Strong Absorption of Hadrons with Hidden and Open Strangeness in Nuclear Matter

We present the first observation of $K^{-}$ and $\varphi$ absorption within nuclear matter by means of ${\pi }^{-}$-induced reactions on C and W targets at an incident beam momentum of $1.7\mathrm{GeV}/c$ studied with HADES at $\mathrm{SIS}18/\mathrm{GSI}$. The double ratio $(K^{-}/K^{+}{)}_{\mathrm{W}}/(K^{-}/K^{+}{)}_{\mathrm{C}}$ is found to be $0.319\pm 0.009(\mathrm{stat}{)}_{-0.012}^{+0.014}(\mathrm{syst})$ indicating a larger absorption of $K^{-}$ in heavier targets as compared to lighter ones. The measured $\varphi /K^{-}$ ratios in ${\pi }^{-}+\mathrm{C}$ and ${\pi }^{-}+\mathrm{W}$ reactions within the HADES acceptance are found to be equal to $0.55\pm 0.04(\mathrm{stat}{)}_{-0.07}^{+0.06}(\mathrm{syst})$ and to $0.63\pm 0.06(\mathrm{stat}{)}_{-0.11}^{+0.11}(\mathrm{syst})$, respectively. The similar ratios measured in the two different reactions demonstrate for the first time experimentally that the dynamics of the $\varphi$ meson in nuclear medium is strongly coupled to the $K^{-}$ dynamics. The large difference in the $\varphi$ production off C and W nuclei is discussed in terms of a strong $\varphi N$ in-medium coupling. These results are relevant for the description of heavy-ion collisions and the structure of neutron stars.

Figure 1

Mass distributions of $K^{-}$ (a) and $K^{+}$ (b) with the corresponding background fits (dashed lines). (c) Invariant mass distribution of $K^{+}{K}^{-}$ pairs normalized to the number of LVL1 events in ${\pi }^{-}+\mathrm{C}$ (pink points) and ${\pi }^{-}+\mathrm{W}$ reactions (blue points). The fit to the uncorrected experimental data consists of the sum of two Gaussians for the $\varphi$ signal together with the background described by a polynominal and Gaussian function (dashed line).

Figure 2

$K^{+}$ and $K^{-}$ differential cross sections in different rapidity regions (see legend). The upper panel corresponds to $K^{-}$ and the lower panel to $K^{+}$. The combined, statistical and systematic, uncertainty and the normalization error are smaller than the symbol size. The dashed lines indicate Boltzmann fits (see text for details).

Figure 3

(a) Cross section of $K^{+}$ and $K^{-}$ in ${\pi }^{-}+\mathrm{C}$ and ${\pi }^{-}+\mathrm{W}$ collisions as a function of rapidity. The shaded bands denote the systematic errors. The open boxes indicate the normalization error. The statistical uncertainties are smaller than the symbol size. The arrow ($y_{CM}$) indicates the $\pi N$ rapidity. Double ratio $(K^{-}/K^{+}{)}_{\mathrm{W}}/(K^{-}/K^{+}{)}_{\mathrm{C}}$ as a function of $p_T$ (b) and the rapidity $y$ (c) without (pink circles) and with $p_T$ extrapolation (orange squares) inside the HADES acceptance. The shaded areas indicate the systematic errors.