Neutrino oscillation occurring because flavor eigenstates are superpositions of mass eigenstates with differing masses is a foundational principle of standard particle physics and does not require citation.
The claim states the standard quantum mechanical mechanism for neutrino oscillations (flavor states as linear combinations of distinct mass eigenstates). This is standard textbook physics and foundational knowledge in particle physics, making it common knowledge where formal citations are not strictly necessary, though papers like [6] and [7] touch upon related aspects of neutrino mass and oscillation.