed signature of QGP formation is the strangeness enhancement effect, which consists in an increase of the ratio of strange to non-strange hadron yields in Pb-Pb collisions with respect to minimum bias pp collisions. This effect has been further investigated within the ALICE collaboration by studying its dependence on the multiplicity of charged particles produced in different collision systems. Results show that the ratios of different strange hadron yields to pion yields increase with the multiplicity of charged particles, revealing a smooth transition between different collision systems, from low multiplicity proton-proton (pp) collisions to high multiplicity central Pb-Pb collisions. This behaviour is striking as different particle production mechanisms are expected to be involved in the different collision systems. The results also show that the strangeness enhancement with multiplicity does not depend on the collision energy at the LHC energies (~TeV) and that it is larger for hadrons with larger strangeness content. The work presented in this thesis addresses the strangeness enhancement effect observed in proton-proton collisions, focusing on the production of the strange meson $\mathrm{K^0_S}$ and of the strange baryon $\Xi^\pm$ in jets and out of jets in pp collisions at $\sqrt{s}=5.02$ TeV and at $\sqrt{s}=13$ TeV. The data were collected by the ALICE experiment during the Run 2 data taking (2015-2018). The aim of this work is to evaluate the contribution to the strangeness enhancement effect given by the mechanisms associated to hadron production in jets (hard processes) and out of jets. For the purpose of separating $\mathrm{K^0_S}$ ($\Xi^\pm$) produced in jets from the ones produced out of jets, the angular correlation between the charged particle with the highest transverse momentum $p_{\mathrm{T}}$ and with $p_\mathrm{T} > 3$ GeV/c (trigger particle) and the $\mathrm{K^0_S}$ ($\Xi^\pm$) produced in the same collision is evaluated. $\mathrm{K^0_S}$ and $\Xi^\pm$ are identified by applying topological and kinematic selections to the variables describing their decay into charged hadrons. The trigger particle is considered as a proxy for the jet axis: $\mathrm{K^0_S}$ ($\Xi^\pm$) produced in the leading jet are found in an angular region centred in the direction of the trigger particle (toward-leading production), whereas $\mathrm{K^0_S}$ ($\Xi^\pm$) produced in an angular region transverse to the trigger particle direction are associated to out-of-jet (transverse-to-leading) production. The toward-leading and the transverse-to-leading yields of $\mathrm{K^0_S}$ ($\Xi^\pm$) are calculated as a function of the multiplicity of charged particles produced in the events with a trigger particle. For both $\mathrm{K^0_S}$ and $\Xi^\pm$, the transverse-to-leading yield increases with the multiplicity of charged particles faster than the toward-leading one, suggesting that the relative contribution of transverse-to-leading processes with respect to toward-leading processes increases with the multiplicity of charged particles produced in the collision. The ratio between the $\Xi^\pm$ and the $\mathrm{K^0_S}$ yields provides insight into the strangeness enhancement effect, since the strangeness content of the $\Xi^\pm$ ($S=2$) is larger than the $\mathrm{K^0_S}$ one ($S=1$). Both the transverse-to-leading and the toward-leading $\Xi^\pm$/$\mathrm{K^0_S}$ yield ratios increase with the multiplicity of charged particles. The transverse-to-leading ratio is larger than the toward-leading one, suggesting that the relative production of $\Xi^\pm$ with respect to $\mathrm{K^0_S}$ is favoured in transverse-to-leading processes. The results as a function of the charged particle multiplicity are compared with the predictions of different phenomenological models, namely EPOS LHC and two different implementations of PYTHIA 8. The results presented in this thesis were approved by the ALICE collaboration and presented in several international conferences. The paper which contains the results presented in this thesis is in preparation. In the last part of this thesis the further developments of this work which will be possible thanks to the large amount of pp collisions which will be collected during the Run 3 data taking are also discussed.