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 0 S , 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 dE/dx. 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 0 S -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 1 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.

On Particle Production in Lead-Gold Collision and Azimuthal Anisotropy at Top SPS Energy

“…V 2 of for − and + produced in Pb-Au collisions at √ NN = 17.3 GeV. The symbols denote the data measured by the CERES Collaboration [17]. The curves denote our results.…”

“…The CERES data [17] and the NA49 data [18] are denoted by the scattered symbols. The theoretical results are denoted by the solid line.…”

“…Figure 2 shows elliptic flows V 2 of 0 produced in Pb-Au collisions at √ NN = 17.3 GeV. The symbols are the data measured by the CERES Collaboration [17] and NA49 Collaboration [18]. Collision centralities are 5.3-13% of geo for CERES and top 13% for NA49 data.…”

“…Differential elliptic flow V 2 ( ) of negative pion candidates after the HBT correction. The CERES data[17] are denoted by the scattered symbols. The theoretical results are denoted by the solid line.…”