Neutrino flavour states and the quantum theory of neutrino oscillations

The definition and derivation of flavour neutrino states in the framework of standard quantum field theory is reviewed, clarifying some subtle points. It is shown that a flavour neutrino state that describes a neutrino produced or detected in a charged-current weak interaction process depends on the process under consideration and is appropriate for the description of neutrino oscillations as well as for the calculation of neutrino production or detection rates. Hence, we have a consistent framework for the description of neutrino oscillations and interactions in neutrino oscillation experiments. The standard flavour neutrino states are obtained as approximations which describe neutrinos in experiments that are not sensitive to the dependence of neutrino interactions on the neutrino mass differences. It is also shown that the oscillation probability can be derived either through the usual light-ray time = distance approximation or through an average of the spacetime-dependent oscillation probability over the unobserved propagation time.