# Neutron-neutron scattering length from the reaction gamma d --> pi^+ nn employing chiral perturbation theory

## TL;DR

This paper proposes a method to accurately determine the neutron-neutron scattering length using gamma d --> pi^+ nn reactions, employing chiral perturbation theory to minimize theoretical uncertainties.

## Contribution

It introduces a high-precision calculation of the transition operator from chiral perturbation theory for this reaction, enabling more accurate extraction of a_nn.

## Key findings

- Theoretical uncertainty can be reduced to 0.1 fm with proper kinematics.
- Chiral perturbation theory provides a reliable framework for this analysis.
- The method improves the precision of neutron-neutron scattering length measurements.

## Abstract

We discuss the possibility to extract the neutron-neutron scattering length a_{nn} from experimental spectra on the reaction gamma d --> pi^+ nn. The transition operator is calculated to high accuracy from chiral perturbation theory. We argue that for properly chosen kinematics, the theoretical uncertainty of the method can be as low as 0.1 fm.