Z. Phys. C - Particles and Fields 38, 97-103 (1988) Z~tsch~ pl*"c~ic~_~, ~ for Physik C and 9 Springer-Verlag 1988 Neutral transverse momentum spectra in 60 and 200 A-GeV + nucleus and proton + nucleus reactions* H. Lrhner Universit~it Mfinster, M/inster, Federal Republic of Germany WA 80 Collaboration R. Albrecht I, T.C. Awes 5, C. Baktash 5, P. Beckmann 4, F. Berger 4, R. Bock 1, G. Claesson 1, L. Dragon 4, R.L. Ferguson 5, A. Franz z, S. Garpman 3, R. Glasow 4, H.A. Gustafsson 3, H.H. Gutbrod 1, K.H. Kampert 4'", B.W. Kolb 1, P. Kristiansson 2, I.Y. Lee 3, H. L6hner 4, I. Lund l'b, F.E. Obenshain 5,6, A. Oskarsson 3, I. Otterlund 3, T. Peitzmann 4, S. Persson 3, F. Plasil 5, A.M. Poskanzer 2, M. Purschke 4, H.G. Ritter 2, R. Santo 4, H.R. Schmidt 1, T. Siemiarczuk ~'v, S.P. Sorensen 5,6, E. Stenlund 3, G.R. Young 5 1 Gesellschaft ffir Schwerionenforschung(GSI), D-6100 Darmstadt, Federal Republic of Germany 2 Lawrence Berkeley Laboratory, Berkeley, CA 94720, USA 3 University of Lund, S-22362 Lurid, Sweden 4 University of Mfinster, D-4400 Mfinster, Federal Republic of Germany 5 Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA 6 University of Tennessee, Knoxville, TN 37996, USA 7 On leave of absence from the Institute of Nuclear Studies, PL-00681 Warsaw, Poland Abstract. Transverse momentum (PT) distributions of inclusive photons and neutral pions at midrapidity are measured with a lead glass calorimeter in 60 and 200 A. GeV 160 + nucleus and proton + nucleus reac- tions. Inclusive photon distributions are compared for central and peripheral reactions. The degree of centra- lity is determined either from the charged particle multiplicity or from the remaining projectile energy in the forward direction. Deviations from a nu- cleus +nucleus interaction model based upon linear extrapolation from p+p reactions are observed in central 160+Au data. The variation of the average transverse momentum is investigated as function of centrality. The target-mass and energy dependence of ;rgOpT distributions are presented. For 160+Au a change of slope in these distributions is observed at * Presented at the 6th International Conference on Ultra-Relativis- tic Nucleus-Nucleus Collisions - Quark Matter 1987, 24-28 August 1987, Nordkirchen, Federal Republic of Germany a Post Doctoral Fellowship from the German Research Community (DFG) b Post Doctoral Fellowship from Swedish Natural Research Science Council pT~0.8 GeV/c compatible with hydrodynamic ex- pansion models. 1 Introduction Oxygen beams of 60 and 200 A-GeV, available now in the CERN-SPS, allow a systematic study of nucle- us-nucleus reactions at ultrarelativistic energies in order to investigate nuclear matter under extreme conditions of density and excitation energy. Accord- ing to QCD lattice calculations [1] a first or second order transition to the deconfined phase of quarks and gluons, the quark-gluon-plasma (QGP), may oc- cur in a finite volume of hadronic matter at energy densities larger than the energy density inside the nu- cleon [2]. First experimental results from 160 + nucleus reactions I-3, 4] show that, in the most cen- tral events, energy densities above 2 GeV/fm 3 are reached. Up to now, cosmic ray data have been the only source of information [5] and the observed rise of their transverse momenta with energy density has been interpreted as due to QGP effects [6]. Since the signature of the QGP is not well established, p + nu-