Final-state interaction theory of cusp formation

The distribution of electrons emitted in atomic collisions often exhibits a cusp-shaped peak centered at a velocity equal to that of the incident projectile. In the case of outgoing ''charged'' projectiles, this effect is assumed to be due to the long-range nature of the Coulomb electron-projectile interaction in the final state. However, measurements made by the Debrecen and Frankfurt groups of cusp electrons in coincidence with neutral H and He ejectiles have confronted the theorist with new questions and apparently conflicting evidence. We review different models which have been proposed to explain these new observations, within the general framework of the theory of final-state interaction. We also analyze the presence of similar effects in other scattering processes.