Search for the weak radiative decay ${\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}\gamma$ at BESIII

The Cabibbo-allowed weak radiative decay ${\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}\gamma$ has been searched for in a sample of ${\mathrm{\Lambda }}_c^{+}{\bar{\mathrm{\Lambda }}}_c^{-}$ pairs produced in $e^{+}{e}^{-}$ annihilations, corresponding to an integrated luminosity of $4.5{\mathrm{f}\mathrm{b}}^{-1}$ collected with the BESIII detector at center-of-mass energies between 4.60 and 4.70 GeV. No excess of signal above background is observed, and we set an upper limit on the branching fraction of this decay to be $\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}\gamma )<4.4\times {10}^{-4}$ at a confidence level of 90%, which is in agreement with Standard Model expectations.