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The FOPI Collaboration at the GSI SIS-18 synchrotron measured charged kaonsfrom central and semi-central collisions of Ni+Ni at a beam energy of 1.91AGeV. We present the distribution of the K$^-$\K$^+$ ratio on the energy vspolar angle plane in the nucleon-nucleon center-of-mass frame, with and withoutsubtraction of the contribution of $\phi$(1020) meson decays to the K$^-$yield. The acceptance of the current experiment is substantially wider comparedto the previous measurement of the same colliding system. The ratio of K$^-$ toK$^+$ energy spectra is expected to be sensitive to the in-medium modificationsof basic kaon properties like mass. Recent results obtained by the HADESCollaboration at 1.23A and 1.76A GeV indicate that after inclusion of the$\phi$ meson decay contribution to the K$^-$ production no difference betweenthe slopes of the K$^-$ and K$^+$ energy spectra is observed withinuncertainties. For our data a linear fit to this ratio obtained aftersubtraction of the $\phi$ meson contribution still shows a decrease withkinetic energy, although a constant value cannot be rejected. The contributionof $\Lambda(1520) \rightarrow p\text{K}^-$ decays estimated from fitting thethermal model to the experimental yields appears to be another factor ofmoderate relevance.

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