Kaonic hydrogen atom and Λ(1405)

A model of the 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 Λ(1405). Our model is in accord with the quark model in which Λ(1405) is regarded as being as "elementary" as the nucleon. It predicts that the K¯N scattering amplitude for I=0 is very small at the threshold, but increases rapidly with energy such that the K¯N scattering data above the threshold can be reproduced.