Understanding the production mechanisms of particles with strangeness in pp collisions with the ALICE experiment at the LHC

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 work presented in this thesis addresses the strangeness enhancement observed in pp collisions, focusing on the production of the strange meson 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. The aim of this work is to evaluate the contribution to the strangeness enhancement effect given by the mechanisms associated with hadron production in jets (hard processes) and out of jets. For the purpose of separating 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_T and with p_T > 3 GeV/c (trigger particle) and the \Ks (\Xi^\pm) produced in the same collision is evaluated. The trigger particle is considered as a proxy for the jet axis: 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 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. For both 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 K^0_S yields provides insight into the strangeness enhancement effect, since the strangeness content of the \Xi^\pm is larger than the K^0_S one. Both the transverse-to-leading and the toward-leading \Xi^\pm/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 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 PYTHIA8. 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.