On the stability of one particle states generated by quantum fields   fulfilling Yang-Feldman equations
  We prove that for a Wightman quantum field $\phi(x)$ the assumptions (i) positivity of the metric on the state space ${\cal H}$ of the theory (ii) the asymptotic condition in the form of Yang-Feldman equations and (iii) Klein-Gordon equation for the outgoing field imply that the states generated by application of the asymptotic fields to the vacuum are stable. We prove by a counter example that this statement is wrong in the case of quantum fields with indefinite metric. 
H. Gottschalk