G Holker

Upper limit for thermal direct photon production in heavy-ion collisions at 60 and 200A·GeV

The production of direct photons has been investigated in reactions ofp and16O projectiles at 60 and 200A·GeV with C and Au nuclei. Photon and π0 spectra have been measured in the pseudorapidity range 1.5≦η≦2.1 for the transverse momentum region 0.4 GeV/c≦pT≦2.8 GeV/c employing the lead-glass spectrometer SAPHIR. An upper limit of 15% at the 90% confidence level for the direct photon signal relative to the neutral pion production is obtained from the comparison of measured photon spectra with Monte Carlo simulations of the hadronic background based on the reconstructed yield of π0 and η mesons. Consequences for a possible phase transition to a quark-gluon plasma are discussed.

Production of η mesons in 200 AGeV/cS+S and S+Au reactions

Abstract Minimum Bias production cross sections of η mesons have been measured in 200 AGeV/cS+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 π0 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 π0 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 π0 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.

Azimuthal anisotropy in S + Au reactions at 200 A GeV

Abstract Azimuthal correlations of photons produced at mid-rapidity in 200 A GeV S + Au collisions have been studied using a preshower photon multiplicity detector in the WA93 experiment. The Fourier expansion method has been employd to estimate the event plane via the anisotropy of the event as a function of centrality. The event plane correlation technique has been used to determine the true event anisotropy, beyond the anisotropy which arises due to finite multiplicity. The VENUS event generator with rescattering and proper simulation of the detector response can explain only a portion of the observed anisotropy. The residual anisotropy is found to be of the order of 5% for semi-central collisions. This suggests that directed collective flow of the produced particles is present at SPS energies.

Azimuthal correlations in the target fragmentation region of high energy nuclear collisions

Abstract Results on the target mass dependence of proton and pion pseudorapidity distributions and of their azimuthal correlations in the target rapidity range −1.73 ≤ η ≤ 1.32 are presented. The data have been taken with the Plastic-Ball detector set-up for 4.9 GeV p+Au collisions at the Berkeley BEVALAC and for 200 A · GeV/ c p-, O-, and S-induced reactions on different nuclei at the CERN-SPS. The yield of protons at backward rapidities is found to be proportional to the target mass. Although protons show a typical “back-to-back” correlation, a “side-by-side” correlation is observed for positive pions, which increases both with target mass and with impact parameter of a collision. The data can consistently be described by assuming strong rescattering phenomena including pion absorption effects in the entire excited target nucleus.

Bose-einstein correlations in the target fragmentation region in 200A GeV16O+nucleus 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.

Event by event measurement of 〈pT〉 of photons in S + Au collisions at 200 A·GeV

The mean transverse momentum of photons has been determined on an event by event basis in S + Au collisions at 200 A.GeV from the ratio of the measured electromagnetic transverse energy (E-T(em)) to the photon multiplicity (N-gamma). The average value obtained is similar to that determined for the same system using spectroscopic techniques. The centrality dependence of the measured values are in agreement with the predictions of the VENUS event generator

Intermittency in 32S+S and 32S+Au collisions at the CERN SPS

Abstract Nonstatistical or “intermittent” fluctuations of charged particle multiplicities have been investigated at the CERN SPS with the WA80 multiplicity array for 32S+S and 32S+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.

Recent results from the WA80 experiment at CERN

Abstract Results from the WA80 experiment on systematics of global observables, dE T / dη and dN ch / dη , as well as on spectra of identified particles are presented. Special attention is given to the target fragmentation region where two-pion correlations and proton distributions have been measured. Incorporation of strong rescattering phenomena into string models is found to provide a good description of most experimental data. Deviations to this are pointed out. The present status of the γ thermal / π 0 ratio as a signal of the quark gluon plasma is discussed.

Target fragmentation in proton and heavy-ion induced reactions at high energies

Results on pseudorapidity distributions, azimuthal correlations, and on two particle interferometry for identified protons and pions measured in the target fragmentation region of high energy proton-nucleus and nucleus-nucleus collisions are presented. Full participation of the whole target nucleus is observed even for 200 GeV p + Au collisions. Incorporation of strong rescattering phenomena into string models, including pion absorption effects in excited target matter, provides a good description of most experimental data.

COMPARISON OF PI(0) AND ETA-SPECTRA FROM S + AU COLLISIONS AT 200 GEV/C

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.