Elliptic flow of Λ hyperons in Pb+Pb collisions at $158A$ GeV

The elliptic flow of Λ hyperons has been measured by the NA49 Collaboration at the CERN-SPS in semicentral Pb+Pb collisions at $158A$ GeV. The standard method of correlating particles with the event plane was used. Measurements of $v_2$ near midrapidity are reported as a function of rapidity, centrality and transverse momentum. Elliptic flow of Λ particles increases both with the impact parameter and with the transverse momentum. It is compared with $v_2$ for pions and protons as well as with model calculations. The observation of significant elliptic flow and its mass dependence suggest strong collective behavior of the matter produced in collisions of heavy nuclei already at the SPS. Scaling properties of elliptic flow of different particle species have been tested at $158A$ GeV. The limited $p_T$ range of the data does not allow for a decisive test of the coalescence model.

Figure 1

Azimuthal distributions of Λ hyperons with respect to the event plane for real events (solid symbols) and mixed-events (open symbols) in two centrality bins. The curves are Fourier expansion fits (see text Eq. (2)).

Figure 2

Elliptic flow of Λ hyperons as a function of rapidity (top) and $p_T$ (bottom). The open points in the top graph have been reflected about midrapidity.

Figure 3

Elliptic flow of Λ hyperons as a function of $p_T$ from mid-central (top) and central (bottom) events measured by the STAR (open symbols) and NA49 (solid symbols) experiments. Curves are hydrodynamical model predictions at RHIC energy for the two different centrality bins.

Figure 4

Elliptic flow for charged pions (circles) and protons (triangles) as a function of $p_T$ from mid-central events measured by the STAR [9] (open symbols) and NA49 [8] (solid symbols) experiments.

Figure 5

Elliptic flow for charged pions (circles), protons (triangles) and Λ hyperons (squares) as a function of $p_T$ from $158A$ GeV Pb+Pb mid-central events measured by the NA49 experiment. Curves are blast wave fits and hydrodynamic model predictions for $\sqrt{s_{\mathrm{NN}}}=17.2$ GeV.

Figure 6

Elliptic flow for charged pions (circles), protons (triangles) and Λ hyperons (squares) scaled by the number of constituent quarks $v_2/n$ as a function of $p_T/n$ (top) and ${\mathit{KE}}_T/n$ (bottom) from $158A$ GeV Pb+Pb mid-central events measured by the NA49 experiment.