Nuclear Physics A544 (1992) 183c-196cNorth-Holland, Amsterdam

1 Introduction

ecent Results from theExperiment at C

A80 Collaûoration

Abstract

a. now at : CERN, CH-1211 Geneva 23, Switzerlandb. now at : Mercedes-Benz, D-7000 Stuttgart, Fed. Rep. of Germanyc. now at: KVI, University of Groningen, NL-9747 AA Groningen, Netherlandsd. University of Tennessee, ICnoxville, Tennessee 37996, USA

0375-9474/92/$05 .00 © 1992 - Elsevier Science Publishers B.V. All rights reserved .

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1. Gesellschaft ff Schwerionenforschung, D-6100 Darmstadt, Fed. Rep. of Germany2. Lawrence Berkeley Laboratory, Berkeley, California 94720. USA3. University of Lund, S-22362 Lund, Sweden4. University of Minster, D-4400 Münster, Fed. Rep. of Germany5. Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA6. Brookhaven National Laboratory, Upton, New York 11973, USA7. Kurchatov Institute of Atomic Energy, Moscow 123182, USSR

Results from the WA80 experiment on systematics of global observables, dETldqand dNchl dry , as well as on spectra of identified particles are presented. Specialattention is given to the target fragmentation region where two-piort coirelationsand proton distributions have been measured . Incorporation of strong rescatter-ing phenomena into string models is found to provide a good description of mostexperimental data . Deviations to this are pointed out. The present status of the'Ythermai/pr

o ratio as a signal of the quark gluon plasma is discussed.

K.H . Kampert4 , R. Albrechtl, T.C . Awes5, P. Beckmann4,a, F. Berger4 , M . Bloomer2,D. Bock4, R. Bock', G. Claesson3, G . Clewing4 , R. Debbes, L. Dragon4,b, A. Eklund3,R.L . Ferguson5, S. Fokin7, A . Franz5,a, S. Garpman3, R. Glasow4, H.A . Gustafsson3,H H. Gutbrod', 0 . Hansen', M. Hartig4 , G. H61ker 4 , J. Idh3, M. Ippolitov7, P. Jacc'.as2,

K . Karadjev7, B.W. Kolb', A. Lebedev7, H. Löhner4, ~, I. Lund',c, V. Manko7, B. Moskowitzs,F.E . Obenshain5 , A. Oskarsson3, I . Otterlund3 , T. Peitzmann4 , F. Plasil5, A.M . Poskanzer2,M. Purschke4, H.G . Fitter 2 , B. Roters4, S . Saini5 , R. Santo4, H.R . Schmidt', R. Schmidt4 ,S .P. Sorensen 5,d, K. Stefens4 , P. Steinhaeuser 4 , E. Stenlund3, D. Stiiken4 , A. Vinogradov7,

H. Wegeners, and G.R. Young5

The aim of high-energy heavy-ion collisions studied at the SPS and AGS acceleratorsis to investigate nuclear matter under conditions of extremely high densities and tem-peratures . Theoretical calculations predict that under such conditions hadronic mattermight undergo a phase-transition to a new form of matter, the quark-gluon plasma

ö84c

P), in which quarks and gluons are deconfined over an extended volume [1] . At)road spectrum of possible plasma signatures has been suggested and was discussedon this conference . Unfortunately, a common characteristic of most of these signaturesis the necessity to distinguish them from the large background created by ordinaryhadronic processes . A thorough understanding of the reaction mechanism is, therefore,an important prerequisite in any search for the QGP. Besides the time evolution of thereaction system, which is a central focus in this field of research, one has to learn aboutthe interaction of produced particles with their high density nuclear environment as wellas among produced particles themselves .

To address such questions, ure will present results from a comprehensive analysis of p-,-, and 32S-induced reactions at bombarding energies of 4.9, 60, and 200 AGeV across

a large variety of target nuclei . Quantities like the transverse energy pseudorapiditydensity, dETIdil , and the charged particle density, dNhldq , have been proven to bevaluable tools for obtaining an understandingof the reaction mechanism . We will discussthese observi-ibles in section 3 . The investigation of the target fragmentation regionprovides a unique tool to extract detailed information about rescattering phenomena.In hadron-nucleus interactions the target nucleons can serve as "detectors" for reactionproducts at distances and time scales not accessible with macroscopic detectors . Thus,,we study rescattering phenomena of produced particles with "cold" matter in section 4.1ky, employing the Plastic-Ball detector which is capable of identifying pions and protons.Two pion correlations, suggested to carry further information about the space-timeevolution of the collision, will be discussed in section 4 .2 . Having provided experimentalevidence for a strong excitation of the target matter, we will show in section 5 thatrescatteringe-ffects as incorporated in the string model VENUS 3.07, allow a quantitativedescription of high PT neutral pion spectra measured at midrapidity in central collisionsof 0, S + nucleus . The analysis of thermal photon production in 0 + Au and S + Auwill be reported in section 6 .

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Kainj) ert / Recent results froin the WA 80 experirrient at CERN

ntal Setull

The experiments were performed with the WA80 experimental setup [2] at the CERNSPS between 1986 and 1990 . In spring 1990 the setup has been consi,!Crably modified toiniprove the acceptance for r' and ?I-mesons . A sketch of the new experimental layoutis depicted in Fig . 1 .

The data presented here were obtained with the detectors and configurations asdescribed briefly below :The Zero-Degree Calorimeter (ZDC) [3] covers the pseudorapidity of q > 6.0 and is usedfor impact parameter selection . The Mid-Rapidity Calorimeter (MIRAC) [4] measuresthe transverse energy with full azimuthal coverage in 2 .4 :5 ?1 :5 5.5 . For 1 .6 :5 37 :52.4 the azimuthal coverage varied from 10% to 100%. The lead-glass calorimeter wall(Saphir + Tower I + Tower 11) [5] comprises 3798 modules covering 1 .27r in azimuthand 2.1 < 71 :5 2 .9 in pseudorapidity. The distance to the target is 9.0 m. (1986-1989 :1278 modules at 3.4 m and 1 .5 :5 q < Ely The front face of the lead-glass wall is fullycovered by two layers of Iarocchi streamer tube detectors with pad readout [6] . Due to

(top view)

zero degreecalorimeter

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K.H . Kampert / Recent results from the WA 80 experiment at CERN

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Figure 1: Sketch of the WAN experimental setup as of 1990

the high granularity (21k -F- 21k pads) and efficiency (E > 93%) the detector can be usedboth as a charged particle veto detector for the photon calorimeter as well as a chargedparticle tracking detector in its own. The Plastic-Ball detector [7] is an azimuthallysymmetric array of 655QE-E telescopes allowing the identification of tight baryons andpositive pious in the pseudorapidity region -1 .7 < rl < 1 .0 .

The minimum bias trigger condition was defined by requiring lesE than 88% of thefull beam energy recorded by the ZDC and at least one charged pal title recorded bythe multiplicity arrays in the interval 1 .3 < 77 < 4.4 .

In this section we will discuss the dependence of the dET/dn distributions on impactparameter, projec".ilP- and target mass, and beam energy. As an example we show inFig . 2 trie daTf , i77 distributions in 200 AGeV "S-induced minimum bias ceilisions onvarious target nuclei . The effects of increasing target mass are very similar to decreasingthe impact parameter for a given system and are manifested in an increasing height,decreasing peak position, and narrower width of the distributions. Superimposed tothe data are results from Gaussian fits . The results of the fits are in general very good

(X2/ndf ::-,, 0.3-0.5), so that the three free parameters, dET/drl~m~,x and a,t exhaustthe full information content in the measured distributions .

Substantial interest has been devoted to the mass- and energy-scaling of the dET/dal

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60

40

20

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ainpert / Recent resultsfrein the WA 80 experitnent at CERN

32

minimum bias

Figure 2:

Target dependence ofdET/d,q in 200 AGeV s2S-inducedminimum bias collisions .

-distributi(:ns . Universal scaling laws have been difficult to obtain as long as the scalingvariables have been chosen as the projectile- and target-masses, Ap and A t . WA80 hastherefore stressed the importance of incorporating the number of participant nucleons ina consistent description of high energy nuclear collisions . To present the data togetherwith a minimum amount of model assumptions, we have chosen to discuss the data interms of a simple. participant-spectator model based on Calaiiber-tbeory. This avoids inparticular any specific comparison to high energy nuclear collision Monte-Carlo models .The basic procedure in this method is to determine the impact parameter from thefore rard energy measured in the ZDC, and calculating the average number of projectile-and target-participants, TV-p and t , respectively, in a geometrical picture for the threedifferent event classes chosen in the following :

where Rp and Rt are the projectile and target radii, R = 1.2 - A''~3 , and Rmb =

vmb/717

with en,,b being the minimum bias cross section .Figure 3 shows the behaviour of the dET/dgj,,,,,,,, and a,, values as a function of the

total number of participants, W = Wp -t- t, for two different projectiles, five differenttargets, each selected for the three event lasses discussed above, and for two bombardingenergies (33 combinations in total) . dE1'/drl Jma, appears to be approximately linearlyproportional to the average number of participants with the coefficient of proportionality

central = b < Rt -- RPintermediate - Rt - p < b < Rt

peripheral = Rt < b < Rmb

K.H. Kampert / Recent results from the WA 80 experiment at CERN

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WA80

1NA80

0 50 t0o

0 50 100

Figure 3: a) Maximum value of dET/dq distributions as a function of thetotal average number of participants W. The straight lines show linear fitsthrough the origin to the data points at 60 and 200 AGeV, respectively. b)Standard deviation e,, of the dET/dr/ distributions as a function of W. Thesolid and dashed lines represent fits according to or,, --.- 0.2 (yP-yt)(1+W"36) .

equal to 0.64 and 0 .90 GeV/nucleon at 60 and 200 A GeV, -espectively. The increase ofdET/drl in Fig. 2 is therefore a simple consequence of the increase of W. The widthof the dET,wj .7 distributions (Fig. 3b) becomes narrower for more central collisions andis most likely caused by an increase in stopping. The variation of the centroid j is morecomplicated and indicates a collective interaction of the projectile participants with alltarget participants, as may be due to the occurrence of rescattering effects, particularlyfor heavy targets [8) .

An important result is that a symmetric description between projectile and targetcan easily be obtained with the participant variables W. and Wt. Based on the obser-vation that the dET/drl distributions for a particular 77-value scales as a power law inboth Wp and Wt a universal parametrization of dET/drl is derived :

G(rl) --

N

exp

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770) 227ren 2 CTn

dET(b) - G(Î)WP("P_")Wdrl

1)

_ia(x) = (1 +exp

fX _ rlo

G(q) can be interpreted as a Gaussian describing the dET/drl distributions for nucleor-nucleon collisions . TV"' describes the projectile and target mass dependence, where theexponent ce has been chosen as a Fermi distribution so that dET/drl scales with the

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lr'i a~re 4 : ~~Tu?tiplicity distributions of charged particles from 32S ~- Al, Gu,Ag, and Au int~,ractions at 200 AGeV incid~: .~t energy. The pseudorapidityinterval covered is -1 .î < i < 4 .C~ .

n~imber of target participa,rats in the target region and is independent of the target inthe projectile region . The projectile dependence is the inverse of this .

This paran2~trizatioii directly relates the dE~ldr~ distributions to the physically rel-evant nuynber of participants and achieves a fairly high accuracy with only five freeparameters. The fit parameters are found to be

1V= .41(yp -yc-1.51) GeV ~l®=0.51(yp -I- ye)~n = 0.24 (yp - yt )

J~ = 0.12 (yp - ya )`with yr and ya being the projectile and target rapidities, respectively. Within the ac-cept~a.agce of 1~~III,AC we fnd an accuracy of bettor than 10(Îo for '35% of all fitted datapoints .

~Zultiplicity measurements at relativistic energies have been performed by several exper-iments usin; a variety of different detectors, like emulsions, stre.~n~~~r chambers, siliconpad detectors and streamer tube arrays. A unique feature of thc~ WA80 experimentalsetup is the close to 4~r acceptance for charged particle multiplicity measureme~lts com-bined +dvith the very high statistics allowing small variations to be ~l~et~ected in an analysisof different reaction systeans . Iüg

statistics is also required for a detailed analysis andunderstanding of small inver

ittency phenomena, as reported by Bloomer et al . on thisc®reference [9 .

Several corrections were applied to the multiplicity data. Some background comes

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K.H . Kampert / Recent results from the IAA 80 experiment at CERN

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32S + Au, A(j, Cu (it 2100 'A' GeV

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Figure 5: Target mass depeirueace of the yield of particles as a function of pseudorapidity.

from both real particles and from noise in the electronics and detectors . Real particlesmay be produced in secondary interactions with the setup but can also come from theunavoidrble beam halo, albedo neutrons from the calorimeters, and from cosmic rays.Further corrections include detection efficiencies and mu:tiple hit corrections and arediscussed in Ref. [10] .

Multiplicity distributions for charged particles from 32S + Al, Cu, Ag, and Au inter-actions at 200 AGeV are shown in figure 4. As the target gets bigger and the thickness ofthe nuclear matter to traverse increases, the total multiplicity also increases . The shapeof the multiplicity distributions is again dominated by the reaction geometry and re-sembles very much the shape of the transverse energy distributions . The pseudorapiditydistributions, dN~h/dr, , have been analyzed in a similar way as the dET/dr0 distribu-tions by fitting a %aussian distribution to the experimental data in 0.0 < 77 < 4.0 . Thetarget fragmentation region in -1 .7 < rl < 0.0 has been excluded from these fits becausethe particle density was too high to get a good description of all data by a singie Gaussian(see also next section) . As a result .ve find increasing charged particle densities withincreasing target mass and centrality . The largest value of dN~h /d;; I�zax ^_- 175 per unitrapidity interval is reached in the most central collisions of S + Au. The peak position,rl, is found to shift backwards in rapidity with increasing target mass and centrality withthe only exception of S + Al where the peak position mover- forward with increasingcentrality as is due to the inverse reaction kinematics . The width, o, is found to narrowwith increasing centrality and decreasing target mass.

The target mass dependence on the yield of particles as a function of pseudorapidityis shown in Fig . 5 for central collisions . The yield is assumed to follow a power-lwu

190e K Kampert / Recent results from the WA 80 experiment at CERN

behaviour on the target mass number At , i .e . dNchldq oc Ai: , where a is determinedas a function of pseudorapidity. The value of a ~-_ 1 in the target fragmentation re-gion indicates a strong excitation of the target matter, i.e. the whole target nucleusparticipates in the reaction, as may be due to rescattering phenomena (see below) . Atmid-rapidity (q ~ 2.8) a value of a ~-_ 1/3 is found which supports the validity of thesimple geometrical picture in this region.

The results of the charged particle multiplicity analysis (see e.g. Fig . 5) have alreadydemonstrated a strong participation and excitation of the target nucleus in heavy-ioncollisions at SPS energies . Having the advantage of a powerful detector for the targetfragmentation region, the Pla3tic-Ball, we will investigate these effects now in moredetail by studying the rapidity and transverse momentum distributions of identifiedbaryons, as well as by correlations among positive pions .

4.1 Pseudorapidity Distributions of Protons in p-Nucleus Reactions

High energy pA collisions should provide a first test of different models in the targetfragmentation region; models without rescattering or without cumulative effects shouldshow almost no protons at backward angles, because they neglect the influence by thesurrounding `spectator'- matter . We have thus compared pseudorapidity distributionsof proton-like fragments (Np-like = Np + Nd + Nt + 2 - Nee) from p-nucleus collisionsfor - 1.7 :5 q :5 1 .0 with the Monte-Carlo event generator VENUS 3 .07 [11] . This modelincorporates rescattehng among strings, secondaries, and spectators in a phenomeno-logical way by introducing a reinteraction radius for meson- and baryon-like strings .Rescattering occurs if any two of these objects come closer than the reinteraction dis-tance . M have tuned the model to reproduce the measured proton yields in 200 AGeVp + Au collisions by increasing the baryon reinteraction distance from the default valueof 1 .4 fm to 1 .7 fm and leaving the meson reinteraction distance unchanged at 0.7 fin .With these parameters the distributions for all targets are rather well reproduced, ascan be seen from Fig . 6 .

While this model can account for the global features of the proton yields with onlyminor modifications, the exact shape of the kinetic energy spectra is not reproduced .VENUS underestimates the proton yield at very backward angles and overestimates theirmean transverse momentum . Recent RQIID calculations [12], on the other hand, arefound to underestimate the slope of the spectra somewhat, so that the data appear tofall between the predictions by the two models .

More details on the target mass dependence of proton and pion yields as well asobservations of azimuthal correlations among proton and pious, suggesting pion absorp-tion phenomena, are reported in the contribution by Schmidt, et a]. to this conference[13] .

3.2

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K.H . Kan:pert / Recent results from the WA 80 experiment at CERN

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Figure 6 : Pseudorapidity distributions of protons measured with the Plas-tic-Ball (left) and from VENUS 3.07 simulations (right) with modified rescat-tering parameters .

Further understanding on tile space-time development of nucleus-nucleus collisions maybe deduced from pion interferometry. We have measured 7r+7r+ correlations in therapidity region --1 .0 < ylab < 1 .0 for p, 160, and 3zS induced reactions on variousnuclei at a bombarding energy of 200 AGeV. The pion source radii have been extractedwitl~ a Lorentz-invariant parametrization and are complemented by measurements ofthe transverse- and longitudinal radii .

A surprising result of the analysis is that the source radii are found to decreasewith increasing target, mass for the oxygen induced data [14] . The transverse radiusfor the C-target (RT ^__ 3.7 fm) is larger than the size of the involved nuclei, indicatingtransverse expansion prior to freeze-out. The values decrease for the heavier targetsdown to RT ^_r 2.4 fm for O + Au, which is comparable to the size of the projectile .The source sizes for the Au target have been studied also for p- and 32S projectilesat the same bombarding energy per nucleon. For sulfur induced reactions we in factfind a rise to RT ^__ 3.2 fm, which is comparable to the growth of the projectile size .The radius for proton induced reactions is found to be RT ti 2.0 fm. This value isintermediate between those of the two existing data sets on hadron nucleus interactionsat comparable energies [15,16] .

While the projectile dependence leads to a variation of the source radii as one mightexpect intuitively, there remains the question of how to understand the variation of theparameters in case of different targets as studied for oxygen induced reactions . From theanalysis discussed in the previous section we found indications for strong rescatteringeffects in the target region . It is then easy to realize that pons observed in the detector

192c

K.H. Kamper., / Recent resultsfrom the WA 80 experiment at CERN

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1 3

096

0+ Au 2 GeV (preliminary)

0. < 10 Kqevic

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40 80 120 160 200 24 , 280 320

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might undergo very different evolutions; they may be directly produced by participantnucleons traversing the target without further interaction, or they may be rescattered inthe target spectator matter after their production. Such a rescattering would alter thevolume measured by interferometry to the freeze-out volume of the excited spectatormatter . The rescattered pious might come from very large volumes or later times,and their correlation could thus be hidden at very small values of Q, hardly accessibleto experiment. It can in fact be demonstrated by simulations [14] that a decrease inthe apparent radius may be caused by an interplay of two different sources, where thesmaller source is fixed at the projectile radius and the size of the second source growsto large values .

In a more detailed analysis of the reaction 0 + Au such a large radius can indeed beobserved . The correlation function of QT for QL < 10 MeV/c and Q0

JE, - E21 < 10XIeV is shown in Fig. 7 together with a Gaussian fit which yields RT

(5.2 ± 0.1) fmand A = 0.74 ± 0.13 . Similar results can be obtained from a two component fit to theunbiased correlation function [14] .

5 Photon and Neutral Pion SiDectra

Figure 7: Two pion correlation func-tion for QL and Q0 < 10 MeV/c in110 + Au reactions . The solid linerepresent a fit to a Gaussian sourcewith RT c-- 5.2 fin .

Transverse momentum distributions of produced particles have revealed characteristicdifferences to a straightforward extrapolation from nucleon-nucleon to nucleus-nucleus

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K.H . Kampert / Recent results from the 1VA 80 experiment at CERN

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193c

collisions . These differences were observed both in the slope of the PT-spectra, as wellas in their centrality dependence. To examine possible explanations of those deviationswe have performed a systematic comparison of inclusive photon- and pro PT-spectra topredictions of the VENUS 3.07 event generator .

Figure 8 shows the experimental 7r° PT-distributions for peripheral and central200 AGeV O + Au events . The neutral pions were reconstructed from the decay photonsmeasured with the highly granulated lead-glass detector Saphir [51 in the pseudorapidity range 1.5 <_ 71 < 2.1 . A particular feature, pointing to the occurrence of collectiveeffects, is the significant increase of the inverse slope of the spectra when goAng fromperipheral to central events . Superimposed on the measured data are results from theVENUS model which were subjected to the experimental trigger conditions an3 accep-

tance of the detector . All data are in units of absolute cross-sections . The irfl-ience of

rescattering effects on these spectra is studied by switching the rescattering mechanismin the model on and off, respectively. The spectra with the reinteraction turned off werefound to resemble very much the predictions of the FRITIOF 1 .6 model.

In peripheral collisions there is only very little difference between the two model

predictions and both provide a rather good description of the experimental data (see

Fig. 8) . However, deviations become substantial for central collisions . Here, the simulated PT-spectrum with rescattering turned off results in a too steep distribution as

compared to the experimental data. Turning the rescattering mechanism on results in

a flattening of the spectra so that they become comparable to the experimental data

with only a slight excess of particles at large transverse momentum. The same findings

as for the 7r°-spectra are obtained from the spectra of inclusive photons .

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rapidity y1 .47 1 .55 1 .64 1 .74 1 .84 1 .96 2.10

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K.H. Kampert / Recent results from the WA 80 experiment at CERN

lating the truncated mean transverse momentum, defined as

f7PTdpdPT(PT) 7,~ -foo_dN dp,r

- c .dPT

G

24 22 .,0 18 16 14

0,ab (de9)Figure 9: (PT)-v,400 as a function of the laboratory angle Blab and of the rapid-ity y for central collisions of 200 AGeV 160 -t- Au. Symbols are as in Fig . 6 .Included as a line is also the dependence as expected from an isotropicallydecaying fireball according to the relation (PT)-y,400(y) « 1/ cosh(y,�, - y) .

The observed change in the slopes can be analyzed in an alternative way by calcu-

In the case of a purely exponential distribution the formula leads to exactly the sameresult as the extraction of the inverse slope parameter . The cutoff momentum has beenchosen to be c = 400 MeV/c in order to specifically study the behaviour at large values ofPT- Plotting (PT)-v,4o0 as a function of the forward energy, EZUC, i .e . as a function ofthe centrality of the collisions, in fact exhibits a rise of the transverse momentum fromabout 185 MeV/c to 215 MeV/c when going from the most peripheral to the most centralevents . The behaviour is qualitatively described by the model with rescattering whileno variation with the impact parameter is observed with the rescattering mechanismturned off.

More information can be obtained by examining the rapidity dependence of thetruncated mean transverse momentum as shown in Fig . 9. We find a rise in (PT)-Y ,400by about 15% when approaching mid-rapidity from y = 1 .5 to y = 2.1 . Now bothpredictions from the VENUS event generator fail to describe the data ; the model predictsno rapidity dependence of the mean transverse momentum in the region covered by theexperiment. Also included in this plot is the rapidity dependence of (PT) as expectedfrom isotropically decaying fireball . The absolute value has been normalized tothe experimental data point at ®lab = 14.5° . This model predicts an increase which

Analysis o

K.H . Kampert / Recent results front the WA 80 experiment at CERN

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is somewhat steeper than the one measured, and may be interpreted as being due toinsufficient stopping (see sect . 3) in the data as compared to the pure fireball model.

Preliminary results for central S + Au reactions show an inverse slope parameter ofthe 7r° PT-spectra of T

225 MeV/c, which is about 10% larger than the value extractedfor central O + Au reactions [17] . Based on the findings from above, we interpret atleast part of this increase caused by the rapidity shift of about 0.6 units between the0- and S-experiment (see sect . 2) .

irect Photons

The production of thermal direct photons has been investigated in reactions of p and 160projectiles at 60 and 200 AGeV with C and Au nuclei . The analysis, discussed in Ref.[18], is based on the 7r° reconstruction and sets an upper limit of Ydirect/70 < 15% on thesignal for all systems in the PT range of 0.4 GeV/c < PT < 2.8 GeV/c. Considering thedifferent sources of errors we expect that the uncertainty ofthe -y/zr° ratio can be reducedto 7% or less for the 200 AGeV S + Au data. The largest improvement originates fromthe much better statistics and from the larger detector coverage improving particularlythe acceptance for r, mesons. Further gain in experimental sensitivity is expected froman optimized cluster algorithm [19], improving the photon reconstruction efficiency,as well as from advanced procedures to determine the absolute value of the photonreconstruction efficiency as a function of particle density.

The data analysis is presently underway and its results will be presented in the nearfuture .

7 Summary and Conclusions

We have shown that the gross features of ultra-relativistic heavy-ion collisions can beunderstood in a simple participant spectator picture with the number of participants be-ing the relevant physical quantity to describe the energy and particle flow of the fireballaround mid-rapidity. The effect of rescattering shows up in very different measurements .While the global cbservables are only marginally affected, the effects are most promi-nent in the target fragmentation region, but show up also in the 7r°-spectra close tomid-rapidity and at large values of PT. It is obviously necessary to include rescatteringeffects in model calculations to achieve a ealistic description of the reaction. All detailsof the experimental data, like the very backward proton rapidity distributions, or therapidity dependence of the (PT) of particles are, however, not explained by the modelsavailable at present . The results from pion interferometry in the target region can beexplained by a two component picture with a very large second component, which againpoints to effects like rescattering .

Both, the accuracy of the experimental data, as well as the quality of the modelshave improved considerably in recent times, so that getting down to such `second order'problems - being ignored initially - seems appropriate . Such an understanding of thevariety of effects in dense hadronic matter is mandatory before any final conclusionsabout the QGP phase transition can be made.

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K.H. Mert / Recent muàM Me" 80 MeMent at CERN=

Partial support by the West German BMFT, GSI, the United States DOE, the SwedishNFX and the CERN PPE Division is gratefully acknowledged.

eMences[11 See this volume and most recent quark Matter proceedings; Nucl . Phys. A525 (1991),

Nucl . Plays. A498 (1989), Z. Phys. C38 (1988) .[2] 4 Albrecht, et al ., =80 Collaboration, Report No. GSI-85-32.[3] G.R . Young, et A., Nucl . Inst . Meth . A279 (1989) 503.[4] T.C . Awes, et al ., Nucl . Inst . Meth . A279 (1989) 479.[5] 11 . Baumeister, et al ., Nucl . Inst . Meth. A292 (1990) 81 .[61 T Albrecht, et al ., Nucl . Inst . Meth . A276 (1989) 131.[7] A. Baden, et al ., Nucl . Inst . Meth. A203 (1982) 189 .[8] T Albrecht, et al ., WA80 Collaboration, Phys. Rev. C (1992), in print.[9] M.A . Bloomer, et al ., WA80 Collaboration, "Intermittency in 32S-FS and 32 S+Au Collisions

at the CERN SPS," these proceedings.[10] T Albrecht, et al ., WA80 Collaboration, in preparation.[111 0 Werner, P. Koch, Phys. Lett . IB242 (1990) 251 .[12] H . Sorge, et al ., Z. Phys . C47 (1990) 629.[13] II .R . Schmidt, et al ., WA80 Collaboration, "Pion Absorption in Excited Nuclear Matter,"

these proceedings.[14] T Albrecht, et al ., WA80 Collaboration, Z . Phys . C (1992) in print.[15] C . De Marzo et a], Phys. Rev., D29 (1984) 363.[16] F. Botterweck, et al ., Z. Phys. 051 (1991) 37 .[17] R. Albrecht, et al., =80 Collaboration, Z . Phys. 047 (1991) 367.[181 R . Albrecht, et al ., WA80 Collaboration, Z. Phys . 051 (1991) 1 .[19] F. Berger, et al ., submitted to Nucl . Instr . Meth.