Transverse energy, charged particle pseudorapidity distributions and photon transverse momentum spectra have been studied as a function of the number of participants (\(N_{part}\)) and the number of ...binary nucleon-nucleon collisions (\(N_{coll}\)) in 158\(\cdot A\) GeV Pb+Pb collisions over a wide impact parameter range. A scaling of the transverse energy pseudorapidity density at midrapidity as \(\sim N_{part}^{1.08 \pm 0.06}\) and \(\sim N_{coll}^{0.83 \pm 0.05}\) is observed. For the charged particle pseudorapidity density at midrapidity we find a scaling as \(\sim N_{part}^{1.07 \pm 0.04}\) and \(\sim N_{coll}^{0.82 \pm 0.03}\). This faster than linear scaling with \(N_{part}\) indicates a violation of the naive Wounded Nucleon Model.
Semi-inclusive triple differential multiplicity distributions of positively charged kaons have been measured over a wide range in rapidity and transverse mass for central collisions of $^{58}$Ni with ...$^{58}$Ni nuclei. The transverse mass ($m_t$) spectra have been studied as a function of rapidity at a beam energy 1.93 AGeV. The $m_t$ distributions of K^+ mesons are well described by a single Boltzmann-type function. The spectral slopes are similar to that of the protons indicating that rescattering plays a significant role in the propagation of the kaon. Multiplicity densities have been obtained as a function of rapidity by extrapolating the Boltzmann-type fits to the measured distributions over the remaining phase space. The total K^+ meson yield has been determined at beam energies of 1.06, 1.45, and 1.93 AGeV, and is presented in comparison to existing data. The low total yield indicates that the K^+ meson can not be explained within a hadro-chemical equilibrium scenario, therefore indicating that the yield does remain sensitive to effects related to its production processes such as the equation of state of nuclear matter and/or modifications to the K^+ dispersion relation.
Three-particle correlations have been measured for identified pi(-) from central 158A GeV Pb+Pb collisions by the WA98 experiment at CERN. A substantial contribution of the genuine three-body ...correlation has been found as expected for a mainly chaotic and symmetric source.
The restoration of chiral symmetry and its subsequent breaking through a phase transition has been predicted to create regions of Disoriented Chiral Condensates (DCC). This phenomenon has been ...predicted to cause anomalous fluctuations in the relative production of charged and neutral pions in high-energy hadronic and nuclear collisions. The WA98 experiment has been used to measure charged and photon multiplicities in the central region of 158 AGeV Pb+Pb collisions at the CERN SPS. In a sample of 212646 events, no clear DCC signal can be distinguished. Using a simple DCC model, we have set a 90% C.L. upper limit on the maximum DCC production allowed by the data.
Anisotropic transverse flow has been studied with a magnetic spectrometer at midrapidity and the Plastic Ball detector at target rapidity in the WA98 experiment. Our preliminary results show the ...existence of directed and elliptic flow in semi-central Pb+Pb collisions. The magnitude of the directed flow is found to be significantly smaller than those observed at the AGS energies.
We have studied the azimuthal angle distributions of identified charged particles near mid-rapidity region in semi-central Pb+Pb collisions at SPS energies. Our preliminary results show that π
+ ...mesons and protons seem to be emitted in the reaction plane, while out-of-plane for
K
+ mesons. This phenomenon might be explained by the existence of another possible effect such as in-medium potential as well as the collective motion.
Neutral pion production in central 158A GeV sup 208Pb+ sup 208 Pb collisions has been studied in the WA98 experiment at the CERN Super Proton Synchrotron. The pisup 0 transverse mass spectrum has ...been analyzed in terms of a thermal model with hydrodynamic expansion. The high accuracy and large kinematic coverage of the measurement allow one to limit previously noted ambiguities in the extracted freeze-out parameters. The results are shown to be sensitive to the shape of the velocity distribution at freeze-out. copyright ital 1999 ital The American Physical Society
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