Wide-acceptance measurement of the $K^{-}/K^{+}$ ratio from Ni+Ni collisions at $1.91A$ GeV

The FOPI Collaboration at the GSI SIS-18 synchrotron measured charged kaons from central and semicentral collisions of Ni+Ni at a beam energy of $1.91A$ GeV. We present the distribution of the $K^{-}/K^{+}$ ratio on the energy vs polar angle plane in the nucleon-nucleon center-of-mass frame, with and without subtraction of the contribution of $\varphi \left(1020\right)$ meson decays to the $K^{-}$ yield. The acceptance of the current experiment is substantially wider compared to the previous measurement of the same colliding system. The ratio of $K^{-}$ to $K^{+}$ energy spectra is expected to be sensitive to the in-medium modifications of basic kaon properties like mass. Recent results obtained by the HADES Collaboration at $1.23A$ and $1.76A$ GeV indicate that after inclusion of the $\varphi$ meson decay contribution to the $K^{-}$ production no difference between the slopes of the $K^{-}$ and $K^{+}$ energy spectra is observed within uncertainties. For our data a linear fit to this ratio obtained after subtraction of the $\varphi$ meson contribution still shows a decrease with kinetic energy, although a constant value cannot be rejected. The contribution of $\mathrm{\Lambda }\left(1520\right)\to p{K}^{-}$ decays estimated from fitting the thermal model to the experimental yields appears to be another factor of moderate relevance.

Figure 1

Raw kinetic energy spectra of charged kaons emitted from central and semicentral collisions of Ni+Ni at a beam energy of $1.91A$ GeV within six bins of $cos{\vartheta }_{NN}$. Red triangles correspond to $K^{+}$ and blue full circles to $K^{-}$. The yield of $K^{-}$ was multiplied by 60.

Figure 2

geant3-based part of the efficiency map on the $E_{NN}^{\text{kin}}\text{−}cos{\vartheta }_{NN}$ plane for detection of charged kaons from Ni+Ni collisions at $1.91A$ GeV within FOPI. Dotted curves mark the geometric boundaries of the MMRPC and Plastic Barrel detectors. See text for details.

Figure 3

Kinetic energy spectra of charged kaons emitted from central and semicentral collisions of Ni+Ni at a beam energy of $1.91A$ GeV for subsequent bins of $cos{\vartheta }_{NN}$. Red triangles correspond to $K^{+}$ and blue full circles to $K^{-}$. The yield of $K^{-}$ was multiplied by 60. Grey boxes denote systematic errors. The best fits of Eq. (1) to the data are shown as dashed lines.

Figure 4

Black circles: kinetic energy distribution of the $K^{-}/K^{+}$ ratio for kaons emitted from central and semicentral collisions of Ni+Ni at $1.91A$ GeV, within six bins of $cos{\vartheta }_{NN}$. Grey boxes denote systematic errors. For the data in left-hand panels the contribution of the $\varphi$ decays to $K^{-}$ production is included. The right-hand panels correspond to the same distribution after subtraction of this contribution from the $K^{-}$ spectra. Open squares: data for central Ni+Ni collisions from the previous experiment [19]. Open triangles: data for central Al+Al collisions [20]. See text for details.