First observation of in-medium effects on phase space distributions of antikaons measured in proton-nucleus collisions

Differential production cross sections of $K^{\pm}$ mesons have been measured in $p$ + C and $p$ + Au collisions at 1.6, 2.5 and 3.5 GeV proton beam energy. At beam energies close to the production threshold, the $K^-$ multiplicity is strongly enhanced with respect to proton-proton collisions. According to microscopic transport calculations, this enhancement is caused by two effects: the strangeness exchange reaction $NY \to K^- NN$ and an attractive in-medium $K^-N$ potential at saturation density.