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...
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|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.
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
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.