  The probability of the event that a neutrino produced in pion decay is
detected in the intermediate $T$ shorter than the life-time $\tau_{\pi}$, $T
\leq \tau_{\pi}$, is sensitive to the absolute mass of the neutrino. With a
newly formulated S-matrix $S[T]$ that satisfies the boundary conditions of the
experiments at a finite $T$, the rate of the event is computed as
$\Gamma_0+\tilde{g}(\omega_{\nu}, {T};\tau_{\pi}) \tilde \Gamma_{1} $, where
$\tilde{g}(\omega_{\nu},{T};\tau_{\pi})$ depends weakly on $\tau_{\pi}$ and
$\omega_{\nu}={m_{\nu}^2c^4}/{(2E_{\nu}\hbar)}$, $c$ is the speed of light.
$\Gamma_0$ is the standard one and the correction, $\tilde{g}(\omega_{\nu},
{T};\tau_{\pi}) \tilde \Gamma_{1} $, reflects relativistic invariance and is
rigorously computed via the light-cone singularity of the system and reveals
the diffraction pattern of a single quantum. The formula explains unsolved
anomalies of neutrino experiments and indicates the heavy neutrino mass, $0.098
\pm 0.022$ or $0.083 \pm 0.026$ {eV}/ $c^2$ for normal or inverted mass
hierarchies, respectively.
