A possibility that the electromagnetic current may have a unitary singlet component has been investigated in connection with V₉→ll̄ decays. The Maki-Hara model of elementary particles seems to be experimentally ruled out. As for the three-triplet model, it has been pointed out that we must have additional 1^- vector mesons ρ^', ω^', and φ^' with nonzero charm quantum numbers in order to saturate a new sum rule.
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Mathur et al. (1969) studied this question.
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