Measurement of $K^{*}(892){}^0$ and $K^0$ mesons in $\mathrm{Al}+\mathrm{Al}$ collisions at 1.9$A$ GeV

A new measurement of subthreshold $K^{*}(892){}^0$ and $K^0$ production is presented. The experimental data complete the measurement of strange particles produced in $\mathrm{Al}+\mathrm{Al}$ collisions at 1.9$A$ GeV measured with the FOPI detector at SIS at GSI (Darmstadt). The $K^{*}(892){}^0/K^0$ yield ratio is found to be $0.0315\pm 0.006(\mathrm{stat}.)\pm 0.012(\mathrm{syst}.)$ and is in good agreement with the transport model prediction. These measurements provide information on the in-medium cross section of $K^{+}$-${\pi }^{-}$ fusion, which is the dominant process in subthreshold $K^{*}(892){}^0$ production.

Figure 1

Invariant mass spectra of $K^{+}{\pi }^{-}$ pairs. The solid histogram and the dashed lines denote the data and the scaled mixed-event background, respectively (upper panel). The lower panel shows the signal after background subtraction. The following characteristics of the signal are shown: number of counts in the signal ($\mathit{S}$), signal-to-background ratio ($\mathit{S}$/$\mathit{B}$), and significance (SIGNIF $=S/\sqrt{S+B}$). The parameters extracted from the fit to the data [mean mass value (MEAN) and width ($\Gamma$)] are also reported.

Figure 2

Invariant mass distribution of ${\pi }^{+}{\pi }^{-}$ pairs. The solid histogram and the dashed lines denote the data and the scaled mixed-event background, respectively (upper panel). The lower panel shows the signal after background subtraction. See caption of Fig. 1 for the abbreviations.

Figure 3

Left: Transverse mass distributions of $K_S^0$ mesons for different rapidity windows after efficiency corrections. The lines represent the Boltzmann function fits. The ranges of the data used to perform the adjustment are shown by full symbols. The values in parentheses denote scaling factors used for the plot. Right: Rapidity density distribution for $K^0,$ where the open symbols are data points reflected with respect to midrapidity. The curve represents a Gaussian fit from which the total yield is extracted (see the text for more details).