Resonance-like structure near the $\hat{I}\cdot d$ threshold in the ${\hat{I}}^{3}d${$\to$}$\ddot{I}{€}^0\hat{I}\cdot d$ reaction

Abstract

To investigate the interaction between the nucleon $N$ and nucleon resonance $N(1535)1/2^{-}$, the $\hat{I}\cdot d$ threshold structure connected to the isoscalar $S$-wave $N$-$N(1535)1/2^{-}$ system has been experimentally studied in the ${\hat{I}}^{3}d${$\to$}$\ddot{I}{€}^0\hat{I}\cdot d$ reaction at incident photon energies ranging from the reaction threshold to 1.15 GeV. A strong enhancement is observed near the $\hat{I}\cdot d$ threshold over the three-body phase-space contribution in the $\hat{I}\cdot d$ invariant-mass distribution. An analysis incorporating the known isovector resonance ${\mathcal{D}}_{12}$ with a spin-parity of $2^{+}$ in the $\ddot{I}{€}^{0}d$ channel reveals the existence of a narrow isoscalar resonance-like structure with $1^{-}$ in the $\hat{I}\cdot d$ system. Using a Flatté parametrization, the mass is found to be $2.427_{-0.006}^{+0.013}$ GeV, close to the $\hat{I}\cdot d$ threshold, and the width is $\left(0.029_{-0.029}^{+0.006}\mathrm{GeV}\right)+\left(0.00_{-0.00}^{+0.41}\right)p_{\hat{I}}\cdot c$, where $p_{\hat{I}}\cdot$ denotes the $\hat{I}\cdot$ momentum in the rest frame of the $\hat{I}\cdot d$ system. The observed structure would be attributed to a predicted isoscalar $1^{-}$ $\hat{I}\cdot NN$ bound state from $\hat{I}\cdot NN$ and $\ddot{I}€NN$ coupled-channel calculation, or an $\hat{I}\cdot d$ virtual state owing to strong $\hat{I}\cdot d$ attraction.

6 pages, 3 figures