  In this II, a probability to detect the neutrino produced in a high-energy
pion decay is shown to receive the large finite-size correction. The neutrino
interacts extremely weakly with matters and is described with a many-body wave
function together with the pion and charged lepton. This wave function slowly
approaches to an asymptotic form, which is probed by the neutrino. The whole
process is described by an S-matrix of a finite-time interval, which couples
with states of non-conserving kinetic energy, and the final states of a broad
spectrum specific to a relativistic invariant system contribute to the positive
semi-definite correction similar to diffraction of waves through a hole. This
diffraction component for the neutrino becomes long range and stable under
changes of the pion's energy.
  Moreover, it has a universal form that depends on the absolute neutrino mass.
Thus a new method of measuring the absolute neutrino mass is suggested.
