  In the charged pion decay, a neutrino is produced in pair with a charged
lepton and they have the same production rate. In this paper we show that
neutrinos have their own space-time correlations in a wide area and are
detected in a different manner from charged leptons, owing to extremely small
mass. The neutrino flux reveals a unique interference effect in the form of
diffraction of non-stationary waves. The diffraction component of the flux
shows a slow position-dependence and leads to an electron neutrino at short
base-line regions. The electron neutrino flux at short distances is attributed
to the neutrino diffraction and the one at long distances is to the normal
flavor oscillation. The former depends upon the average mass-squared $\bar
m^2_\nu$ and the latter depends upon the mass-squared difference $\delta
m^2_\nu$. The LSND and the two neutrino experiment (TWN) measure $\bar m^2_\nu$
and the other experiments measure $\delta m^2_\nu$. Hence they are consistent
with each other. The neutrino diffraction would supply valuable information on
the absolute neutrino mass.
