Correlations between positive pions are investigated in the target fragmentation region of 200A GeV 160 + nucleus collisions. The pions are measured with the Plastic Ball detector in the WA80 experiment at the CERN SPS. The target mass dependence of the radii and the correlation strength extracted by interferometry is studied. A new approach to the fit of the correlation function is introduced. The correlation strength and both invariant and transverse radii increase with decreasing target mass. The transverse radius for t60 + C reactions appears to be much larger than the geometrical radius of the nuclei involved. For the Au target only a small fraction of the measured pions contributes to the apparent correlation. Hints for a much larger second component in 1 6 0 + A u reactions are observed. Rescattering phenomena may provide a clue to understand these phenomena.

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.

We have studied one- and two-dimensional scaled factorial moments in S-32+S and S-32+Au collisions at 200 GeV/nucleon in a high statistics electronic measurement at the CERN SPS using pad-readout streamer tubes. We observe no intermittency signal beyond that produced by folding the FRITIOF event generator with a detailed model of our detector. The systematic effects of detector response, two-track separation, and finite statistics in a factorial moment analysis are discussed in detail. Even though the observed signal contains measurable distortions due to these experimental effects, we show that we are sensitive to intermittency. As an alternative method, a two-particle correlation function analysis was applied to the same data to measure correlated particle production at small scales. We show that this method does not suffer as much as the factorial moment analysis does from distortions due to the limited two-track resolution of the detector. The correlation functions also agree with the predictions of FRITIOF filtered through our detector simulation, down to the limit of the two-track resolution. Since FRITIOF models nucleus-nucleus collisions by the superposition of nucleon-nucleon collisions, we conclude that there is no evidence in our data of the kinds of collective behavior predicted to give strong intermittency in heavy ion collisions.

Correlations between positive pions are investigated in the target fragmentation region of 200A GeV16O+nucleus collisions. The pions are measured with the Plastic Ball detector in the WA80 experiment at the CERN SPS. The target mass dependence of the radii and the correlation strength extracted by interferometry is studied. A new approach to the fit of the correlation function is introduced. The correlation strength and both invariant and transverse radii increase with decreasing target mass. The transverse radius for16O+C reactions appears to be much larger than the geometrical radius of the nuclei involved. For the Au target only a small fraction of the measured pions contributes to the apparent correlation. Hints for a much larger second component in16O+Au reactions are observed. Rescattering phenomena may provide a clue to understand these phenomena.

We have studied one and two-dimensional intermittency in S+S collisions at 200 GeV/nucleon in a high statistics electronic measurement at the CERN SPS using pad-readout streamer tubes. We observed no intermittency signal beyond that produced by folding the Fritiof event generator with a detailed model of our detector. Even though the observed signal contains significant distortions due to experimental effects, we show that we are sensitive to intermittency in the collision.

Nonstatistical or "intermittent" fluctuations of charged particle multiplicities have been investigated at the CERN SPS with the WA80 multiplicity array for S-32+S and S-32+Au collisions of varying centrality. Within the phase space domain studied there is no evidence for intermittency in these collisions beyond that accounted for by FRITIOF filtered through a full detector simulation.