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.Comment: 12 pages, 2 figures; v4: more elaborate parametrizations of dipion spectra implemented, discussion extende