We present the first direct Nf=2 lattice QCD computation of two- and three-π+ 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 Mπa0=−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.
@article{arxiv.2008.03035,
title = {Scattering of two and three physical pions at maximal isospin from lattice QCD},
author = {Matthias Fischer and Bartosz Kostrzewa and Liuming Liu and Fernando Romero-López and Martin Ueding and Carsten Urbach},
journal= {arXiv preprint arXiv:2008.03035},
year = {2021}
}