Does large-N QCD lie at the pion bootstrap kink?
In plain words
Bounds on how massless pions can scatter when the number of colors is large show a sharp corner. Whether QCD with many colors lies exactly at that corner is unknown.
Precise statement
Albert and Rastelli (JHEP 08 (2022) 151) bounded ratios of chiral-Lagrangian Wilson coefficients for massless pions in large-N confining gauge theories, using tree-level meson exchange, crossing, unitarity and Regge behavior, and found a kink on the exclusion boundary. Determine whether the large-N QCD values of these ratios (from lattice QCD at large N or other controlled methods) coincide with the kink, and what meson spectrum the extremal amplitude has.
What would settle it
Large-N lattice determination of the relevant Wilson-coefficient ratios compared with the kink location.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
Albert and Rastelli (2022) raised and critically examined the identification; Albert, Henriksson, Rastelli and Vichi (arXiv:2312.15013, 2023) found, under a spin-2 spectral assumption, an extremal amplitude with an apparent Regge trajectory close to real meson masses; no large-N lattice comparison settles it as of 2026.
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