Direct comparison of phase-space distributions of K- and K+ mesons in heavy-ion collisions at SIS energies -- evidence for in-medium modifications of kaons?

The ratio of K^- to K⁺-meson yields has been measured in the systems ⁹⁶Ru + ⁹⁶Ru at 1.69 A GeV, ⁹⁶Ru+⁹⁶Zr at 1.69 A GeV, and ⁵⁸Ni+⁵⁸Ni at 1.93 A GeV incident beam kinetic energy. The yield ratio is observed to vary across the measured phase space. Relativistic transport-model calculations indicate that the data are best understood if in-medium modifications of the kaons are taken into account. Received: 30 November 1998 / Accepted: 20 December 2000