Etude de la production de baryons et mésons étranges dans les collisions proton-proton avec l’expérience ALICE auprès du LHC

The ALICE experiment at LHC is dedicated to investigate the transition of matter from the hadron gaz to the Quark and Gluons Plasma in which partons are deconfined. Ultra-relativistic heavy-ion collisions offer indeed the possibility to create extreme temperature and pressure conditions which are required to reach a deconfined phase. Elementary collisions such as proton-proton are of great importance since they are regarded as the hadronic reference. The aim of this thesis was to prepare the analysis of strange baryon and meson production mechanisms in proton-proton collisions at the LHC energies by the measurement of lambda$ and K0s with ALICE. Strange particles are a major tool to probe the matter created. The behaviour of the lambda/K0s ratio at intermediate transverse momentum in high energy proton-proton collisions, that we have studied with several theoritical models, could also sign the presence of collective phenomena. Up to now, these phenomena have been observed only heavy-ion collisions.