Resonance-like structure near the $\eta d$ threshold in the $\gamma{d}${$\to$}$\pi^0\eta{d}$ reaction

TL;DR

This study observes a resonance-like structure near the $ ext{η}d$ threshold in the $ ext{γ}d o ext{π}^0 ext{η}d$ reaction, suggesting possible existence of an $ ext{η}NN$ bound or virtual state due to strong attraction.

Contribution

First experimental evidence of a narrow isoscalar resonance-like structure near the $ ext{η}d$ threshold in the $ ext{γ}d o ext{π}^0 ext{η}d$ reaction, supporting predictions of an $ ext{η}NN$ bound or virtual state.

Findings

Strong enhancement near the $ ext{η}d$ threshold in the invariant-mass distribution.

Identification of a narrow isoscalar resonance-like structure with $1^-$.

Mass close to the $ ext{η}d$ threshold with a very narrow width.

Abstract

To investigate the interaction between the nucleon $N$ and nucleon resonance $N(1535)1/2^-$, the $\eta d$ threshold structure connected to the isoscalar $S$-wave $N$-$N(1535)1/2^-$ system has been experimentally studied in the $\gamma{d}${$\to$}$\pi^0\eta{d}$ reaction at incident photon energies ranging from the reaction threshold to 1.15 GeV. A strong enhancement is observed near the $\eta d$ threshold over the three-body phase-space contribution in the $\eta d$ invariant-mass distribution. An analysis incorporating the known isovector resonance $\mathcal{D}_{12}$ with a spin-parity of $2^+$ in the $\pi^0d$ channel reveals the existence of a narrow isoscalar resonance-like structure with $1^-$ in the $\eta d$ system. Using a Flatt\'e parametrization, the mass is found to be $2.427_{-0.006}^{+0.013}$ GeV, close to the $\eta d$ threshold, and the width is $\left(0.029_{-0.029}^{+0.006}{\rm\ GeV}\right)+\left(0.00_{-0.00}^{+0.41}\right) p_\eta c$, where $p_\eta$ denotes the $\eta$ momentum in the rest frame of the $\eta d$ system. The observed structure would be attributed to a predicted isoscalar $1^-$ $\eta NN$ bound state from $\eta NN$ and $\pi NN$ coupled-channel calculation, or an $\eta d$ virtual state owing to strong $\eta d$ attraction.