Perturbative application of next-to-leading order pionless EFT for A≤3 nuclei in a finite volume

Lattice quantum chromodynamics (LQCD) calculations with physical pion mass would revolutionize nuclear physics by enabling predictions based on the fundamental theory of the strong force. To bridge the gap between finite-volume LQCD results and free-space physical observables, two primary extrapolation methods have been employed so far. The traditional approach relies on the L\"uscher formula and its extensions, while a recent alternative employs effective field theories (EFTs) fitted directly to the finite volume data. In this study, we fit pionless EFT with perturbative inclusion of the next-to-leading order to finite-volume energies generated from a phenomenological NN interaction. The theory is then used to extrapolate the finite-volume results into free space as well as to predict new few-body observables. As a benchmark, we also apply the L\"uscher formalism directly to the finite-volume data. Through a comprehensive analysis, we explore the characteristics of order-by-order predictions of the pionless EFT fitted within a finite volume, investigate the limitations of the different extrapolation techniques used, and derive recommended box sizes required for reliable predictions.