Multi-differential studies to explore strangeness enhancement in pp collisions with ALICE at the LHC

Abstract

The relative production rate of (multi-)strange hadrons in high multiplicity hadronic interactions is enhanced with respect to the one measured at lower multiplicities and reaches values observed in heavy-ion collisions. The microscopic origin of this striking phenomenon, historically interpreted as a signature of quark-gluon plasma (QGP) formation in heavy-ion collisions, is still unknown: is it related to soft particle production or to hard scattering events, such as jets? Is it related to final particle multiplicity only or does it also depend on initial state effects? The ALICE experiment has addressed these questions by performing dedicated measurements in pp collisions at $\sqrt{s}=13$ TeV. To separate strange hadrons produced in jets from those produced in soft processes, the angular correlation between high-${p_{\mathrm{T}}}$ charged particles and strange hadrons has been exploited. The near-side jet yield and the out-of-jet yield of $\mathrm{K^0_S}$ and $\Xi$ have been studied as a function of the multiplicity of charged particles produced in pp collisions. Moreover, a multi-differential analysis has been carried out to disentangle the contribution of final state multiplicity from the one of effective energy available for strange particle production. The effective energy has been estimated by subtracting the energy measured in the Zero Degree Calorimeters (ZDC) from the centre-of-mass energy. The results suggest that soft (i.e. out-of-jet) processes are the dominant contribution to strange particle production and that initial state properties do not play a significant role in strangeness production, which is mainly driven by final particle multiplicity.