First Evidence of Directed Flow at 158 A GeV Pb + Pb Collisions in WA98 Experiment

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, Φ0, is determined for each event using the observed protons in the Plastic Ball. A clear azimuthal asymmetry of fragments is observed with respect to Φ0. 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 pt spectra of particles measured in the mid-rapidity spectrometer has been studied in conjunction with the Plastic Ball, and different pt slopes with respect to Φ0 for proton and π+ are observed.