Determination of s- and p-wave I = 1/2 Kπ scattering amplitudes in N_f = 2 + 1 lattice QCD

The elastic I=1/2, s- and p-wave kaon-pion scattering amplitudes are calculated using a single ensemble of anisotropic lattice QCD gauge field configurations with N_f=2+1 flavors of dynamical Wilson-clover fermions at m_π=230 MeV. A large spatial extent of L=3.7 fm enables a good energy resolution while partial wave mixing due to the reduced symmetries of the finite volume is treated explicitly. The p-wave amplitude is well described by a Breit–Wigner shape with parameters m_K^⁎ /m_π=3.808(18) and g_K^⁎Kπ ^BW=5.33(20) which are insensitive to the inclusion of d-wave mixing and variation of the s-wave parametrization. An effective range description of the near-threshold s-wave amplitude yields m_πa₀=−0.353(25).