The Feynman--Hellmann approach to computing matrix elements in lattice QCD by first adding a perturbing operator to the action is described using the transition matrix and the Dyson expansion formalism. This perturbs the energies in the two-point baryon correlation function, from which the matrix element can be obtained. In particular at leading order in the perturbation we need to diagonalise a matrix of near-degenerate energies. While the method is general for all hadrons, we apply it here to a study of a Sigma to Nucleon baryon transition vector matrix element.
@article{arxiv.2305.05491,
title = {Feynman--Hellmann approach to transition matrix elements and quasi-degenerate energy states},
author = {M. Batelaan and K. U. Can and R. Horsley and Y. Nakamura and P. E. L. Rakow and G. Schierholz and H. Stüben and R. D. Young and J. M. Zanotti},
journal= {arXiv preprint arXiv:2305.05491},
year = {2023}
}