Interpolating non spreading wave packets in QFT and neutrino oscillation problem

Consistent constructive generalization of the introduced recently relativistic covariant wave packet is deduced on the grounds of general principles of quantum field theory with careful extension to the higher spins. This state is uniquely defined as a so called interpolating state, which has the both correct limits to the states localized in momentum space and/or in coordinate space. The wave packet is unambiguously determined by analytical properties of Wightman function in complex coordinate space, defining a representation of nonhomogeneous complex Lorentz group. It appears as only possible careful and natural relativistic generalization of Gaussian wave packet but contains covariant particle (antiparticle) states only with positive (negative) energy sign and propagates without their mixing and without spreading in relativistically covariant sense. The analytical continuation in coordinate space thus specifies a universal way to wave-packet's construction for massive particles with arbitrary spin. Specific simultaneous zero-mass and zero-width limit naturally reduces this packet from 3+1 to 1+1 space-time dimensions, leaving a one-parametric freedom. Within intermediate wave packet approach to the neutrino oscillation phenomena the respectively generalized expression for two-flavor oscillations of leptonic charge of electronic neutrino is given.For diagrammatic treatment of oscillation in terms of those wave packets a covariant meaning of "pole integration" is elucidated with the help of Huygens' principle in terms of the reduced to mass shell composite wave functions closely related to overlap functions for the neutrino creation/detection processes.