Interpolating wave packets and composite wave functions in QFT and neutrino oscillation problem

A consistent constructive covariant description of neutrino flavour transition amplitude in vacuum is presented. To this end a special generalized relativistic wave packet is constructed with correct extension onto the higher spins. This packet is uniquely defined as an `interpolating' wave packet, which by means of relativistically invariant `width' accurately interpolates between the states localized in momentum space and in coordinate space. The wave packet is unambiguously determined by analytical properties of Wightman functions in complex coordinate space naturally connected with its minimization properties. The packet gives natural relativistic generalization of non relativistic Gaussian wave packet but it contains covariant states of particle (antiparticle) only with positive (negative) energy sign and propagates without their mixing and without changing of its relativistically invariant width. For the diagrammatic treatment of oscillation with the use of these wave packets for external particles, the notion of covariant composite wave function for intermediate neutrino is introduced. It strictly and naturally connects both oscillation pictures, giving an effective language for detailed description of this process, and resolves the problems with causality and with covariant equal time prescription for the intermediate neutrino picture. It is closely related to overlap function of neutrino creation/detection vertices, elucidating a covariant meaning of the `pole integration' procedure. Their space-time asymptotic behaviour in narrow-packets approximation naturally conforms with such approximation of one-packet state and with the asymptotic behaviour of oscillation amplitude. The respective overlap function is explicitly calculated for two-packet example of pion decay vertex. Its correspondence and difference with previous approximate calculations is analyzed.