Observation of the electromagnetic radiative decays of the \boldmath{$\Lambda(1520)$} and \boldmath{$\Lambda(1670)$} to \boldmath{$\gamma\Sigma^0$}

Abstract

Using $(10087\pm 44)\times10^6$ $J/\psi$ events collected with the BESIII detector, we report the first observation of the electromagnetic radiative decays of the $\Lambda(1520)$ and $\Lambda(1670)$ to $\gamma\Sigma^0$, with a statistical significance of $16.6\sigma$ and $23.5\sigma$, respectively. The ratio of the branching fractions $\frac{\mathcal{B}(\Lambda(1520)\to\gamma\Lambda)}{\mathcal{B}(\Lambda(1520)\to\gamma\Sigma^0)}$ is determined to be $2.88\pm0.27(\text{stat.})\pm0.21(\text{syst.})$, which is in good agreement with flavor SU(3) symmetry. The branching fraction of $\Lambda(1520)\to\gamma\Sigma^0$ is measured to be $\mathcal{B}(\Lambda(1520)\to\gamma\Sigma^0)=(2.95\pm0.28(\text{stat.})\pm0.56(\text{syst.}))\times 10^{-3}$, corresponding to a partial width of $\Gamma(\Lambda(1520)\to\gamma\Sigma^0)=(47.2\pm4.5(\text{stat.})\pm9.0(\text{syst.}))$ 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 $\gamma\Sigma^0$ mass spectrum around 1.67 GeV/$c^2$, attributed to the $\Lambda(1670)$. The product branching fraction $\mathcal{B}(J/\psi\to\bar\Lambda\Lambda(1670)+c.c.)\times\mathcal{B}(\Lambda(1670)\to\gamma\Sigma^0)$ is measured for the first time as $(5.39\pm0.29(\text{stat.})\pm 0.44(\text{syst.}))\times 10^{-6}$. However, no corresponding structure is seen in the $\gamma\Lambda$ mass spectrum, so an upper limit on the product branching fraction $\mathcal{B}(J/\psi\to\bar\Lambda\Lambda(1670)+c.c.)\times\mathcal{B}(\Lambda(1670)\to\gamma\Lambda)$ is determined to be $5.97\times10^{-7}$ at the 90\% confidence level.