Two-index SU(N) theories: QCD, Orientifolds, Super Yang-Mills, Lattice and Steven Weinberg's $\pi \pi$ scattering legacy

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 $1\mathrm{/}{N}^2$ 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.