Isotensor πππ Scattering with a ρ-Resonant Subsystem from QCD.

This Letter presents a lattice quantum chromodynamics (QCD) determination of πππ scattering amplitudes for the isospin-2 channel with angular momentum and parity J^{P}=1^{+}. The calculation is performed using unphysically heavy light-quark masses, corresponding to a pion mass of m_{π}≈400  MeV, for which the ρ meson manifests as a narrow resonance. The analysis employs a previously developed formalism that nonperturbatively relates the finite-volume spectra of two- and three-pion systems to their infinite-volume scattering amplitudes. We combine earlier lattice QCD results for ππ systems with isospins 1 and 2 with new determinations of the isospin-2 three-pion finite-volume spectrum, obtained for three cubic, periodic lattice volumes with box length ranging from ≈4/m_{π} to 6/m_{π}. These combined data constrain the low-lying partial waves of the infinite-volume three-pion K matrix, from which we solve a set of coupled integral equations to extract the corresponding scattering amplitudes.