Pion-proton correlations in central Pb+Au collisions at 158 AGeV

The 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 sameand the opposite-sign particles exhibit a minimum and a peak, respectively, at low relative momentum q: = p2 − p1, caused by the mutual Coulomb interaction. A difference between the average freeze-out position or time of two particle species reveals itself as an asymmetry of the correlation function at small q [1]. The high statistics of the CERES data set and the power of the two-dimensional analysis allow to study how the observed displacement between the sources of pions and protons evolves with the transverse momentum. The CERES spectrometer at the CERN SPS was upgraded in 1998 with a cylindrical Time Projection Chamber (TPC), which allows to measure charged hadrons and leptons with the momentum resolution of ∆p/p = 2%⊕ 1% · p/GeV/c. For the analysis presented here mainly information from the TPC were used. More detailed information about experimental setup one will find in [3, 4]. In the 2000 run about 32 millions central Pb+Au collision at energy of 158 AGeV events were recorded. A subset of this statistics was used in this analysis (about 23 M). The tracks chosen for the analysis came mainly from the vertex and fulfill a set of cuts. To avoid inefficiency due to the twotrack resolution a minimum opening angle requirement was applied to pairs from the signal and the background distribution. Background distribution was generated using event mixing. Only events with similar centrality and originating from the same reaction target were mixed. Particle identification was based on the information from the TPC. The correlation functions were calculated for ππ, πp, and πp pairs in the pair center of mass frame in five bins of pair transverse momentum. In this frame, variables q‖, q⊥ are defined as q‖ = qout, q⊥ = √