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

Abstract

Differential elliptic flow spectra v 2 ( p T ) of π − , K S 0 , p , Λ have been measured at s NN = 17.3 GeV around midrapidity by the CERN-CERES/NA45 experiment in mid-central Pb + Au collisions (10% of σ geo ). The p T 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 d E / d x . At higher p T , proton elliptic flow is derived as a constituent, besides π + and K + , of the elliptic flow of positive pion candidates. This retrieval requires additional inputs: (i) of the particle composition, and (ii) of v 2 ( p T ) of positive pions. For (i), particle ratios obtained by NA49 are adapted to CERES conditions; for (ii), the measured v 2 ( p T ) of negative pions is substituted, assuming π + and π − elliptic flow magnitudes to be sufficiently close. The v 2 ( p T ) spectra are compared to ideal-hydrodynamics calculations. In synopsis of the series π − – K S 0 – p – Λ , flow magnitudes are seen to fall with decreasing p T progressively even below hydro calculations with early kinetic freeze-out ( T f = 160 MeV ) leaving not much time for hadronic evolution. The proton v 2 ( p T ) data show a downward swing towards low p T with excursions into negative v 2 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.