Measurement of the In-Medium $K^0$ Inclusive Cross Section in ${\pi }^{-}$-Induced Reactions at $1.15\mathrm{GeV}/c$

The $K^0$ meson production by ${\pi }^{-}$ mesons of $1.15\mathrm{GeV}/c$ momentum on C, Al, Cu, Sn, and Pb nuclear targets was measured with the FOPI spectrometer at the Schwer-Ionen-Synchrotron accelerator of GSI. Inclusive production cross sections and the momentum distributions of $K^0$ mesons are compared to scaled elementary production cross sections and to predictions of theoretical models describing the in-medium production of kaons. The data represent a new reference for those models, which are widely used for interpretation of the strangeness production in heavy-ion collisions. The presented results demonstrate the sensitivity of the kaon production to the reaction amplitudes inside nuclei and point to the existence of a repulsive $KN$ potential of $20\pm 5\mathrm{MeV}$ at normal nuclear matter density.

Figure 1

Upper plot: The $K_S^0$ invariant mass distribution in ${\pi }^{-}+\mathrm{Pb}$ reactions. Lower plot: Geometrical acceptance of the detector in the ${\pi }^{-}+\mathrm{Pb}$ experiment in terms of rapidity versus transverse momentum of reconstructed $K_S^0$ mesons.

Figure 2

The $K^0$ inclusive production cross section (squares) as a function of the mass number of the target nucleus. The solid line represents the fit with a power law function. The hatched area corresponds to the sum of the cross sections of the elementary processes scaled according to the transverse size of the target nuclei. QMC model predictions at $\rho ={\rho }_0$ [6] (dashed-dotted line) are scaled with the same prescription, whereas HSD transport-model calculations (dashed line) yield absolute predictions.

Figure 3

The ratio of $K^0$ ($K^{+}$) yields produced by pions (protons) on heavy and light targets plotted as a function of the momentum $p$ in the lab system. The full squares depict the yield ratio of $K_S^0$ produced on Pb and C targets in this experiment. A similar ratio of $K^{+}$ yields measured in proton-induced reactions on Au and C targets is represented by full circles [11]. The results of the HSD model with different strengths of the $KN$ potential are depicted by solid (black), dashed (red), and dotted (blue) lines.