Unified Approach to Neutrino Oscillation and Decay Probabilities of Neutrino-Source Hadrons

In the framework of quantum field theory, we examine what physical implications are included in the expectation values of the flavor neutrino charges at the time $x^0$ with respesct to the state $\ket{\Psi(x^0)}$, which is taken so as to coincide with the neutrino source state such as a charged pion at the time $x^0_I$ ($< x^0$).