Recherche de matière étrange (exotique) dans les expériences STAR et ALICE auprès des collisionneurs d’ions lourds ultra-relativistes RHIC et LHC

Ultra-relativistic heavy ion collisions offer the possibility to create conditions of temperature and density that could lead nuclear matter to a state of deconfined partons, the quark-gluon plasma. Strange baryon production is one of the essential observables to understand the mechanisms involved in the medium. Furthermore, theories predict a possible production of strange dibaryons, still hypothetic particles, from which one could draw important inferences in nuclear physics and astrophysics. The experiments STAR at RHIC, and, soon, ALICE at LHC, allow one to search for strange baryons and dibaryons. The STAR sensitivity to the metastable dibaryon $\rm {H^0}$ in the $\Lambda p \pi^-$ decay mode was calculated thanks to a dedicated simulation. The search for the $\rm {H^0}$, and for the $\Xi^- p$ resonance as well, was performed in the STAR Au+Au data at $\sqrt{s_{NN}}=62.4$ and $200~GeV$ energies. Within the framework of the preparation of ALICE to the first Pb+Pb data, the detector ability to identify strange baryons $\Lambda$, $\Xi$ and $\Omega$, was estimated via several simulations. So as to favour the reconstruction efficiency in a large range of transverse momentum while keeping a reasonable S/B ratio, the influence of the geometrical selections and the size of the reconstruction zone was emphasized. The ALICE sensitivities to the metastable strange dibaryons ${\rm H^0}$ and $(\Xi^0p)_b$ and to the $\Lambda\Lambda$ resonance were calculated as well.