abstract
- The first evidence of directed flow at 158 A GeV Pb + Pb collisions is observed using the Plastic Ball detector in WA98 experiment. The azimuthal angle of the total transverse energy, Φ_0, is determined for each event using the observed protons in the Plastic Ball. A clear azimuthal asymmetry of fragments is observed with respect to Φ_0. The degree of azimuthal asymmetry is found to be largest in mid-central collisions, which is consistent with the picture of the collective flow. The p_t spectra of particles measured in the mid-rapidity spectrometer has been studied in conjunction with the Plastic Ball, and different p_t slopes with respect to p_t for proton and π^+ are observed.