Speaker
Description
The $\eta d$ threshold structure $\mathcal{D}_{\eta d}$ with a spin parity of $1^-$ has been experimentally studied in the $\gamma d \to \pi^0 \eta d$ reaction at $E_\gamma \lt 1.15$ GeV. The structure is connected to the isoscalar $S$-wave two-body system between the nucleon $N$ and nucleon resonance $N(1535)1/2^-$, or the chiral partner candidate of $N$. By incorporating the known isovector resonance $\mathcal{D}_{12}$ with a spin-parity of $2^+$ in the $\pi^0 d$ channel, we decompose the $\eta d$ and $\pi^0 d$ invariant-mass spectra into the $\eta \mathcal{D}_{12}$ and $\pi^0 \mathcal{D}_{\eta d}$ contributions. From the analysis of $\pi^0$ and $\eta$ angular distributions, we determine the spin-parity of the first intermediate $\eta \mathcal{D}_{12}$ and $\pi^0 \mathcal{D}_{\eta d}$ systems. The spin-parity of the major component is found to be $0^-$ in the $\pi^0 \mathcal{D}_{\eta d}$ system, suggesting that some nucleon resonance contributes to $\pi^0 \eta$ photoproduction on the nucleon. It should be noted that $\Delta(1700)3/2^-$ is the main contributor for the elementary $\pi^0 \eta$ photoproduction. The talk will be based on [T. Ishikawa et al., arXiv:2105.10887].
