Estimating the two-particle K-matrix for multiple partial waves and decay channels from finite-volume energies

An implementation of estimating the two-to-two K -matrix from finite-volume energies based on the Lüscher formalism and involving a Hermitian matrix known as the “box matrix” is described. The method includes higher partial waves and multiple decay channels. Two fitting procedures for estimating the K -matrix parameters, which properly incorporate all statistical covariances, are discussed. Formulas and software for handling total spins up to S = 2 and orbital angular momenta up to L = 6 are obtained for total momenta in several directions. First tests involving ρ -meson decay to two pions include the L = 3 and L = 5 partial waves, and the contributions from these higher waves are found to be negligible in the elastic energy range.