Y. AKIBA, D. BEAVIS, P. BEERY, H.C. BRITT, B. BUDICK, C. CHASMAN, Z. CHEN, C.Y. CHI, Y.Y. CHU, V. CIANCIOLO, B.A. COLE, J.B. COSTALES, H.J. CRAWFORD, J.B. CUMMING R. DEBBE, J. ENGELAGE, S.Y. FUNG, M. GONIN, S. GUSHUE, H. HAMAGAKI, O. HANSEN, R.S. HAYANO, S. HAYASHI, S. HOMMA, H.HUANG,H. KANEKO, J. KANG, S. KAUFMAN, W. KEHOE, K. KURITA, R.J. LEDOUX, MJ. LEVINE, Y. MIAKE, D. MORRISON, R.J. MORSE, B. MOSKOWITZ, S. NAGAMIYA, M.N. NAMBOODIRI, T. NAYAK, J. OLNESS, C.G. PARSONS, L.P. REMSBERG, D. ROEHRICH, P. ROTHSCHILD, H. SAKURAI, T.C. SANGSTER, R. SETO, R. SOLTZ, S.G. STEADMAN, G.S.F. STEPHANS, T. SUNG, S. TANAKA, Y. TANAKA, M.J. TANNENBAUM, J. THOMAS, J.H. VAN DIJK, F. VIDEB^K, O. VOSSNACK, V. VUTSADAKIS, F.Q. WANG, Y. WANG, H.E. WEGNER, D.S. WOODRUFF, Y.D. WU, AND W. ZAJC

We report measurements of proton emission at target rapidities for minimum bias and central collisions of $14.6A\mathrm{GeV}/c$ ${}^{28}\mathrm{Si}$ with Al, Cu, and Au nuclei as well as minimum bias and central collisions of $11.7A\mathrm{GeV}/c$ ${}^{197}\mathrm{Au}$ with Au nuclei. Results for deuteron emission are also reported for the $\mathrm{S}\mathrm{i}+\mathrm{A}\mathrm{u}$ reaction. The spectra span the laboratory angular range of $50\ifmmode^\circ\else\textdegree\fi{}<~\ensuremath{\theta}<~130\ifmmode^\circ\else\textdegree\fi{}$ and kinetic energy range of $40\mathrm{MeV}<~{E}_{\mathrm{kin}}<~225\mathrm{MeV}.$ Inverse slopes of proton spectra and proton $dN/d\ensuremath{\eta}$ values in the kinetic energy range $50\mathrm{MeV}<~{E}_{\mathrm{kin}}<~110\mathrm{MeV}$ are reported. The inverse slopes are 40--80 MeV for the various systems, generally increasing with increasing pseudorapidity. The $dN/d\ensuremath{\eta}$ values for $A+A$ collisions within the restricted kinetic energy interval are compared to those for protons from $p+\mathrm{Au}$ in the literature. All pseudorapidity distributions have very similar shapes. The experimental results have been compared to the predictions of the nucleon-nucleon collision models ARC and RQMD. The predictions made by these two models for the distribution of protons at target rapidities are very similar to each other. However, there are significant differences between the model predictions and the experimental results in the details of the spectral slopes and the proton yields for different trigger conditions.

A search for the production of direct photons in S+Au collisions at 200 A GeV has been carried out in the CERN-WA80 experiment. For central collisions the measured 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 statistical error of stat =0.8% and a systematic error of 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. (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. ) Deceased Directly radiated thermal photons have long been considered an interesting penetrating probe with which to study the early phase of the hot and dense matter produced in ultra-relativistic nucleus-nucleus collisions. Single \direct" photons are expected at high transverse momentum, pT , from well-known hard QCD processes, but also possibly in the pT region 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 size of the nuclear volume, photons produced throughout all stages of the collision will be observable in the nal state. Thus, it is believed that the emitted photons should provide information about the initial conditions of the hot dense system and thereby provide evidence for the possible formation of a Quark Gluon Plasma (QGP). The search for direct photon production in ultra-relativistic nucleus-nucleus collisions has been a major emphasis of the WA80 experiment at CERN. First results from WA80 found no excess photon yield beyond that attributable to resonance decays in central collisions of O+Au at 200 A GeV, setting an upper limit of = 0 < 15% [2]. The preliminary results of the 1990 WA80 S+Au photon analysis showed no signi cant excess in peripheral collisions, while an excess at about the 2 level was seen in central collisions [3]. Although preliminary, these results have generated a great deal of theoretical interest [4, 5, 6, 7, 8]. In this Letter we report the nal results of the WA80 S+Au direct photon analysis, we compare the nal results to theoretical calculations, and discuss the implications towards the possible formation of a QGP. The WA80 experimental setup for the 1990 run period with 200 A GeV S beams was upgraded from that used for the previous run periods with O and S beams [2, 3, 9]. The direct photon sensitivity for this data set, relative to the O data [2], was improved by several factors [3] including an increased data sample, an increased detector coverage, a coverage closer to mid-rapidity, and improved analysis techniques. The WA80 photon spectrometer consisted of a nely segmented electromagnetic calorimeter composed of 3798 lead-glass modules with photomultiplier tube readout. The lead-glass was arranged into three independently calibrated arrays, of roughly equal size. Two of the arrays consisted of TF1 lead-glass of 4 cm x 4 cm x 40 cm (15 X0) [10] deployed as towers to the left and right of the beam axis. The third array, located below the beam axis, was the SAPHIR lead-glass detector [11] used in the WA80 O run period [2] which consisted of SF5 lead-glass modules of 3.5 cm x 3.5 cm x 46 cm (18 X0). The entire photon spectrometer 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 CPV consisted of streamer tubes with charge-sensitive pad readout, with pads of dimension similar to the lead-glass modules [12]. For the direct photon analysis the total event sample of 6:27 10 events was divided into various centrality classes based on the measured transverse energy. The total transverse 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 to 2:4 . In this Letter, results are presented for the most peripheral events corresponding to 31% mb and the most central events corresponding to 7:4% mb, with mb = 3600 mb [15]. This central event class corresponds to the complete geometrical overlap of the S nucleus with the Au target, with an average of 107 participating nucleons (to be compared to an average of 5.6 participating nucleons for the peripheral event class), in contrast to the less restrictive centrality condition of 25% mb used in the preliminary analysis [3, 14, 15]. In the WA80 experiment, the 0 and yields have been measured simultaneously

The E-802 Collaboration at the BNL-AGS has measured charged particle multiplicity distributions from central (ZCAL) collisions of $^{16}\mathrm{O}$+Cu at 14.6A GeV/c as a function of the pseudorapidity interval \ensuremath{\delta}\ensuremath{\eta}\ensuremath{\ge}0.1 in the range 1.2\ensuremath{\le}\ensuremath{\eta}\ensuremath{\le}2.2. The fluctuations of these distributions as a function of the pseudorapidity interval have been studied by the method of normalized factorial moments and also by directly fitting the measurements to negative binomial distributions (NBD). Excellent fits to NBD were obtained in all \ensuremath{\delta}\ensuremath{\eta} bins, allowing, for the first time, a systematic formulation of the subject of intermittency in terms of distributions to complement the description based on normalized factorial moments. In agreement with all previous measurements of NBD fits to multiplicity distributions in hadron and lepton reactions, the k parameter of the NBD fit for central $^{16}\mathrm{O}$+Cu collisions is found to exhibit an apparently linear increase with the \ensuremath{\delta}\ensuremath{\eta} interval, albeit with a much steeper slope than for other reactions, and a nonzero intercept, k(0)\ensuremath{\ne}0. The evolution of the NBD parameter k(\ensuremath{\delta}\ensuremath{\eta}) is used to determine the two-particle short-range rapidity correlation length for central $^{16}\mathrm{O}$+Cu collisions, \ensuremath{\xi}=0.18\ifmmode\pm\else\textpm\fi{}0.05, which is much shorter than the value \ensuremath{\xi}\ensuremath{\sim}1--3 for hadron collisions, but this is a quantitative rather than a qualitative difference. These results lead to a simple and elegant explanation of the intermittency formalism, without resort to fractals, for all reactions, which demystifies intermittency---for $^{16}\mathrm{O}$+Cu central collisions, intermittency is nothing more than the apparent statistical independence of the multiplicity in small pseudorapidity bins, \ensuremath{\delta}\ensuremath{\eta}\ensuremath{\sim}0.2, due to the surprisingly short two-particle rapidity correlation length.

Minimum Bias production cross sections of η mesons have been measured in 200 AGeV/c S+Au and S+S collisions at the CERN SPS by reconstructing the η→γγ decay. The measurements have been made over the rapidity range 2.1 ≤ y ≤ 2.9 using the leadglass spectrometer of WA80. Within the statistical and systematical uncertainties the spectral shapes of π^0 and η mesons yields are identical when their invariant differential cross section is plotted as a function of the transverse mass. The relative normalization of the η to π^0 transverse mass spectra is found to be 0.53 ± 0.07 for S+Au and 0.43 ± 0.15 for S+S reactions. Extrapolation to full phase space leads to an integrated cross section ratio of η to π^0 mesons of 0.147 ± 0.017 (stat.)± 0.015 (syst.), and 0.120 ± 0.034 (stat.)± 0.022 (syst.) for S+Au and S+S collisions, respectively.

The investigation of highly excited and compressed nuclear matter created in relativistic heavy ion collisions requires probes sensitive to the different stages of the development of the system. Among the characteristic signals listed in table 1, hadrons mainly probe the late stage of the reaction and their final spectra are influenced by their last scatterings. Electromagnetic probes, on the other hand, escape from the reaction zone without rescattering and probe the very early stage of the reaction, where temperatures and densities are highest. Electromagnetic probes are therefore particularly useful to study the possible formation of a Quark-Gluon Plasma, which is expected only at extreme values of temperature and density. The disadvantage of electromagnetic probes is that the signals are usually small and have to be disentangled from a tremendous physical background requiring a very high experimental precision and detailed treatment of the various background sources. In the case of photons this background consists mainly of photons from γ decaying hadrons like π0 and η.

Direct thermal photons in the pT range of 0–5 GeV/c are expected to provide a sensitive probe of the hot dense matter formed in the early stage of relativistic heavy ion collisions. The production of single photons in 200 A GeV S + Au reactions has been investigated using the 3800 element Pbglass calorimeter of CERN experiment WA80. Neutral π0 and η cross sections have been measured via their two-photon decay branch yields. In a first analysis of the WA80 results, a slight excess photon yield above that which may be accounted for by hadronic decays was observed for central collisions. A report on the status of the reanalysis of this data is presented.

Neutral pi0 and eta spectra have been obtained over a wide transverse momentum range for various centralities of S + Au collisions at 200 GeV/nucleon. The analysis was done of the full statistics of the WA80 experiment at CERN using the two-photon invariant mass spectra. Compared to the previous analysis [1, 2] the pi0 and eta spectra were obntained over an extended p(t) range of 0.2 < p(t)(pi0) < 4.4 GeV/c and 0.4 < p(t)(eta) < 3.0 GeV/c.

Results from the experimental program with light ion beams and heavy target nuclei at the CERN SPS could demonstrate the occurrence of an unprecedented state of high density in hadronic matter. The thermal nature of the hadronic system has been investigated by analyzing spectra and production ratios of hadrons which reveal a large degree of rescattering of primary and secondary hadrons.Thermal photons from elementary quark-gluon interactions are considered a promising signal for the occurrence of a phase transition to the quark-gluon plasma. The predictions for thermal photons from elementary parton interactions are discussed and compared to the thermal emission rate of photons from a hot hadronic gas. Recent results from the photon spectrometers in heavy ion experiments are presented. Production cross sections of pi0 and eta mesons are determined and the projectile and target mass dependence is discussed. An upper limit for the single photon yield was determined for central O+Au reactions. Recent S+Au reactions exhibit an excess of photons over the yield expected from hadronic decays. The spectral shape of the expected single photon signal is discussed which might reveal the temperature of hot matter and indicate a phase transition.

Antiproton production cross sections have been measured in minimum bias p+Be, p+Al, p+Cu, and p+Au collisions at 14.6 GeV/c with the E-802 spectrometer at the Brookhaven Alternating Gradient Synchrotron. The antiproton multiplicity at laboratory rapidities 1.0 to 1.6 shows almost no dependence upon the target mass. The p¯ production for p+Be is somewhat lower than an estimation based upon 19–24 GeV/c p+p data. In this rapidity interval the antiproton yield scales by a factor of 28±6 from p+Au to central Si+Au collisions.Received 25 November 1992DOI:https://doi.org/10.1103/PhysRevC.47.R1351©1993 American Physical Society

Measurements of collisions of 14.5 GeV/c per nucleon 28Si ions with nuclear targets using the BNL E802 spectrometer are presented. A description of the experiment and preliminary particle ratios are discussed.

The scientific foundation for RHIC is of course intimately connected with the ideas of a possible phase transition from the confined hadronic state of quark matter, to a state where quarks (and gluons) can move freely over distances many (several) times the nucleonic diameter (quark-gluon plasma). An important part of the rationale for the choice of energy of RHIC, relies on the idea of transparency developing from full ''stopping'' with increasing bombarding energy. It is also important that energy densities of may times that of the ground state of cold nuclear matter can be reached in ion-ion collisions. In this talk, we will examine whether the two latter premises seem to hold true in view of the data from A+A collisions at the brookhaven AGS-Tandem Complex and at the CERN-SPS. Finally, a few comments are made on measured slopes of hadronic p/perpendicular/ spectra. 18 refs., 2 figs., 1 tab.