  We have studied the structure of K- pp by solving this system in a
variational treatment, starting from ansatz that Lambda(1405) is a K-p
quasi-bound state, Lambda* with mass 1405 MeV/c2. The structure of K-pp reveals
a molecular feature, namely, the K- in an "atomic center", Lambda*, plays a key
role in producing strong covalent bonding with the other proton. Deeply bound
Kbar nuclear systems are formed by this "super-strong" nuclear force due to
migrating real bosons, Kbar, a la Heitler-London-Heisenberg, which
overcompensates the stiff nuclear incompressibility. Theoretical background of
the Lambda (1405) ansatz is discussed in connection with the double-pole
picture of Lambda (1405) based on chiral SU(3) dynamics. Detailed analysis
reveals single-pole nature of the observable Lambda (1405). There are two kinds
of Sigma pi invariant masses experimentally observable, the usual T22 invariant
mass and the conversion T21 invariant mass. It is of vital importance to
determine whether the Lambda* mass is 1405 MeV or 1420 MeV. The T21 invariant
mass from K- absorption at rest in deuteron can provide decisive information
about this Lambda* mass problem.
