Study Of (Multi-)strange Particle Production With ALICE At The CERN-LHC

The Quark-Gluon Plasma(QGP)was predicted to exist at a sufficiently high temperature and energy density by the theory of strong interactions,Quantum ChromoDynamics(QCD)around 50 years ago.The QGP is a new state of matter,where quarks and gluons are not confined inside hadrons,and is supposed to have dominated the early universe a few microseconds after the Big Bang.It is crucial for understanding its subsequent evolution to understand the QGP properties.High energy heavy-ion collisions provide us with the unique opportunity to study the QGP in a laboratory.The droplet of the QGP created in collision rapidly expands and cools down until at a temperature of about 150 MeV,at which the deconfined partons hadronize into the ordinary,color-neutral matter.Systematic study of particle production allows us to investigate the properties of the initial partonic phase and the hadronization process.A Large Ion Collider Experiment(ALICE)is one of the four large experiments(ALICE,ATLAS,CMS,and LHCb)at the Large Hadron Collider(LHC)of the European Organization for Particle Physics(CERN).The main target of ALICE is to study the properties of QGP.The interpretation of heavy-ion results and extraction of the QGP properties requires studies of particle production in small collision systems,such as protonproton(pp)and proton-nucleus(pA).The measurements in these small collision systems were thought of as a necessary foundation to quantify the initial and final state effects in the case of the so-called cold nuclear matter.However,during the last decade,the study of small systems has gained increased interest as a research field in its own right.In particular,observations similar to those made in heavy-ion collisions have been observed in pp and p-Pb collisions where the formation of QGP was not expected.These include,for example,the long-range angular correlations.The magnitude of the effect increases smoothly with system size and particle multiplicity from pp,p-Pb to Pb-Pb collisions.Another new feature observed in high-multiplicity pp and p-Pb collisions is the enhancement of the baryon-to-meson ratios,p/π and Λ/KS0,at intermediate transverse momentum pT(～3 GeV/c)which is qualitatively similar to that observed in Pb-Pb collisions.Moreover,the strange to non-strange hadron ratio increases continuously as a function of charged-particle multiplicity density from low-multiplicity pp to highmultiplicity p-Pb collisions and reaches the values observed in Pb-Pb collisions.These observations suggest the existence of a common underlying mechanism determining the chemical composition of particles produced by small to large collision systems.On the other hand,measurements of jet production at mid-rapidity in small systems do not exhibit nuclear modifications.The enhancement of baryon-to-meson ratios in the intermediate pT region(24 GeV/c)has been linked to the interplay of radial flow and parton recombination.In addition to these effects,one can expect that particle production in this region results from the hard fragmentation of partons in the high pT range.This is due to the steeply falling power-law spectrum characteristic of parton production.This socalled "leading particle effect" was described in terms of a "trigger bias".Studying the ratios of particles associated with jets allows us to gain new insights into the origin of the baryon-to-meson ratio enhancement.In this thesis,the production of strange hadrons(KS0,Λ,Ξ±,and Ω±),baryonto-meson ratios(Λ/KS0,Ξ/KS0,and Ω/KS0)and baryon-to-baryon ratios(Ξ/Λ,Ω/Λ,and Ω/Ξ)associated with jets and the underlying event are measured in pp collisions at(?)=13 TeV and p-Pb collisions at(?)=5.02 TeV with the ALICE detector at the LHC.The inclusive production of the same particle species and the corresponding ratios are also reported.The production of the multi-strange hadrons,Ξ±and Ω±,and their ratios in jets and in the underlying event are measured for the first time.In both pp and p-Pb collisions,the baryon-to-meson and baryon-to-baryon ratios measured in jets differ from those in inclusive particle production for low and intermediate hadron transverse momentum(0.6-6 GeV/c).Ratios measured in the underlying events are in turn similar to those from inclusive particle production.The event multiplicity dependence is further investigated in p-Pb collisions.In contrast to what is observed in underlying events,there is non-significant event multiplicity dependence for particle production in jets.In particular,they demonstrate that the fragmentation of jets alone is insufficient to describe the strange and multi-strange particle production in hadronic collisions at LHC energies.All of the above results are compared with PYTHIA 8 predictions with three color-reconnection implementation modes.The simulations reproduce fairly well for the spectra of A,Kg and the ratio of Λ/KS0 in data.However,large discrepancies between the data and simulations are observed in the spectra and ratios including multi-strange baryons.This study provides new constraints on particle production mechanisms in Monte-Carlo simulations.