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The coefficient of (σ₁+σ₂)·^n in the pp scattering amplitude at 310 Mev can be completely accounted for by a strongly interacting neutral vector meson (or a sharp three-pion resonance in the $T=0$, $J=1$ state) of mass ≈3μ_π-4μ_π.
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J. J. Sakurai (1960) studied this question.
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