The Role of Lambda(1405) in Kaon-Proton Interactions

S-wave $K^-p$ scattering into various channels near threshold are analyzed in heavy-baryon chiral perturbation theory with $\Lambda (1405)$ introduced as an independent field. This is the approach that predicted the critical density $2<=\rho_c/\rho_0 <=3$ for negatively charged kaon condensation. We show that chiral perturbation expansion treating the $\Lambda (1405)$ as elementary is consistent with {\it all} threshold data including a double-charge-exchange process suppressed at leading order of chiral expansion in the absence of the $\Lambda (1405)$. We also discuss S-wave $K^+ p$ scattering phase shifts at low energy.