First-principles modelling of the magnetic structure of the lightest nuclear systems using effective field theory without pions

Abstract: The strong interaction, i.e., quantum chromodynamics at the low energy nuclear regime, is notoriously known to be challenging for predictive modeling. Here, we use the simplest possible nuclear effective field theory (EFT), and show that in the case of the magnetic structure of nuclear systems with $A=2$ and $A=3$ nucleons, it is highly precise as well as predictive. The theoretical framework is the pionless EFT (\pilesseft), of point nucleons with contact interactions, expanded consistently up to next-to-leading order (NLO) in perturbation theory, i.e., including only eleven low-energy parameters, and augmented by a novel Bayesian analysis of theoretical uncertainties. The theory accurately predicts the shell structure reflected in the values of the magnetic moments and reactions of these nuclei within $\approx 1\%$ calculated theoretical uncertainty. We show that this perfect prediction originates in implicit a-posteriori properties of the calculation, particularly an unexpectedly small expansion parameter, as well as a vanishing contribution from the two-body isoscalar current.