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in heavy ion reactions particle production at high transverse momentum is expected to result from parton scatterings with large momentum transfer . in n+n reactions these hard scatterings
dominate particle production above @xmath2  gev/@xmath3 @xcite .
hard scattering in heavy ion collisions will occur in the early stage of the reaction , well before a quark - gluon plasma is expected to form .
thus , fast partons produced in hard scatterings interact with the extended medium that is subsequently produced in a heavy ion reaction .
the interaction of partons with hot and dense nuclear matter leads to an energy loss of the partons .
this parton energy loss is considered as a potential signature of a quark - gluon plasma formation since the energy loss in a medium of deconfined quarks and gluons is expected to be higher than in hadronic matter @xcite .
an observable consequence of parton energy loss is the suppression of hadron production at large transverse momenta relative to a baseline expectation in the absence of energy loss .
the construction of this baseline necessarily relies on model assumption . for the description of
hard scattering processes the nucleons of the two colliding nuclei can be approximated as incoherent superpositions of partons . in this picture the particle yields in a heavy ion reaction are expected to scale with the number @xmath4 of inelastic nucleon - nucleon collisions , since the cross - section for hard processes is small .
thus , the results on neutral pion production are presented in terms of the following ratio of invariant yields @xmath5 : @xmath6 this ratio is usually denoted as the nuclear modification factor . in the absence of nuclear modifications to hard scattering @xmath7 will be unity .
calculations based on perturbative qcd are a second possibility for the construction of a baseline expectation .
these calculations usually also phenomenologically model the multiple initial scatterings of the partons in a a+a reaction . by just comparing the hadron yield in a+a reaction with the scaled n+n reference , as in equation  [ eq : raa ]
, the effect of multiple parton scattering is not taken into account .
multiple parton scattering can explain the enhancement of high-@xmath1 hadron production in p+a reactions relative to a @xmath4-scaled n+n reference .
this anomalous enhancement is usually referred to as the cronin - effect . in order to fix
the strength of initial parton scattering in the modeling of a+a reactions data for p+a reactions at the same energy are very important .
the wa98 experiment at the cern sps and the phenix experiment at rhic reconstruct neutral pions on a statistical basis via their decay into two photons .
the photon detector leda of the wa98 experiment consists of 10.080 lead glass modules and covers the pseudorapidity interval @xmath8 .
after the last wa98 data taking in 1996 the wa98 lead glass detector was brought from cern to the brookhaven national laboratory and is now part of the phenix experiment .
phenix consists of two separate electromagnetic calorimeters : the lead glass detector ( pbgl ) and a lead - scintillator ( pbsc ) sampling calorimeter .
both detectors cover the pseudorapidity interval @xmath9 .
the pbgl and pbsc data were analyzed separately .
figure  [ fig : pi0cen ] shows the agreement of the final @xmath10 spectra @xcite .
the number @xmath4 of inelastic binary nucleon - nucleon collisions in wa98 and in phenix is determined within a glauber model framework @xcite .
the glauber - model is based on a purely geometric picture of a nucleus - nucleus collision : nucleons travel on straight - line trajectories and a collision between two nucleons takes place if their distance in the plane transverse to the beam axis is smaller than a certain value given by the inelastic nucleon - nucleon cross section .
the inelastic nucleon - nucleon cross section increases from @xmath11  mb at cern sps energies ( @xmath12  gev ) to @xmath13  mb at the rhic energy of @xmath14  gev .
the neutral pion yields in central pb+pb collisions at @xmath15  gev and in central au+au collisions at @xmath16  gev are shown in figure  [ fig : pi0cen ] @xcite .
the yields are normalized to @xmath17 and @xmath18 , respectively .
the measured @xmath10 spectra are compared to results for nucleon - nucleon reactions .
the n+n references is a parameterization that is based on an interpolation of existing data to the respective center - of - mass energy @xcite .
one observes that the @xmath10 spectrum in n+n reactions at the rhic energy is significantly flatter than the corresponding spectrum at the cern sps energy .
the curvature of the @xmath10 spectrum in a+a reactions , however , changes only moderately when going from sps to rhic .
figure  [ fig : cen_pp ] shows the ratio @xmath7 as defined in equation  [ eq : raa ] .
the phenix pbgl and pbsc results were averaged for this plot .
a striking difference between the results for the sps and rhic energy can be seen .
the ratio @xmath7 for @xmath15  gev is qualitatively in line with expectations from the cronin effect : @xmath7 increases with @xmath1 and the neutral pion yield increases stronger than @xmath4 above @xmath19  gev/@xmath3 . at @xmath16  gev , however , @xmath7 is basically flat .
moreover , @xmath7 is significantly below unity over the entire @xmath1 range .
thus , high-@xmath1 neutral pion production at rhic energies is suppressed compared to the @xmath4-scaled n+n reference .     as defined in equation  [ eq : raa ] for neutral pion spectra at cern sps and rhic energy . for the wa98 data
points the error due to the uncertainty of the n+n reference and due to the uncertainty of @xmath4 is indicated by brackets .
the errors bars of the phenix data points indicate the statistical errors .
the systematical errors , indicated by shaded areas , include the uncertainties of the n+n reference and of @xmath4 .
, height=264 ]    one can now replace the n+n reference by the @xmath4-normalized @xmath10 spectrum measured in peripheral a+a collisions .
this is depicted in figure  [ fig : cen_per ] .
the number of binary nucleon - nucleon collisions for the peripheral samples are @xmath20 and @xmath21 .
figure  [ fig : cen_per ] indicates that the @xmath4 normalized peripheral @xmath10 spectrum at rhic is very similar to the respective n+n spectrum .
in contrast , the comparison of the sps results in figure  [ fig : cen_pp ] and figure  [ fig : cen_per ] shows that the shape of the @xmath10 spectrum changes significantly when going from n+n to peripheral pb+pb collisions with @xmath22 .
spectra at sps and rhic energy .
each spectrum is normalized to the number of n+n collisions.,height=264 ]
a pqcd calculation from wang described in @xcite is compared to the central @xmath10 spectrum for pb+pb collisions at @xmath15  gev/@xmath3 in figure  [ fig : pqcd]a . in the range around @xmath23
gev/@xmath24 the pqcd calculation overpredicts the experimental data by around 30% . in a recent publication
also lvai et al .
compare their pqcd calculations to wa98 data @xcite . in agreement with wang s calculation
they find that their pqcd calculation predicts more neutral pions at high @xmath1 than actually measured .
thus , one can conclude that a possible parton energy loss effect ( `` jet quenching '' ) is not ruled out at sps energies .
figure  [ fig : pqcd]b shows a comparison of the phenix neutral pion spectrum in central au+au collisions with a pqcd calculation from wang @xcite .
the standard pqcd calculation without parton energy loss effects clearly fails to describe the data points . by introducing a parton energy loss parameter a reasonably description of the data
can be reached . in @xcite
it is argued that when the expansion of the fireball is appropriately taken into account the energy loss of @xmath25  gev / fm in an expanding system effectively corresponds to a much higher energy loss in a static system .
this implies that the energy loss in the initial fireball produced in a a+a collision at rhic is significantly higher than in cold nuclear matter .
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transverse momentum spectra for neutral pions have been measured by the wa98 experiment in @xmath0=17.3  gev pb+pb collisions at the cern sps and by the phenix experiment in @xmath0=130  gev au+au collisions at rhic . 
 the neutral pion yields in central collisions for both reaction systems are compared to scaled transverse momentum spectra for nucleon - nucleon reactions at the respective energy . at sps energies neutral pion production at high @xmath1 
 is enhanced compared to the n+n reference while at rhic a significant suppression of high-@xmath1 neutral pions is observed .     * 
 high-@xmath1 neutral pion production + in heavy ion collisions at sps and rhic *   + k.  reygers for the wa98 and the phenix collaboration + _ institut fr kernphysik der universitt mnster + wilhelm - klemm - str .  9 , d-48149 mnster , germany +