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the relativistic heavy ion collider ( rhic ) at brookhaven national laboratory started regular operations in june 2000 , opening new frontiers in the study of hadronic matter under unprecedented conditions of temperature and energy density .
the research is focused on the phase transition associated with quark deconfinement and chiral symmetry restoration expected to take place under these conditions .
phenix is one of the four experiments at rhic .
it is designed to cover the entire time - scale of the interaction , from initial hard scattering to final state interactions by simultaneous measurement of a wide range of probes in the same detector .    here , the results on neutral pion production in the @xmath3  gev / c transverse momentum range are presented .
high @xmath4 hadrons are produced primarily by initial hard scattering of partons , and their rate of production in @xmath5 collisions can be calculated in perturbative qcd , which simply scales to nuclear collisions by the relative number of binary nucleon - nucleon collisions . however , in the case of colliding nuclei , there are several nuclear effects that can modify the spectra of hadrons even in the ordinary nuclear medium , such as shadowing and @xmath4 broadening . in addition , there are predictions  @xcite that if the medium becomes very dense , the scattered partons may lose considerable energy via gluon bremsstrahlung or `` jet quenching '' . according to those predictions , this effect would cause a significant depletion of the spectra at high @xmath4 .
the phenix detector consists of an axial - field magnet surrounded by two central arms ( called east and west ) , each one subtending @xmath6 in azimuth and @xmath7 units of pseudorapidity .
two spectrometers at forward angles ( @xmath8 ) around the beam axis serve to identify and track muons . during the year 2000
data taking run , only a subset of the detectors of the central arm were read out and analyzed .
trigger and basic event characterization was provided by two sets of beam - beam counters ( bbc ) covering @xmath9 in azimuth and @xmath10 , as well as by two zero - degree calorimeters located @xmath11 m from the collision point and covering @xmath12  @xcite .
the primary interaction trigger is generated by the coincidence between the beam - beam counters which detect @xmath13% of the nuclear interaction cross section of @xmath14 barns .
another trigger is generated by a coincidence between the two zero - degree calorimeters which are sensitive to unbound spectator neutrons from the nuclear interactions or from coulomb dissociation .
the correlation between the bbc and zdc signals are used to determine the centrality of the collision .
they are also used to establish the number of participant nucleons ( @xmath15 ) and binary collisions ( @xmath16 ) using a glauber - model calculation  @xcite .
the central arm spectrometers consist of a multiplicity vertex detector ( mvd ) , drift chambers ( dc ) , two layers of pad chambers ( pc ) , a gas - filled ring imaging cherenkov detector ( rich ) , a time expansion chamber ( tec ) , and a high granularity electromagnetic calorimeter ( emc ) consisting of eight sectors , each covering @xmath17 and @xmath18 .
six of the calorimeter sectors are lead - scintillator sampling calorimeters ( pbsc ) , while two sectors consist of lead glass calorimeters ( pbgl ) .
the data presented here are obtained from two pbsc sectors in the west arm spectrometer , which have @xmath19 energy resolution and @xmath20  mm position resolution for electromagnetic showers in a low multiplicity environment .
the energy calibration was established using minimum ionizing particles , maintained with a laser monitoring system , and verified from the data using @xmath21 matching for identified electrons in low multiplicity events , as well as with the mass of the @xmath0 .
the systematic error on the overall energy scale is less than 1.5%  @xcite .
only events taken at full magnetic field and satisfying the primary interaction trigger , as explained above , are analyzed .
additional cuts include the measured event vertex position ( @xmath22  cm ) , consistency between the zdc and bbc interaction time measurement .
altogether , 1.17 million events passed these cuts and form the sample referred to as `` minimum bias '' . based upon the correlation of the measured bbc charge and zdc energy , centrality classes were established as fractions of the total nuclear cross section . in this analysis
`` central '' refers to the 0 - 10% most central collisions , while `` peripheral '' means the upper 60 - 80% range of @xmath23 . as `` peripheral '' .
this is now replaced by the upper 60 - 80% of @xmath23 where the sample is much cleaner and the results are less biased by the inefficiencies of the trigger in the most peripheral collisions .
the centrality selection for the `` central '' spectra is the same ( 0 - 10% most central ) , but the points and the systematic errors have been revised . ]
neutral pions have been measured using their @xmath24 decay mode . in this measurement
, the invariant mass of the photon pairs has to be within a narrow ( @xmath25 ) window around the observed @xmath0 mass , thus allowing for less stringent photon identification cuts , which reduces the systematic errors on efficiency losses due to these cuts .
each cluster found in the calorimeter is subject to a timing cut : it has to arrive within 2.5 ns with respect to the expected time - of - flight of a photon coming from the event vertex ( tof cut ) .
this cut eliminates slow hadrons , in particular anti - neutrons which are a major source of neutral clusters in the 1 - 2 gev energy range .
in addition , the shape of each cluster is compared to the known and parametrized shape of electromagnetic showers , and the @xmath26 of the difference between the observed and predicted shower shape is calculated @xcite .
a @xmath27 cut is then applied to the showers .
both cuts are designed to keep the photon efficiency as high as possible .
therefore the accepted clusters have a significant contribution for other particles which is removed by the backround subtraction method described below .
furthermore , the efficiency of both cuts depends on the event multiplicity .
this was verified from the data by comparing @xmath0 peak contents at a given @xmath4 extracted with different photon identification cuts , as well as by studying the effect of the cuts on well identified electrons .
the @xmath28 invariant mass is calculated from all pairs of clusters in an event passing the photon identification cuts .
the combinatorial background is estimated using an event mixing method , which after proper normalization , is subtracted from the invariant mass distribution .
the @xmath0 reconstruction efficiency is calculated by the following procedure .
first , the effect of a second particle contributing to the same cluster is investigated ( overlaps ) .
simulated single electromagnetic showers are merged both in real and simulated events . for different single shower energies and event centralities , the distributions of the ratios of the measured and original energies are stored ( referred to as photon energy and event centrality - dependent @xmath29 `` smearing '' functions ) .
these functions are then used in a fast monte carlo simulation as follows .
neutral pions are generated with the expected @xmath4 distribution and allowed to decay .
for those cases , when both decay photons reach the calorimeter , their respective energies are randomized with the appropriate @xmath29 smearing function , and the invariant mass is calculated using the randomized energies .
the resulting simulated line - shapes are compared to line - shapes obtained from the data after mixed event subtraction .
they agree very well , and the same cuts are applied to the data and the simulations to establish the efficiency .
simulations are also used to determine the background from particles striking the pole - tips and structural elements of detectors in front of the calorimeter .
an additional source of background arises from those @xmath0 s produced close to ( but not at ) the collision vertex which reconstruct in the calorimeter with the proper invariant mass , increasing the true @xmath0 yield .
this background is also estimated using simulations ( hijing 1.35  @xcite ) .
the calculated contribution of non - vertex but properly reconstructed @xmath0 is @xmath30 at @xmath31 and gradually decreases to @xmath32 at @xmath33 .
this yield has been subtracted from the measured @xmath0 yield .
the semi - inclusive transverse momentum distribution of @xmath0 in peripheral ( upper 60 - 80% of @xmath23 ) and 10% most central au+au collisions is shown in figure  [ fig : plot_pt_ua1 ] . at high
@xmath34 the peripheral spectrum is limited by statistics .
error bars include both statistical and systematics errors .
both spectra are compared to @xmath4 spectra derived from nucleon - nucleon data . since there is no measurement of @xmath0 production in @xmath5 at @xmath2  gev , this reference spectrum
is derived from ua1  @xcite and cdf  @xcite charged hadron spectra .
first , the available data are fitted with a function @xmath35 , then the fit parameters @xmath36 are interpolated to rhic energy .
the result is divided by @xmath37 mb for the yield and by 1.6 to obtain the pion content from the unidentified charged spectra .
( the actual parameter values are @xmath38 , @xmath39 and @xmath40 . )
this parametrized curve is then multiplied by the estimated mean number of binary nucleon - nucleon collisions ( @xmath41 and @xmath42 in peripheral and central collision , respectively ) .
the systematic error on the number of collisions is indicated by the two bands .    in peripheral collisions
the scaled @xmath5 parametrization describes the results very well , but it significantly overpredicts the measured spectrum in central collisions , particularly at higher @xmath4 . the observed deficit in the @xmath0 yield is even more surprising if one takes into account that at 3 - 4 gev / c a cronin - type enhancement due to @xmath34 broadening above the scaled @xmath5 distribution would be expected .
the same deficit can be seen in figure  [ fig : ratio_centperiph2 ] without referring to parametrized @xmath5 results .
both the central and the peripheral spectra are normalized by the respective number of binary collisions , then divided point - by - point .
the central / peripheral ratios are shown as triangles , and the error bars are statistical .
the solid line gives the upper limit on the ratio if the systematic errors of the @xmath0 spectra are added in quadrature .
the dashed line adds ( in quadrature ) the systematic error on the number of collisions ( @xmath16 ) .
the central / peripheral ratio , normalized by @xmath16 , is expected to be one in the case of simple scaling with @xmath16 .
however , the measured ratio is much smaller .
the expected range of the central / peripheral ratio with a cronin - effect included is also shown .
figure  [ fig : ptdist_wang_cent10 ] shows the results for both the peripheral and central collisions compared to three theoretical calculations  @xcite ( curves ) .
the solid lines are a straightforward pqcd calculation for @xmath5 , with simple scaling to au - au collisions by the mean number of binary collisions @xcite .
the dotted lines are a calculation where effects of nuclear shadowing and @xmath4 broadening are added , and result in a change of slope , suppressing the soft part of the spectrum and enhancing the hard scattering part ( cronin effect ) .
the calculation plotted with dashed lines adds a constant @xmath43 parton energy loss to the shadowing and @xmath4 broadening .
the peripheral data are consistent with all three scenarios
. however , the central data are well below the first and second ( pqcd and shadowing / cronin ) curve , but they are not inconsistent with the third scenario that includes a parton energy loss .
transverse momentum spectra for neutral pions in central and peripheral @xmath2 gev au+au collisions have been presented .
the peripheral spectrum is consistent with the simple scaling of @xmath5 collisions with the mean number of binary nucleon - nucleon collisions . in the central spectra ,
a significant deficit with respect to this point - like scaling is observed at high transverse momenta .    9 x.n .
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transverse momentum spectra for identified @xmath0 s in the range 1  gev / c  @xmath1  4  gev / c have been measured by the phenix experiment in au - au collisions at @xmath2 gev . 
 the spectra from peripheral nuclear collisions are consistent with the simple expectation of scaling the spectra from p+p collisions by the average number of nucleon - nucleon binary collisions . 
 the spectra from central collisions and the ratio of central / peripheral spectra are significantly suppressed when compared to point - like scaling .