# The Beam-Target Helicity Asymmetry for $\vec{\gamma} \vec{n} \rightarrow \pi^- p$ in the {\bf{$N^*$} Resonance Region

## TL;DR

This paper presents the first measurements of beam-target double-polarization asymmetries in gamma-neutron interactions producing pions, providing new insights into nucleon resonance structures and updating resonance photo-couplings.

## Contribution

It reports novel polarization asymmetry data in gamma-neutron reactions and incorporates these into partial wave analyses, significantly revising resonance parameters.

## Key findings

- First measurement of beam-target asymmetries in gamma-neutron reactions
- Data led to significant updates in nucleon resonance photo-couplings
- Results improve understanding of nucleon resonance structures

## Abstract

We report the first beam-target double-polarization asymmetries in the $\gamma + n(p) \rightarrow \pi^- + p(p)$ reaction spanning the nucleon resonance region from invariant mass $W$= $1500$ to $2300$ MeV. Circularly polarized photons and longitudinally polarized deuterons in $H\!D$ have been used with the CLAS detector at Jefferson Lab. The exclusive final state has been extracted using three very different analyses that show excellent agreement, and these have been used to deduce the {\it{E}} polarization observable for an effective neutron target. These results have been incorporated into new partial wave analyses, and have led to significant revisions for several $\gamma nN^*$ resonance photo-couplings.