Lattice QCD Evidence that the Lambda(1405) Resonance is an   Antikaon-Nucleon Molecule
  For almost 50 years the structure of the Lambda(1405) resonance has been a mystery. Even though it contains a heavy strange quark and has odd parity, its mass is lower than any other excited spin-1/2 baryon. Dalitz and co-workers speculated that it might be a molecular state of an antikaon bound to a nucleon. However, a standard quark-model structure is also admissible. Although the intervening years have seen considerable effort, there has been no convincing resolution. Here we present a new lattice QCD simulation showing that the strange magnetic form factor of the Lambda(1405) vanishes, signaling the formation of an antikaon-nucleon molecule. Together with a Hamiltonian effective-field-theory model analysis of the lattice QCD energy levels, this strongly suggests that the structure is dominated by a bound antikaon-nucleon component. This result clarifies that not all states occurring in nature can be described within a simple quark model framework and points to the existence of exotic molecular meson-nucleon bound states. 
Jonathan M. M. Hall, Waseem Kamleh, Derek B. Leinweber, Benjamin J.   Menadue, Benjamin J. Owen, Anthony W. Thomas, Ross D. ...
hep-lat nucl-th