Measurement of Branching Fraction and Transition Magnetic Moment of the Hyperon Dalitz Decay ${\mathrm{\Sigma }}^0\to \mathrm{\Lambda }{e}^{+}{e}^{-}$

Based on a data sample of 10 billion $J\mathrm{/}\psi$ events collected with the BESIII detector operating at the BEPCII collider, the Dalitz decay ${\mathrm{\Sigma }}^0\to \mathrm{\Lambda }{e}^{+}{e}^{-}$ is studied experimentally for the first time. The ${\mathrm{\Sigma }}^0$ hyperons are produced through the process $J\mathrm{/}\psi \to {\mathrm{\Sigma }}^0{\bar{\mathrm{\Sigma }}}^0$ and analyzed using a double-tag method. The absolute branching fraction is measured to be $\mathcal{B}({\mathrm{\Sigma }}^0\to \mathrm{\Lambda }{e}^{+}{e}^{-})=(6.34\pm {0.25}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}}\pm {0.23}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.}})\times {10}^{-3}$. This result shows a $2\sigma$ discrepancy from the theoretical calculation quoted in the PDG, where the uncertainties are statistical and systematic, respectively. In addition to the branching fraction, the transition magnetic moment $\mu$ is determined to be $(1.74\pm {0.03}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}}\pm {0.09}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.}}){\mu }_N$, where ${\mu }_N=e\mathrm{/}(2m_p)$ represents the nucleon magnetic moment, providing valuable insight into the intrinsic structure of the ${\mathrm{\Sigma }}^0$ hyperon.