Strange meson production in Al+Al collisions at 1.9A GeV

The production of K${}^{+}$, K${}^{-}$ and $\phi$(1020) mesons is studied in Al+Al collisions at a beam energy of 1.9A GeV which is close or below the production threshold in NN reactions. Inverse slopes, anisotropy parameters, and total emission yields of K${}^{\pm }$ mesons are obtained. A comparison of the ratio of kinetic energy distributions of K${}^{-}$ and K${}^{+}$ mesons to the HSD transport model calculations suggests that the inclusion of the in-medium modifications of kaon properties is necessary to reproduce the ratio. The inverse slope and total yield of $\varphi$ mesons are deduced. The contribution to K${}^{-}$ production from $\varphi$ meson decays is found to be [17 $\pm$ 3 (stat) ${}_{-7}^{+2}$ (syst)] %. The results are in line with previous K${}^{\pm }$ and $\varphi$ data obtained for different colliding systems at similar incident beam energies.