A search for the production of direct photons in S+Au collisions at 200\cdotA~GeV has been carried out in the CERN-WA80 experiment. For central collisions the measured photon excess at each p_T, averaged over the range 0.5~GeV/c~ \leq p_T \leq 2.5~GeV/c, corresponded to 5.0\% of the total inclusive photon yield with a statistical error of \sigma_{\rm stat}=0.8\% and a systematic error of \sigma_{\rm syst}=5.8\%. Upper limits on the invariant yield for direct photon production at the 90\%~C.L. are presented. Possible implications for the dynamics of high-energy heavy-ion collisions are discussed

Albrecht, R

Antonenko, V G

Awes, T C

Barlag, C

Berger, F

Bloomer, M A

Blume, C

Bock, D L

Bohne, E M

Bucher, D

Böck, R K

Claesson, G

Claussen, A

Clewing, G

Debbe, R

Dragon, L

Dubovik, Yu

Eklund, A

Fokin, S L

Franz, A

Garpman, S I A

Glasow, R

Gustafsson, Hans Åke

Gutbrod, H H

Hansen, O

Hölker, G

Idh, J

Ippolitov, M S

Jacobs, P

Kampert, K H

Karadzhev, K

Kolb, B W

Lebedev, A

Lund, I

Löhner, H

Man'ko, V I

Moskowitz, B E

Nikolaev, S

Nystrand, J

Obenshain, F E

Oskarsson, A

Otterlund, I

Peitzmann, Thomas

Plasil, F

Poskanzer, A M

Purschke, M L

Ritter, H G

Roters, B

Saini, S

Santo, R

Schlagheck, H

Schmidt, H R

Stankus, P

Steffens, K

Steinhäuser, P

Stenlund, E

Stüken, D

Söderström, K

Sørensen, S P

Vinogradov, A

Wegner, H

Young, G R

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN{PPE/95{18613 December 1995LIMITS ON THE PRODUCTION OF DIRECT PHOTONS IN 200AGeV32S+Au COLLISIONSR. Albrecht1), V. Antonenko2), T. C. Awes3), C. Barlag4), F. Berger4), M. Bloomer5),C. Blume4), D. Bock4), R. Bock1), E. -M. Bohne4), D. Bucher4), G. Claesson6),A. Claussen4), G. Clewing4), R. Debbe7), L. Dragon4), Yu. Dubovik2), A. Eklund6),S. Fokin2), A. Franz8), S. Garpman6), R. Glasow4), H.-A. Gustafsson6), H. H. Gutbrod1),O. Hansen7), G. Holker4), J. Idh6), M. Ippolitov2), P. Jacobs5), K. H. Kampert4),K. Karadjev2), B. W. Kolb1), A. Lebedev2), H. Lohner9), I. Lund1), V. Manko2),B. Moskowitz7), S. Nikolaev2), J. Nystrand6), F. E. Obenshain3), A. Oskarsson6),I. Otterlund6), T. Peitzmann4), F. Plasil3), A. M. Poskanzer5), M. Purschke4),H.-G. Ritter5), B. Roters4), S. Saini3), R. Santo4), H. Schlagheck4), H. R. Schmidt1),K. Soderstrom6), S. P. Srensen3;8), P. W. Stankus3), K. Steens4), P. Steinhaeuser4),E. Stenlund6), D. Stuken4), A. Vinogradov2), H. Wegner7;)and G. R. Young3)WA80 CollaborationAbstractA search for the production of direct photons in S+Au collisions at 200A GeVhas been carried out in the CERN-WA80 experiment. For central collisions themeasured photon excess at each pT, averaged over the range 0.5 GeV/c  pT2.5 GeV/c, corresponded to 5.0% of the total inclusive photon yield with a statisticalerror of stat=0.8% and a systematic error of syst=5.8%. Upper limits on theinvariant yield for direct photon production at the 90% C.L. are presented. Possibleimplications for the dynamics of high-energy heavy-ion collisions are discussed.(Submitted to Physical Review Letters)1)Gesellschaft fur Schwerionenforschung, D-64291 Darmstadt, Germany.2)Kurchatov Institute of Atomic Energy, Moscow 123182, Russia.3)Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831.4)University of Munster, D-48149 Munster, Germany.5)Lawrence Berkeley Laboratory, Berkeley, California 94720.6)University of Lund, S-22362 Lund, Sweden.7)Brookhaven National Laboratory, Upton, New York 11973.8)University of Tennessee, Knoxville, Tennessee 37996.9)Kernfysisch Versneller Instituut, University of Groningen, NL-9747 AA Groningen, Netherlands.)DeceasedDirectly radiated thermal photons have long been considered an interesting pene-trating probe with which to study the early phase of the hot and dense matter produced inultra-relativistic nucleus-nucleus collisions. Single \direct" photons are expected at hightransverse momentum, pT, from well-known hard QCD processes, but also possibly in thepTregion below several GeV/c due to thermal radiation from the hot dense matter [1].Since the mean free path of the produced photons is considerably larger than the sizeof the nuclear volume, photons produced throughout all stages of the collision will beobservable in the nal state. Thus, it is believed that the emitted photons should pro-vide information about the initial conditions of the hot dense system and thereby provideevidence for the possible formation of a Quark Gluon Plasma (QGP).The search for direct photon production in ultra-relativistic nucleus-nucleus colli-sions has been a major emphasis of the WA80 experiment at CERN. First results fromWA80 found no excess photon yield beyond that attributable to resonance decays in cen-tral collisions of16O+Au at 200A GeV, setting an upper limit of direct=0< 15% [2].The preliminary results of the 1990 WA8032S+Au photon analysis showed no signicantexcess in peripheral collisions, while an excess at about the  2 level was seen in centralcollisions [3]. Although preliminary, these results have generated a great deal of theoreticalinterest [4, 5, 6, 7, 8]. In this Letter we report the nal results of the WA8032S+Au directphoton analysis, we compare the nal results to theoretical calculations, and discuss theimplications towards the possible formation of a QGP.The WA80 experimental setup for the 1990 run period with 200A GeV32S beamswas upgraded from that used for the previous run periods with16O and32S beams [2, 3, 9].The direct photon sensitivity for this data set, relative to the16O data [2], was improvedby several factors [3] including an increased data sample, an increased detector coverage,a coverage closer to mid-rapidity, and improved analysis techniques. The WA80 photonspectrometer consisted of a nely segmented electromagnetic calorimeter composed of3798 lead-glass modules with photomultiplier tube readout. The lead-glass was arrangedinto three independently calibrated arrays, of roughly equal size. Two of the arrays con-sisted of TF1 lead-glass of 4 cm x 4 cm x 40 cm (15 X0) [10] deployed as towers to theleft and right of the beam axis. The third array, located below the beam axis, was theSAPHIR lead-glass detector [11] used in the WA8016O run period [2] which consisted ofSF5 lead-glass modules of 3.5 cm x 3.5 cm x 46 cm (18 X0). The entire photon spectrome-ter provided coverage of from 1/10 to 1/2 of full  over the rapidity range of 2:1  y  2:9.Immediately in front of the photon spectrometer was a double-layer charged-particle veto(CPV) counter which covered the lead-glass region of acceptance. Each layer of the CPVconsisted of streamer tubes with charge-sensitive pad readout, with pads of dimensionsimilar to the lead-glass modules [12].For the direct photon analysis the total event sample of 6:27  106events wasdivided into various centrality classes based on the measured transverse energy. The totaltransverse energy was measured in the WA80 mid-rapidity calorimeter [13] which had full coverage over the pseudo-rapidity range 2:9    5:5 and partial coverage extending to2:4  . In this Letter, results are presented for the most peripheral events correspondingto 31% mband the most central events corresponding to 7:4% mb, with mb= 3600mb [15]. This central event class corresponds to the complete geometrical overlap of the Snucleus with the Au target, with an average of 107 participating nucleons (to be comparedto an average of 5.6 participating nucleons for the peripheral event class), in contrast to theless restrictive centrality condition of 25% mbused in the preliminary analysis [3, 14, 15].In the WA80 experiment, the 0and  yields have been measured simultaneously1with the inclusive photon yield, obs, via their two-photon decay branches, in the samepTand rapidity region for each event class used in the excess photon search [9]. This isessential in order to minimize systematic error due to the known centrality dependenceof the meson pTspectra. In this analysis the direct photon excess is determined on astatistical basis: once the spectrum of photons expected from background sources, bkgd,has been calculated based on the measured 0and  yields (which nominally account forabout 98% of bkgd) with estimates of the small photon contributions from other radiativedecays, one may in principle extract the photon excess as excess obs  bkgd. However,since the large majority of photons observed at a given pToriginate from the decay of0's at nearly the same pT, it is in practice more convenient to work with the ratios(=0)obsand (=0)bkgd, which are less sensitive to systematic error [2, 3]. Furthermore,it is useful to study the ratio (=0)obs=(=0)bkgdwhich indicates the fraction of photonsobserved relative to the expected decay background and should have a value of 1 if thereare no excess photons. (Note: by construction the calculated background 0yield shouldbe identical to the measured 0yield). This ratio is convenient since in this quantity bothstatistical and systematic errors are roughly constant over the pTregion of interest.Since the low pTthermal photon excess is expected to be small in comparison tothe known background sources, with likely signal/background ratios of 10% or less [1], itis imperative to minimize and accurately determine possible sources of systematic error.As indicated below, it is possible to use the WA80 data sample itself to estimate and limitmost sources of systematic error. For example, the energy dependence of the measured 0mass peak has been used to set limits on the non-linearity of the energy scale. The varioussources of systematic error are listed in Table 1 together with estimated upper limits ontheir values for the central and peripheral data sets presented in this Letter. Completedetails of the systematic error estimation will be presented in a forthcoming publication.Individual showers were identied as a cluster of adjacent lead-glass modules withenergy deposit. Overlapping showers were identied and separated using an algorithmbased on nding local maxima and apportioning the energy deposited in each module ina self-consistent manner [10]; though it is inevitable that some showers will have theirenergy modied, or be lost completely, due to the eects of overlap. These eects can,however, be understood in situ by using the data itself. A large number of showers in thecalorimeter (or pairs of showers) resulting from single particles produced at the target{ including , 0, , , K, p, n, p, or n { were simulated using GEANT v3.15 (theGEANT simulation parameters were adjusted so that the simulated EM showers matchedtest beam results). The energy deposited in each module from these simulated single-particle events was added to the energies in a real data event, and then both the originaland the superimposed event were put through the identical analysis chain to reconstructhits in the detector. Comparison of the reconstructed showers with the input GEANTparticles provided the information necessary for the eciency determination. Single, sep-arated showers can be labeled as photons or non-photons using dierent sets of criteriaof varying restrictiveness. The dierent photon identication criteria applied were to use:A) all showers, B) only showers with small lateral prole, C) only showers without anoverlapping hit in either CPV layer, D) only showers without an overlapping hit in bothCPV layers. The dierent criteria give rise to dierent photon and 0identication e-ciencies and dierent background contamination corrections. In a consistent analysis, allphoton identication methods should give the same nal result. The variation of the nalresult with photon identication method, for both single photon and 0measurements,has been used to check for systematic errors in the yield determination.2To determine the eciency and hadron-contamination-corrected inclusive photonyield, two dierent approaches were used, one using the CPV and the other without.Various cross-checks were also explored, leading to a nal estimate for the systematicuncertainty of the inclusive photon yield measurement of  2% (see Table 1).The major source of systematic error in the search for excess photons is the uncer-tainty in the 0yield extraction (see Table 1). The yield extraction is complicated by thelarge photon multiplicity in nucleus-nucleus collisions which leads to modications of the0mass peak due to shower overlap. This is simultaneously accompanied by a decreasing0peak-to-background ratio resulting from the increasing number of two-photon com-binatorial background pairs. The centrality dependent combinatorial background in thetwo-photon mass distribution has been determined by an event-mixing method in whichphoton pairs are combined from articial events which have been produced by takingphotons from dierent unrelated events of the same event class. This event-mixed massdistribution closely resembles the shape of the combinatorial background distribution inboth mand pT. Still, in order to accurately match the combinatorial background distri-butions, it was found necessary to apply a small correction to the event-mixed distributionsby multiplying by a weakly linear function in mand pT. Typically this correction wasless than 0:5% over the 0mass t region.Fig. 1 shows the ratio (=0)obs=(=0)bkgdas a function of pTfor peripheral andcentral collisions in 200A GeV32S+Au reactions. The nal result, shown by the lledcircles, has been obtained with the 0yield determined with all identied showers con-sidered to be photons (photon identication method A) above). The variation of the nalresult with the photon identication method used in the 0yield extraction is shown bythe open points for the case of central collisions. This variation, in which the 0identi-cation eciency varies by more than a factor of two, gives an indication of the level ofsystematic error which may be attributed to the 0yield extraction (see Table 1). Thetotal systematic error estimate is corroborated by the results of an independent com-plete  and 0analysis shown by the light-shaded squares. This analysis was performedwithout use of the CPV and featured independent methods and calculations of yields,eciencies, and backgrounds. A t to the nal ratio with a constant value over the range0.5 GeV/c  pT 2.5 GeV/c gives an average direct photon excess over backgroundsources of 3:7%  1:0% (statistical) 4:1 (systematic) for peripheral collisions and anexcess of 5:0%  0:8% (statistical) 5:8% (systematic) for central collisions. These dataare on average consistent within 1 with no direct photon excess in both peripheral andcentral collisions. The larger statistical errors and smaller pTcoverage for the peripheralevent class is a result of the much lower photon multiplicity. The dierence between thepreliminary WA80 result [3] and the present, nal result is attributed to the above noteddiculties in the 0yield extraction.The preliminary WA80 direct photon result generated a great deal of theoreticalattention with various hydrodynamical model calculations [4, 5, 6, 7, 8]. Most of the modelcalculations were able to t the preliminary WA80 photon excess yield under various,rather standard QGP formation scenarios [5, 6, 7, 8]. Calculations were also presentedfor scenarios in which a QGP did not form, and these over-predicted the observed excessphoton yield [5, 7] (the greater magnitude in non-QGP scenarios is due to the higherinitial temperature resulting from the fewer degrees of freedom in the hadronic matter).Both of these qualititative observations remain true in light of the nal result. Withknowledge of the measured excess photon yield, and using the statistical and systematicerrors of the measurement summed quadratically, an upper limit can be calculated at each3pTfor the excess photon yield per event. Upper limits, at the 90% condence level, on theinvariant yield of excess photons per central32S+Au collision are shown in Fig. 2. Theselimits are similar in magnitude to the excess photon yields reported in the preliminaryanalysis; accordingly, the theoretical predictions of the scenarios with QGP formationremain consistent with the nal upper limits, while the predictions of the scenarios withoutQGP formation, that is, with few hadronic degrees of freedom, remain ruled out by thenal upper limits.This can be seen in Fig. 2 where the upper limit data are compared with thecalculations of Ref. [5] for both the QGP and pure hadron gas scenarios, and with thepure hadron gas calculations of [7]. While no prediction for direct photon production canbe considered denitive at the present time, the present upper limits on direct photonproduction rule out a simple thermal hadron gas description of the32S+Au collision withinthe context of these particular model calculations. It will be important to compare theupper limits with predictions of non-thermal models such as cascade calculations.This work was supported jointly by the German BMFT and DFG, the U.S. DOE,the Swedish NFR, the Humboldt Foundation, the International Science Foundation underContract N8Y000, the INTAS under Contract INTAS-93-2773, and by the ORNL Postdoc-toral Research Associates Program administered jointly by ORISE and ORNL. The Kur-chatov team would like to thank Gesellschaft fur Schwerionenforschung (Darmstadt, Ger-many) and Oak Ridge National Laboratory (Oak Ridge, USA) for their support. ORNLis managed by Lockheed Martin Energy Systems under contract DE-AC05-84OR21400with the U.S. Department of Energy.References[1] See for example, P. V. Ruuskanen, Nucl. Phys. A 544, 169c (1994) and J. Kapusta,Nucl. Phys. A 566, 45c (1994), and references therein.[2] R. Albrecht et al., Z. Phys. C 51, 1 (1991).[3] R. Santo et al., Nucl. Phys. A 566, 61c (1994).[4] E. V. Shuryak and L. Xiong, Phys. Lett. B 333, 316 (1994).[5] D. K. Srivastava and B. Sinha, Phys. Rev. Lett. 73, 2421 (1994).[6] J. J. Neumann, D. Siebert, and G. Fai, Phys. Rev. C 51, 1460 (1995).[7] A. Dumitru, U. Katscher, J. A. Maruhn, H. Stocker, W. Greiner, and D. H. Rischke,Phys. Rev. C 51, 2166 (1995).[8] N. Arbex, U. Ornik, M. Plumer, A. Timmermann, and R. M. Weiner, Phys. Lett. B354, 307 (1995).[9] R. Albrecht et al., Phys. Lett. B 361 14 (1995).[10] F. Berger et al., Nucl. Instrum. Methods Phys. Res. Sect. A 321, 152 (1992).[11] H. Baumeister et al., Nucl. Instrum. Methods Phys. Res. Sect. A 292, 81 (1990).[12] R. Albrecht et al., Nucl. Instrum. Methods Phys. Res. Sect. A 276, 131 (1989).[13] T. C. Awes et al., Nucl. Instrum. Methods Phys. Res. Sect. A 279, 497 (1989).[14] G. Clewing, Doctoral Thesis, University of Munster. (1993).[15] G. Holker, Doctoral Thesis, University of Munster. (1993).Centrality independent.40.80.911.11.21.3± σsyst.0.80.911.11.21.30 0.5 1 1.5 2 2.5 3 3.5 4± σsyst.All Showers  (Final Result)Shower Size CutVeto OR CutVeto AND CutAnalysis II (no Veto Cut)Central(7.4% σmb )Peripheral(31% σmb )200 A GeV 32S + AupT (GeV/c)(γ/pio)Observed  /  (γ/pio)BackgroundFigure 1: The ratio (=0)obs=(=0)bkgdas a function of transverse momentum for periph-eral and central collisions of 200A GeV32S+Au. The errors on the data points (shown forthe solid points only) indicate the statistical errors only. The shaded regions indicate thetotal estimated pT-dependent systematic errors which bound the region corresponding tono photon excess.510-610-510-410-310-210-111010 20 0.5 1 1.5 2 2.5 3 3.5200 A GeV 32S + AuCentral  (7.4% σmb )WA8090% C.L. LimitSrivastava and SinhaBjorken Hydro., τi=1 fm/cTc=160 MeV, Tf=100 MeV Hadron gas, Ti=408 MeV QGP, Ti=203 MeVDumitru, et al. Hadron gas, 3-fluid Hydro.Pt (GeV/c)1/N Events  Ed3 N/d3 p   (c3/GeV2 )Figure 2: Upper limits at the 90% condence level on the invariant excess photon yieldper event for the 7:4% mbmost central collisions of 200A GeV32S+Au. The solid curveis the calculated thermal photon production expected from a hot hadron gas taken fromRef. [5]. The dashed curve is the result of a similar hadron gas calculation taken fromRef. [7]. The dotted curve is the calculated thermal photon production expected in thecase of a QGP formation also taken from Ref. [5].6Table 1: Various sources of systematic error in the WA80 200A GeV32S+Au direct photonanalysis specied as a percentage of (=0)obs=(=0)bkgd. The dependence of the errorson pTis indicated.Source of Error Peripheral Collisions (31% mb) Central Collisions (7:4% mb)pT< 1:5GeV=c pT> 1:5GeV=c pT< 1:5GeV=c pT> 1:5GeV=c reconstruction eciency 1.0 1.0 2.0 2.00yield extraction and eciency 2.0 3.0 4.0 5.0Detector acceptance0.5 0.5 0.5 0.5Energy non-linearity2.0 1.0 2.0 1.0Binning eects1.0-0.0 0.0 1.0-0.0 0.0Charged vs. neutral shower separation1.0 1.0 1.0 1.0 conversion correction1.0 1.0 1.0 1.0Neutrons 1.5 0.5 1.5 0.5Other neutrals, e.g. n;K0L1.0 0.5 1.0 0.5= ratio, mt-scaling 1.5 1.5 1.5 1.5Other radiative decays 0.5 0.5 0.5 0.5Total: (quadratic sum) 4.2 4.0 5.7 5.97

Limits on the production of direct photons in 200 A GeV$^{32}$S + Au collisions

Limits on the production of direct photons in 200 A GeV $^{32}$ S + Au collisions

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Limits on the production of direct photons in 200 A GeV32^{32}32S + Au collisions

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Limits on the production of direct photons in 200 A GeV $^{32}$ S + Au collisions