Probability Collectives for Unstable Particles
  Unstable particles, together with their stable decay products, constitute probability collectives which are defined as Hilbert spaces with dimension higher than one, nondecomposable in a particle basis. Their structure is considered in the framework of Birkhoff-von Neumann's Hilbert subspace lattices. Bases with particle states are related to bases with a diagonal scalar product by a Hilbert-bein involving the characteristic decay parameters (in some analogy to the $n$-bein structures of metrical manifolds). Probability predictions as expectation values, involving unstable particles, have to take into account all members of the higher dimensional collective. E.g., the unitarity structure of the $S$-matrix for an unstable particle collective can be established by a transformation with its Hilbert-bein. 
Heinrich Saller (MPI fuer Physik)