Kaonic hydrogen atom andΛ(1405)

A model of the $\overline{K}N$ interaction at low energies is proposed such that the recently observed, surprisingly small energy shift associated with the $2p-1s$ x rays from $K^{-}p$ atoms can be explained. The essential difference between this and conventional models lies in the interpretation of the origin of $\Lambda \mathrm{}\left(1405\right)\mathrm{}$. Our model is in accord with the quark model in which $\Lambda \mathrm{}\left(1405\right)\mathrm{}$ is regarded as being as "elementary" as the nucleon. It predicts that the $\overline{K}N$ scattering amplitude for $I=0\mathrm{}$ is very small at the threshold, but increases rapidly with energy such that the $\overline{K}N$ scattering data above the threshold can be reproduced.