I review and improve on how two-index SU(N) gauge-fermion theories help access salient information about the large N vacuum and spectrum of QCD, super Yang Mills and meson-meson scattering. The interplay with recent lattice simulations will be employed to deduce the size of 1/N² corrections. Through the meson-meson scattering analysis I will honor Steven Weinberg's memory by showing how two-index extrapolations naturally accommodate the appearance of tetraquarks states crucial to unitarize meson-meson scattering at low energies.
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Francesco Sannino (2024) studied this question.
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