System-size dependence of strangeness production in high-energy A+A collisions and percolation of strings

We argue that the shape of the system-size dependence of strangeness production in nucleus-nucleus collisions can be understood in a picture that is based on the formation of clusters of overlapping strings. A string percolation model combined with a statistical description of the hadronization yields a quantitative agreement with the data at sqrt s_NN = 17.3 GeV. The model is also applied to RHIC energies

System size and centrality dependence of the balance function in A+A collisions at sqrt[sNN]=17.2 GeV

Electric charge correlations were studied for p+p, C+C, Si+Si, and centrality selected Pb+Pb collisions at sqrt[sNN]=17.2 GeV with the NA49 large acceptance detector at the CERN SPS. In particular, long-range pseudorapidity correlations of oppositely charged particles were measured using the balance function method. The width of the balance function decreases with increasing system size and centrality of the reactions. This decrease could be related to an increasing delay of hadronization in central Pb+Pb collisions

System size and centrality dependence of the balance function in A + A collisions at sqrt s NN = 17.2 GeV

Electric charge correlations were studied for p+p, C+C, Si+Si and centrality selected Pb+Pb collisions at sqrt s_NN = 17.2$ GeV with the NA49 large acceptance detector at the CERN-SPS. In particular, long range pseudo-rapidity correlations of oppositely charged particles were measured using the Balance Function method. The width of the Balance Function decreases with increasing system size and centrality of the reactions. This decrease could be related to an increasing delay of hadronization in central Pb+Pb collisions

Observation of enhanced subthreshold K+ production in central collisions between heavy nuclei

In the very heavy collision system 197Au+197Au the K+ production process was studied as a function of impact parameter at 1 GeV/nucleon, a beam energy well below the free N-N threshold. The K+ multiplicity increases more than linearly with the number of participant nucleons and the K+/ pi + ratio rises significantly when going from peripheral to central collisions. The measured K+ double differential cross section is enhanced by a factor of 6 compared to microscopic transport calculations if secondary processes (Delta N-->K Lambda N and Delta Delta -->K Lambda N) are ignored

Azimuthally anisotropic emission of pions in symmetric heavy-ion collisions

Triple differential cross sections d3 sigma /dp3 for charged pions produced in symmetric heavy-ion collisions were measured with the KaoS magnetic spectrometer at the heavy-ion synchrotron facility SIS at GSI. The correlations between the momentum vectors of charged pions and the reaction plane in 197Au+197Au collisions at an incident energy of 1 GeV/nucleon were determined. We observe, for the first time, an azimuthally anisotropic distribution of pions, with enhanced emission perpendicular to the reaction plane. The anisotropy is most pronounced for pions of high transverse momentum in semicentral collisions

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

Energy dependence of Lambda and anti-Lambda production at CERN-SPS energies

Rapidity distributions for Lambda and anti-Lambda hyperons in central Pb-Pb collisions at 40, 80 and 158 AGeV and for K 0 s mesons at 158 AGeV are presented. The lambda multiplicities are studied as a function of collision energy together with AGS and RHIC measurements and compared to model predictions. A different energy dependence of the Lambda/pi and anti-Lambda/pi is observed. The anti-Lambda/Lambda ratio shows a steep increase with collision energy. Evidence for a anti-Lambda/anti-p ratio greater than 1 is found at 40 AGeV