How to measure the Pomeron phase in diffractive dipion photoproduction
  The study of charge asymmetry of pions in the high-energy process gamma p -> pi+ pi- p (e p -> e pi+ pi- p) at very small dipion momenta offers a method to measure the phase of the forward hadronic (quasi)elastic amplitude gamma p -> rho p. We estimate potential of such measurements at HERA. 
I. F. Ginzburg (1), I. P. Ivanov (1 and 2), ((1) Sobolev Institute of   Mathematics, Novosibirsk, Russia, (2) INFN, Cosenza,...