Observation of the electromagnetic radiative decays of the \boldmath{$\hat{I}›(1520)$} and \boldmath{$\hat{I}›(1670)$} to \boldmath{${\hat{I}}^3\hat{I}{\pounds }^0$}

Abstract

Using $(10087\pm 44)\times 10^6$ $J/\ddot{I}ˆ$ events collected with the BESIII detector, we report the first observation of the electromagnetic radiative decays of the $\hat{I}›(1520)$ and $\hat{I}›(1670)$ to ${\hat{I}}^3\hat{I}{\pounds }^0$, with a statistical significance of $16.6\ddot{I}ƒ$ and $23.5\ddot{I}ƒ$, respectively. The ratio of the branching fractions $\frac{\mathcal{B}(\hat{I}›(1520)\to {\hat{I}}^3\hat{I}›)}{\mathcal{B}(\hat{I}›(1520)\to {\hat{I}}^3\hat{I}{\pounds }^0)}$ is determined to be $2.88\pm 0.27(\text{stat.})\pm 0.21(\text{syst.})$, which is in good agreement with flavor SU(3) symmetry. The branching fraction of $\hat{I}›(1520)\to {\hat{I}}^3\hat{I}{\pounds }^0$ is measured to be $\mathcal{B}(\hat{I}›(1520)\to {\hat{I}}^3\hat{I}{\pounds }^0)=(2.95\pm 0.28(\text{stat.})\pm 0.56(\text{syst.}))\times 10^{-3}$, corresponding to a partial width of $\hat{I}“(\hat{I}›(1520)\to {\hat{I}}^3\hat{I}{\pounds }^0)=(47.2\pm 4.5(\text{stat.})\pm 9.0(\text{syst.}))$ keV, which is inconsistent with predictions from the relativized constituent quark model and the Algebraic model. Additionally, we observe a clear resonant structure in the ${\hat{I}}^3\hat{I}{\pounds }^0$ mass spectrum around 1.67 GeV/$c^2$, attributed to the $\hat{I}›(1670)$. The product branching fraction $\mathcal{B}(J/\ddot{I}ˆ\to \bar{\hat{I}}›\hat{I}›(1670)+c.c.)\times \mathcal{B}(\hat{I}›(1670)\to {\hat{I}}^3\hat{I}{\pounds }^0)$ is measured for the first time as $(5.39\pm 0.29(\text{stat.})\pm 0.44(\text{syst.}))\times 10^{-6}$. However, no corresponding structure is seen in the ${\hat{I}}^3\hat{I}›$ mass spectrum, so an upper limit on the product branching fraction $\mathcal{B}(J/\ddot{I}ˆ\to \bar{\hat{I}}›\hat{I}›(1670)+c.c.)\times \mathcal{B}(\hat{I}›(1670)\to {\hat{I}}^3\hat{I}›)$ is determined to be $5.97\times 10^{-7}$ at the 90\% confidence level.