Elliptic flow of charged pions, protons and strange particles emitted in Pb + Au collisions at top SPS energy

Differential elliptic flow spectra image of image, image, p, Λ have been measured at image around midrapidity by the CERN-CERES/NA45 experiment in mid-central Pb + Au collisions (10% of image). The image range extends from about 0.1 GeV/c (0.55 GeV/c for Λ) to more than 2 GeV/c. Protons below 0.4 GeV/c are directly identified by image. At higher image, proton elliptic flow is derived as a constituent, besides image and image, of the elliptic flow of positive pion candidates. This retrieval requires additional inputs: (i) of the particle composition, and (ii) of image of positive pions. For (i), particle ratios obtained by NA49 are adapted to CERES conditions; for (ii), the measured image of negative pions is substituted, assuming image and image elliptic flow magnitudes to be sufficiently close. The image spectra are compared to ideal-hydrodynamics calculations. In synopsis of the series image, flow magnitudes are seen to fall with decreasing image progressively even below hydro calculations with early kinetic freeze-out (image) leaving not much time for hadronic evolution. The proton image data show a downward swing towards low image with excursions into negative image values. The pion-flow isospin asymmetry observed recently by STAR at RHIC, invalidating in principle our working assumption, is found in its impact on proton flow bracketed from above by the direct proton flow data, and not to alter any of our conclusions. Results are discussed in perspective of recent viscous hydrodynamics studies which focus on late hadronic stages.