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We present a detailed study of chemical freeze-out in p-p, C-C, Si-Si and Pb-Pb collisions at beam momenta of 158A GeV as well as Pb-Pb collisions at beam momenta of 20A, 30A, 40A and 80A GeV. By analyzing hadronic multiplicities within the statistical hadronization model, we have studied the parameters of the source as a function of the number of the participating nucleons and the beam energy. We observe a nice smooth behaviour of temperature, baryon chemical potential and strangeness under-saturation parameter as a function of energy and nucleus size. Interpolating formulas are provided which allow to predict the chemical freeze-out parameters in central collisions at centre-of-mass energies √ s NN > ∼ 4.5 GeV and for any colliding ions. Specific discrepancies between data and model emerge in particle ratios in Pb-Pb collisions at SPS between 20A and 40A GeV of beam energy which cannot be accounted for in the considered model schemes.

We present a detailed study of chemical freeze-out in nucleus-nucleus collisions at beam energies of 11.6A, 30A, 40A, 80A, and 158A GeV. By analyzing hadronic multiplicities within the statistical hadronization approach, we have studied the strangeness production as a function of center-of-mass energy and of the parameters of the source. We have tested and compared different versions of the statistical model, with special emphasis on possible explanations of the observed strangeness hadronic phase space undersaturation. We show that, in this energy range, the use of hadron yields at midrapidity instead of in full phase space artificially enhances strangeness production and could lead to incorrect conclusions as far as the occurrence of full chemical equilibrium is concerned. In addition to the basic model with an extra strange quark nonequilibrium parameter, we have tested three more schemes: a two-component model superimposing hadrons coming out of single nucleon-nucleon interactions to those emerging from large fireballs at equilibrium, a model with local strangeness neutrality and a model with strange and light quark nonequilibrium parameters. The behavior of the source parameters as a function of colliding system and collision energy is studied. The description of strangeness production entails a nonmonotonic energy dependence of strangeness saturation parameter ␥ S with a maximum around 30A GeV. We also present predictions of the production rates of still unmeasured hadrons including the newly discovered ⌰ + ͑1540͒ pentaquark baryon.

Measurements of charged pion and kaon production in central PbϩPb collisions at 40, 80, and 158 A GeV are presented. These are compared with data at lower and higher energies as well as with results from pϩp interactions. The mean pion multiplicity per wounded nucleon increases approximately linearly with s NN 1/4 with a change of slope starting in the region 15-40 A GeV. The change from pion suppression with respect to p ϩp interactions, as observed at low collision energies, to pion enhancement at high energies occurs at about 40A GeV. A nonmonotonic energy dependence of the ratio of K ϩ to ϩ yields is observed, with a maximum close to 40A GeV and an indication of a nearly constant value at higher energies. The measured dependences may be related to an increase of the entropy production and a decrease of the strangeness to entropy ratio in central PbϩPb collisions in the low SPS energy range, which is consistent with the hypothesis that a transient state of deconfined matter is created above these energies. Other interpretations of the data are also discussed.

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The most recent data of NA49 on hadron production in nuclear collisions at CERN SPS energies are presented. Anomalies in the energy dependence of pion and kaon production in central Pb+Pb collisions are observed. They suggest that the onset of deconfinement is located at about 30 AGeV. Large multiplicity and transverse momentum fluctuations are measured for collisions of intermediate mass systems at 158 AGeV. The need for a new experimental programme at the CERN SPS is underlined. * invited talk presented at Quark Matter 2004 IntroductionSince the first Pb-run at the CERN SPS (at 158 AGeV) in 1994 the NA49 experiment [1] collected a large set of data on hadron production in nuclear collisions. The data taking period ended in 2002 with Pb-runs at 20 and 30 AGeV. Several research programmes were undertaken by the NA49 Collaboration. Selected results of two of them, the energy scan programme and the system size dependence programme, are reported here. Other recent results of NA49 on the energy dependence of particle ratio fluctuations [2], (multi)strange hyperon production [3] and two pion correlations [4] were also presented during this conference. Finally our evidence for pentaquark candidates in p+p interactions at 158 AGeV was discussed [5].The energy scan programme was motivated by the hypothesis [6] that the onset of the deconfinement phase transition is located between the top SPS and AGS energies. Within this project data on hadron production in central Pb+Pb collisions at 20, 30, 40, 80 and 158 AGeV were recorded. In this report we show the first results obtained at 20 AGeV which extend the previously measured hadron systematics [7,8] to the full SPS energy range from 20 to 158 AGeV.The aim of the system size dependence programme is to study how the properties of strongly interacting matter change with its volume. Data on p+p, C+C, Si+Si and minimum bias Pb+Pb collisions at 158 AGeV and 40 AGeV were collected. Here we show new results on system size dependence of electric charge correlations and multiplicity fluctuations at 158 AGeV.