  We investigate radiation-reaction effects for a charged scalar particle
accelerated by an external potential realized as a space-dependent mass term in
quantum electrodynamics. In particular, we calculate the position shift of the
final-state wave packet of the charged particle due to radiation at lowest
order in the fine structure constant alpha and in the small h-bar
approximation. We show that it disagrees with the result obtained using the
Lorentz-Dirac formula for the radiation-reaction force, and that it agrees with
the classical theory if one assumes that the particle loses its energy to
radiation at each moment of time according to the Larmor formula in the static
frame of the potential. However, the discrepancy is much smaller than the
Compton wavelength of the particle. We also point out that the electromagnetic
correction to the potential has no classical limit. (Correction. Surface terms
were erroneously discarded to arrive at Eq. (59). By correcting this error we
find that the position shift according to the Lorentz-Dirac theory obtained
from Eq. (12) is reproduced by quantum field theory in the hbar -> 0 limit. We
also find that the small V(z) approximation is unnecessary for this agreement.
See Sec. VII.)
