Is the analysis of flow at the CERN Super Proton Synchrotron reliable? art. no. 034902

Several heavy ion experiments at SPS have measured azimuthal distributions of particles with respect to the reaction plane. These distributions are de duced from two-particle azimuthal correlations under the assumption that th ey result solely from correlations with the reaction plane. In this paper, we investigate other sources of azimuthal correlations: transverse momentum conservation, which produces back-to-back correlations, resonance decays s uch as Delta --> p pi, HBT correlations, and final state interactions. Thes e correlations increase with impact parameter: most of them vary with the m ultiplicity N like 1/N. When they are taken into account, the experimental results of the NA49 Collaboration at SPS are significantly modified. These correlations might also explain an important fraction of the pion directed flow observed by WA98. Data should be reanalyzed taking into account carefu lly these nonflow correlations.