Observation of the Electromagnetic Radiative Decays of the Λ⁡(1520) and Λ⁡(1690) to 𝛾⁢Σ0

Using $(10087 \pm 44)\times10^{6}$ $J/\psi$ events collected with the BESIII detector, we present a study of resonant structures in the $J/\psi \to \Lambda\bar{\Lambda}\gamma$ and $\Lambda\bar{\Sigma}^{0}\gamma$ processes, reporting the first observation of the electromagnetic radiative decays $\Lambda(1520)\to\gamma\Sigma^{0}$ and $\Lambda(1690)\to\gamma\Sigma^{0}$, with statistical significances of $16.6\sigma$ and $23.2\sigma$, respectively. The ratio of branching fractions $$\frac{\mathcal{B}[\Lambda(1520)\to\gamma\Lambda]}{\mathcal{B}[\Lambda(1520)\to\gamma\Sigma^{0}]} = 3.19 \pm 0.34\,(\mathrm{stat}) \pm 0.19\,(\mathrm{syst}),$$ is determined to be consistent with flavor SU(3) symmetry within two standard deviations. The branching fraction $$\mathcal{B}[\Lambda(1520)\to\gamma\Sigma^{0}] = (2.66 \pm 0.28\,(\mathrm{stat}) \pm 0.50\,(\mathrm{syst})) \times 10^{-3},$$ corresponding to a partial width $$\Gamma[\Lambda(1520)\to\gamma\Sigma^{0}] = (46.8 \pm 6.6\,(\mathrm{stat}) \pm 9.3\,(\mathrm{syst}))~\mathrm{keV},$$ is found to be inconsistent with predictions from the relativized constituent quark model and the Algebraic model. In addition, a clear resonant structure is observed in the $\gamma\Sigma^{0}$ invariant mass spectrum near $1.7~\mathrm{GeV}/c^{2}$, attributed to the $\Lambda(1690)$. The product branching fraction $$\mathcal{B}[J/\psi \to \Lambda\bar{\Lambda}(1690)+\mathrm{c.c.}] \times \mathcal{B}[\Lambda(1690)\to\gamma\Sigma^{0}] = (6.05 \pm 0.42\,(\mathrm{stat}) \pm 0.39\,(\mathrm{syst})) \times 10^{-6},$$ is measured for the first time. However, no corresponding structure is observed in the $\gamma\Lambda$ invariant mass spectrum, and an upper limit on the product branching fraction $$\mathcal{B}[J/\psi \to \Lambda\bar{\Lambda}(1690)+\mathrm{c.c.}] \times \mathcal{B}[\Lambda(1690)\to\gamma\Lambda] < 5.08 \times 10^{-7}$$ is determined at the $90\%$ confidence level.