Francesca Bellini (Università di Bologna)
The production of hadrons containing one or more strange quarks has been measured in heavy-ion collisions for more than 30 years to investigate the formation of the deconfined medium known as Quark Gluon Plasma (QGP) and its thermal properties. Measurements in systems such as proton-nucleus (p-A) or proton-proton (pp) collisions have been traditionally used as a baseline, but a the recent observation of strangeness enhancement in high-multiplicity pp collisions by the ALICE experiment at the CERN Large Hadron Collider has opened new perspectives.
ALICE has recently reported on the enhanced production of strange hadrons relative to pions in high-multiplicity pp collisions [1]. From low to high multiplicity events, strange hadron production increases smoothly and continuously until it reaches the values measured in Pb-Pb collisions. The trend, more pronounced for multi-strange baryons, is remarkably similar to the one observed in p–Pb collisions, indicating that the phenomenon is related to the final system created in the reaction.
After a short historical excursus through the milestones in the study of strangeness production as a QGP signature, the results on strangeness production at the LHC will be presented. The new multiplicity-dependent measurement will be discussed, also in comparison with model predictions. As the observed strangeness enhancement adds to a set of previous measurements showing that pp collisions exhibit characteristic features known from high-energy heavy-ion collisions, the questions are open: is it strangeness enhancement not a unique feature of QGP formation? Can high-multiplicity pp collisions provide information on the onset of deconfinement?