  The neutrino produced in the pion decay reveals a new diffraction phenomenon
due to many-body interactions in an intermediate time region when wave
functions of the parent and daughters overlap. Because of diffraction, the
probability to detect the neutrino involves a large finite-size correction that
depends on the neutrino mass, $m_{\nu}$ and energy, $E_{\nu}$, the speed of
light, $c$, and the distance $L$ between the positions of the initial pion and
final neutrino, ${m_{\nu}^2c^4 L}/{(2E_{\nu}\hbar)}$.
  The correction vanishes for the charged leptons and is finite for the
neutrino at a macroscopic distance, $L$, of near-detector regions in ground
experiments. A new method for determining the absolute neutrino mass is
proposed.
