# Scattering of two and three physical pions at maximal isospin from lattice QCD

7 Aug 2020  ·  Matthias Fischer, Bartosz Kostrzewa, Liuming Liu, Fernando Romero-López, Martin Ueding, Carsten Urbach ·

We present the first direct $N_f=2$ lattice QCD computation of two- and three-$\pi^+$ scattering quantities that includes an ensemble at the physical point. We study the quark mass dependence of the two-pion phase shift, and the three-particle interaction parameters. We also compare to phenomenology and chiral perturbation theory (ChPT). In the two-particle sector, we observe good agreement to the phenomenological fits in $s$- and $d$-wave, and obtain $M_\pi a_0 = -0.0481(86)$ at the physical point from a direct computation. In the three-particle sector, we observe reasonable agreement at threshold to the leading order chiral expansion, i.e.\@ a mildly attractive three-particle contact term. In contrast, we observe that the energy-dependent part of the three-particle quasilocal scattering quantity is not well described by leading order ChPT.

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## Categories

High Energy Physics - Lattice High Energy Physics - Phenomenology