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ultra - relativistic heavy - ion collisions produce dense matter which is expected to form at sufficiently high energy densities a deconfined phase of quarks and gluons , the quark gluon plasma .
a necessary condition for such a phase transition is local equilibrium which might be achievable through rescattering of produced particles .
hints for thermalization can most easily identified by studying the observables as function of centrality .
the high @xmath1 pion production is expected to be dominated by hard parton scattering but has recently been shown to be also explainable by a thermal model with hydrodynamic expansion .
the comparison of the wa98 @xmath2 data with hydrodynamic models provides constraints on the partition of excitation energy in terms of temperature and an average flow velocity .
such a finite thermalized system without any external pressure will necessarily expand and the thermal pressure will generate collective motion which will be reflected in the momentum spectra of the final hadrons .
thus a part of the thermal excitation energy will be converted into collective motion of the hadrons .
there seem to be qualitative changes in the behaviour of heavy - ion reactions once a certain system size is attained .
strangeness production is enhanced in s+s reactions compared to p+p , but seems to saturate for even larger nuclei@xcite .
recent results from the wa98 experiment@xcite show significant change of the shape of the @xmath2 @xmath1 spectrum in peripheral pb+pb collisions compared to p+p data . however , from semi - central pb+pb reactions with about 50 participating nucleons up to the most central reactions the shape remains unchanged . in the present analysis
the centrality has been selected with the transverse energy @xmath3 measured in the calorimeter mirac .
[ scaling ] shows the scaling behaviour of the charged particle multiplicity @xmath4 with the number of participants @xmath5 .
it can be seen that @xmath4 follows a power law with the number of participants .
the extracted exponent from the data is @xmath6 . on the bottom part of fig .
[ scaling ] the same analysis performed with venus 4.12 @xcite is displayed . while the simulation result also obeys roughly a power law scaling , the agreement is not as good , and the scaling exponent appears to be significantly larger than that obtained from the experimental data .
the neutral meson spectra are mainly influenced by thermal and chemical freeze - out in the final state . in the analysis of central reactions of pb+pb at 158 @xmath0gev
it is seen that both predictions of perturbative qcd @xcite and hydrodynamical parameterizations @xcite can describe the measured neutral pion spectra very well .
it is particularly astonishing to observe that on the one hand the pqcd calculation gives a good description also at relatively low momenta while on the other hand the hydrodynamical parameterization would yield a sizable contribution even at very high momenta .
[ hydro]a shows a comparison of the neutral pion spectra to a fit of a hydrodynamical parameterization including transverse flow and resonance decays @xcite . using the default gaussian profile the best fit
is obtained with @xmath7 and @xmath8 .
[ hydro]b shows the best fit parameters as a filled circle  the corresponding @xmath9 contour is also shown .
the figure also contains the @xmath10 allowed region from the @xmath11 dependence of the transverse radii extracted by negative pion interferometry with the wa98 negative tracking arm @xcite .
the interferometry constraints are very similar to those given in @xcite  they favour relatively large transverse flow velocities
. such large velocities are only compatible with the neutral pion spectra , if one assumes a very different spatial profile
. however , this would result in rather low temperatures , thus these parameters are very sensitive to the used profile@xcite .
if the initial state of the evolution is azimuthally asymmetric , as in semi - central heavy - ion collisions , this property will be reflected in the azimuthal asymmetry of the final state particle distributions .
the strength of the collective flow will yield information on the nuclear equation of state during the expansion .
collective flow development follows the time evolution of pressure gradients in the hot , dense matter .
thus , collective flow can serve as a probe to provide information on the initial state and to which extent the reaction zone might me thermalized . in particular , the formation of a quark gluon plasma during the early stages of the collision is expected to result in reduced pressure gradients due to a softer nuclear equation of state which results in a reduced collective motion@xcite .      fig .
[ flowcent ] shows the centrality dependence of the directed flow in terms of the average transverse momentum @xmath12 . for protons the maximum directed flow
is observed in reactions with intermediate centrality .
the corresponding impact parameter of @xmath13@xcite results twice as large as observed for ags energies@xcite .
since the observed @xmath14 of pions is positive it indicates that the pions are preferentially emitted away from the target spectators , this is called anti - flow@xcite .
the rapidity dependence of directed flow is given in fig .
[ flowy ] .
in addition , pion data measured with the tracking arm in the wa98 experiment@xcite at midrapidity and data near midrapidity measured by the na49 collaboration@xcite are shown .
the maximum flow is observed in the fragmentation regions , while it rapidly decreases near midrapidity .
the data follow a gaussian distribution .    hence for a complete description of the rapidity distribution of the collective flow the formerly used slope at midrapidity ( e.g. @xmath15 ) is not sufficient .
it is more reasonable to use the three parameters of the gaussian distribution to describe the data .
the peak position reflects the beam momentum , the peak height gives the strength of the flow and the width of the distribution provides information on how much the participants and the spectators are involved in the collectivity .
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the wa98 experiment at the cern sps measures pb+pb collisions at 158 @xmath0gev . 
 the scaling properties of the charged particle multiplicity at midrapidity with the number of participants are studied using the spmd and the mirac detector . 
 neutral pion spectra obtained from the leda detector are compared to hydrodynamical parametrizations . 
 the collective flow in the target fragmentation region has been studied using the plastic ball detector . the results exhibit a strong dependence on centrality and rapidity .