Lineshape of the Λ(1405) hyperon measured through its Σ0π0 decay

The pp ! pK + Y 0 reaction has been studied for hyperon masses m(Y 0 ) � 1540MeV/c 2 at COSY-Julich by using a 3.65GeV/c circulating proton beam incident on an in- ternal hydrogen target. Final states comprising two protons, one positively charged kaon and one negatively charged pion have been identified with the ANKE spectrom- eter. Such configurations are sensitive to the production of the ground stateand � 0 hyperons as well as the � 0 (1385) and �(1405) resonances. Applying invariant- and missing-mass techniques, the two overlapping excited states could be well sep- arated, though with limited statistics. The shape and position of the �(1405) distri- bution, reconstructed cleanly in the � 0 � 0 channel, are similar to those found from other decay modes and there is no obvious mass shift. This finding constitutes a challenging test for models that predict �(1405) to be a two-state resonance.