Nonadiabatic effects in nuclear reactions and the problem of determination of the proton electromagnetic form factor in the nonphysical region

It is shown that in nuclear reactions a considerable cancellation takes place between some nonadiabatic effects associated with the nonstatic nature of nucleons, coupled with the fact that they are bound and with off-shell properties of the elementary amplitudes. A techique is developed which allows us to study the above effects at low, medium and high energies in both elastic and inelastic scattering. Parameters which characterize the cancellation of nonadiabatic effects are obtained. It is shown that a certain choice of the energy variable in the elementary amplitude is advantageous in describing quasielastic reactions. This choice corresponds to the collision of an incident particle with a free nucleon whose momentum equals the Fermi momentum of the nucleon being knocked out. The results are applied to study the possibility of extracting the proton electromagnetic form factor in the nonphysical region from the anti pd -> e+e-nsub(s) data. It is found that due to the cancellation the amplitude of the virtual annihilation effectively enters the pole diagram at q2>=4m2 independently of the kinematical range of the external variables. The near-threshold behaviour of the amplitude obtained when extracting it from the anti pd-e+e-nsub(s) data by means of the commonly used 'quasifree' approximation (pole graph) is predicted. (orig.)