  We developed a microscopic model for antikaon absorption on two nucleons in
nuclear matter. The absorption is described within a meson-exchange picture and
the primary $K^-N$ interaction strength is derived from state-of-the-art chiral
coupled channel meson-baryon interaction models. We took into account the
medium modification of the $K^-N$ scattering amplitudes due to the Pauli
correlations. We derived the $K^-NN$ as well as $K^-N$ optical potentials as
functions of nuclear matter density including the real part of the $K^-NN$
potential. We calculated the $K^-$ single- and two-nucleon absorption fractions
and branching ratios for various mesonic and non-mesonic channels. We confirmed
the crucial role of in-medium effects in our calculations. Our results are in
very good agreement with available experimental data from old bubble chamber
experiments as well as with the latest results from the AMADEUS collaboration.
