An Excitation Function of K${}^{-}$ and K${}^{+}$ Production in Au+Au Reactions at the AGS

Mid-rapidity spectra and yields of K${}^{-}$ and K${}^{+}$ have been measured for Au+Au collisions at 4, 6, 8, and 10.7 AGeV. The K${}^{-}$ yield increases faster with beam energy than for K${}^{+}$ and hence the K${}^{-}$/K${}^{+}$ ratio increases with beam energy. This ratio is studied as a function of both $\sqrt{s}$ and $\sqrt{s}$-$\sqrt{s_{th}}$ which allows the direct comparison of the kaon yields with respect to the production threshold in p+p reactions. For equal $\sqrt{s}$ - $\sqrt{s_{th}}$ the measured ratio K${}^{-}$/K${}^{+}$=0.2 at energies above threshold in contrast to the K${}^{-}$/K${}^{+}$ ratio of near unity observed at energies below threshold. The use of the K${}^{-}$/K${}^{+}$ ratio to test the predicted changes of kaon properties in dense nuclear matter is discussed.