System-Size Dependence of Strangeness Production in Heavy-Ion Collisions at 158 AGeV

Strangeness enhancement in A+A collisions relative to p+p interactions as a signal for the transition to a deconfined state of strongly interacting matter was recently searched for mostly in high-energy collisions of heavy nuclei such as central Pb+Pb or Au+Au. The expectation is that in these large systems with about 360 participating nucleons such a transition is more likely because of a longer lifetime and higher energy density. However, earlier studies with lighter beams had demonstrated that already in S+S with 54 participants strangeness is significantly enhanced. In this work strange-particle production is studied as function of system size in symmetric central A+A collisions at 158 AGeV. Using the NA49 spectrometer at the CERN-SPS, yields and kinematic distributions of kaons, K*(892), the phi-meson and, for reference purposes, also of pions are measured in minimum-bias and inelasticity-selected p+p interactions, and in central C+C and Si+Si collisions. Together with earlier data for central S+S and Pb+Pb the results present a complete picture of the evolution of strangeness enhancement as function of system size. The data show a continuous increase of the strange-particle abundances in dependence on system size, with a fast rise in small systems and a saturation already for about 60 participating nucleons if comparing central A+A collisions only. On the basis of the present data and using microscopic models for A+A collisions an attempt is made to localize the origin of strangeness enhancement and to understand its evolution. For several reasons, rescattering is found to be an unlikely explanation, in particular for the lighter systems. The idea that the high string excitations - obtained in A+A collisions as a consequence of sequential N+N interactions - are responsible is dismissed on the basis of inelasticity-controlled p+p data. On the other hand, the geometry dependence indicated by a comparison of the central A+A data with those for peripheral Pb+Pb suggests that the density of the primary inelastic interactions in space-time plays a decisive role, because it is found to act as a scaling variable for the strangeness enhancement in all systems. The final conclusion of this work is that a high collision density leads to formation of coherent partonic (sub-)systems comprising several strings whose hadronization can be described statistically. Then, these systems and/or their hadronization must be subject to the phenomenon of canonical strangeness suppression respectively of grand-canonical strangeness enhancement. This would explain both the strangeness enhancement itself and its system-size dependence