# On the convergence of nuclear effective field theory with perturbative pions

## TL;DR

This paper investigates the convergence of nuclear effective field theory with perturbative pions, developing analytical tools to compute high-order Feynman graphs and identifying channels where the expansion converges well or poorly.

## Contribution

It introduces methods to analytically compute high-order iterated pion exchange graphs across all angular momentum channels, clarifying convergence issues in nuclear EFT.

## Key findings

- Convergence is satisfactory in most partial waves except low angular momentum channels.
- The perturbative expansion of a 1/r^3 potential is generally well-behaved.
- Results support exploring methods to extend EFT validity, especially in problematic channels.

## Abstract

The classic paper by Fleming, Mehen and Stewart cast doubts on the convergence of spin-triplet nucleon-nucleon partial wave scattering amplitudes when following the proposal of Kaplan, Savage and Wise to construct nuclear effective field theory around the unitary fermion limit with perturbative pion exchange. FMS identified the subclass of iterated one-pion exchange potential graphs as the cause of this poor convergence, which they showed persisted in the chiral limit. Theoretical tools are developed here to compute these Feynman graphs analytically to high order in all angular momentum channels simultaneously, examining the amplitudes computed to seven loops in the $L=J$ channels, and three loops in the coupled $L=J\pm1$ channels. One finds that there is nothing pathological about the perturbative expansion of a $1/r^3$ potential in general, and that the expansion converges satisfactorily in all partial waves except those with the lowest angular momentum, particularly the ${}^3P_0$ and the coupled ${}^3S_1-{}^3D_1$ channels. The results corroborate work by Birse, which suggests possible avenues to explore for improving the range of validity of the EFT expansion.