Observation of Different Azimuthal Emission Patterns of $K^{+}$ and of $K^{-}$ Mesons in Heavy-Ion Collisions at $1–2A\mathrm{GeV}$

Azimuthal distributions of ${\pi }^{+}$, $K^{+}$, and $K^{-}$ mesons have been measured in $\mathrm{Au}+\mathrm{Au}$ reactions at $1.5A\mathrm{GeV}$ and $\mathrm{Ni}+\mathrm{Ni}$ reactions at $1.93A\mathrm{GeV}$. In semicentral collisions at midrapidity, ${\pi }^{+}$ and $K^{+}$ mesons are emitted preferentially perpendicular to the reaction plane in both collision systems. In contrast for $K^{-}$ mesons in $\mathrm{Ni}+\mathrm{Ni}$ reactions, an in-plane elliptic flow was observed for the first time at these incident energies.

Figure 1

Azimuthal distribution of ${\pi }^{+}$ and $K^{+}$ mesons for semicentral $\mathrm{Au}+\mathrm{Au}$ collisions at $1.5A\mathrm{GeV}$. The data are corrected for the resolution of the reaction plane and refer to impact parameters of $5.9\mathrm{fm}<b<10.2\mathrm{fm}$, rapidities of $0.3<y/y_{\mathrm{beam}}<0.7$, and momenta of $0.2\mathrm{GeV}<p_t<0.8\mathrm{GeV}/c$. The lines are fits with function (1) resulting in the values for $v_1$ and $v_2$ as given in the figure.

Figure 2

Azimuthal distribution of ${\pi }^{+}$, $K^{+}$, and $K^{-}$ mesons for semicentral $\mathrm{Ni}+\mathrm{Ni}$ collisions at $1.93A\mathrm{GeV}$. The data are corrected for the resolution of the reaction plane and refer to impact parameters of $3.8\mathrm{fm}<b<6.5\mathrm{fm}$, rapidities of $0.3<y/y_{\mathrm{beam}}<0.7$, and momenta of $0.2\mathrm{GeV}/c<p_t<0.8\mathrm{GeV}/c$. The lines are fits with function (1) resulting in the values for $v_1$ and $v_2$ as given in the figure.

Figure 3

Comparison of the data from $\mathrm{Ni}+\mathrm{Ni}$ reactions at $1.93A\mathrm{GeV}$ with IQMD model calculations [19]. The data are corrected for the resolution of the reaction plane and refer to impact parameters of $3.8\mathrm{fm}<b<6.5\mathrm{fm}$, rapidities of $0.3<y/y_{\mathrm{beam}}<0.7$, and momenta of $0.2\mathrm{GeV}/c<p_t<0.8\mathrm{GeV}/c$.