Centrality dependence of strangeness production in heavy-ion collisions as a geometrical effect of core–corona superposition

Abstract

It is shown that data on strange particle production as a function of centrality in Au-Au collisions at √ s NN = 200 GeV can be explained with a superposition of emission from a hadron gas at full chemical equilibrium (core) and from nucleon-nucleon collisions at the boundary (corona) of the overlapping region of the two colliding nuclei. This model nicely accounts for the enhancement of φ meson and strange particle production as a function of centrality observed in relativistic heavy ion collisions at that energy. The enhancement is mainly a geometrical effect, that is the increasing weight of the core with respect to corona for higher centrality, while strangeness canonical suppression in the core seems to play a role only in very peripheral collisions. This model, if confirmed at lower energy, would settle the long-standing problem of strangeness under-saturation in relativistic heavy ion collisions, parametrized by γS. Furthermore, it would give a unique tool to locate the onset of deconfinement in nuclear collisions both as a function of energy and centrality if this is to be associated to the onset of the formation of a fully equilibrated core.

Transverse-momentum spectra of strange particles produced in Pb + Pb collisions atsNN=2.76TeV in the chemical nonequilibrium model

“…The appearance of γ s can be explained, for example, in the so called core-corona model [44][45][46][47], where a superposition of two sources of particle production is taken into account: single nucleon-nucleon (NN) collisions and a fully equilibrated source.…”

System-size and centrality dependence of charged kaon and pion production in nucleus-nucleus collisions at 40AGeV and 158AGeV beam energy

“…The reduction of relative strangeness production towards even smaller system sizes has been discussed in the framework of the core-corona model in the previous paragraph. An implementation of this ansatz into the statistical model and a detailed comparison of model calculations to RHIC data can be found in [25]. In order to describe the data satisfactorily it is essential to use a volume dependence which is not just proportional to N w .…”

“…The fraction of single scatterings is calculated using straight line geometry as described in the Glauber model [24]. This approach was recently successfully applied to the systemsize dependence of strangeness production at RHIC energies [25,26] and for the above mentioned hyperon production at SPS [6]. The common features of such model calculations at fixed energy are a fast increase of relative strangeness production with system size for small reaction volumes (below approximately 60 participating nucleons) and eventual saturation for large system sizes.…”

Λpolarization in peripheral heavy ion collisions

“…(2) in Ref. [19]) (N P C /2)/(3.6N P ) = 15/360 0.042. This implies that at the top RHIC energy, and even more so at the LHC energy where the fraction of corona collisions is lower, at most only about 4% of hyperons come from NN collisions, and that their contribution to the measured polarization, at very low x F , is at most 0.04 × 0.05 = 0.002, far below the signal level.…”

“…In order to estimate the impact of this background on the hydrodynamically originated polarization, one should estimate the number of single NN collisions in the corona as a function of the number of participants nucleons N P in peripheral nuclear collisions. A calculation carried out by one of the authors [19] with the Glauber Monte-Carlo model at √ s N N = 200 GeV shows that for peripheral collisions with N P 100 the number of nucleons undergoing single collisions in the corona is N P C 30. According to a STAR measurement [20], for N P 100, at midrapidity the multiplicity is approximately 3.6N P times the one in pp collisions at the same energy.…”