The effect of the in-medium ${\mathrm{\Theta }}^{+}$ pentaquark on the kaon optical potential

The kaon nuclear optical potential is studied including the effect of the ${\mathrm{\Theta }}^{+}$ pentaquark. The one-nucleon contribution is obtained using an extension of the J"ulich meson-exchange potential as bare kaon-nucleon interaction. Significant differences between a fully self-consistent calculation and the usually employed low-density $T\rho$ approach are observed. The influence of the one-nucleon absorption process, $KN\to {\mathrm{\Theta }}^{+}$, on the kaon optical potential is negligible due to the small width of the pentaquark. In contrast, the two-nucleon mechanism, $KNN\to {\mathrm{\Theta }}^{+}N$, estimated from the coupling of the pentaquark to a two-meson cloud, provides the required amount of additional kaon absorption to reconcile with data the systematically low $K^{+}$-nucleus reaction cross sections found by the theoretical models.