Schematic diagrams showing possible quark propagation channels in pion-pion scattering.

From the article:

Isoscalar $\pi \pi$ Scattering and the $\sigma$ Meson Resonance from QCD
Raul A. Briceño, Jozef J. Dudek, Robert G. Edwards, and David J. Wilson (for the Hadron Spectrum Collaboration)
Phys. Rev. Lett. 118, 022002 (2017)

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