The comparison of $K^+$ and $K^-$ spectra at low transverse momentum in light symmetric heavy ion reactions at energies around 2 AGeV allows for a direct experimental determination of the strength of the $K^+$ as well as of t he $K^-$ nucleus potential. Other little known or unknown input quantities like the production or rescattering cross sections of $K^+$ and $K^-$ mesons do not spoil this signal. This result, obtained by simulations of these reactio ns with the Isospin Quantum Molecular Dynamics (IQMD) model, may solve the longstanding question of the behaviour of the $K^-$ in hadronic matter and especially whether a $K^-$ condensate can be formed in heavy ion collisions.
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