NUKLEONIKA 2006, 51(Suppl. 3):s47-s50

Two-dimensional pion-proton correlation functions were analyzed in the central Pb+Au collision data taken by CERES at the top SPS energy. The correlations between the same- and the opposite-sign particles exhibit a minimum and a peak, respectively, at low relative momentum, caused by the mutual Coulomb interaction. The clearly visible asymmetry of the peak in the direction parallel to the pair momentum indicates that protons, on average, freeze-out at larger radii or at earlier times than pions. A similar result was obtained previously by the STAR experiment and was connected to the transverse flow. The high statistics of the CERES data set and the power of the two-dimensional analysis allow one to study how the observed displacement between the sources of pions and protons evolves with the transverse momentum.