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 ${\mathrm{\Lambda }}_c^{+}{\overline{\mathrm{\Lambda }}}_c^{-}$ pairs produced in $e^{+}{e}^{-}$ annihilations, corresponding to an integrated luminosity of $4.5{\mathrm{fb}}^{-1}$ 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.

Figure 1

The lowest-order Feynman diagram of the ${\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}\gamma$ decay.

Figure 2

Distributions of $M_{\mathrm{BC}}$ for the different ST channels of the ${\overline{\mathrm{\Lambda }}}_c^{-}$ at $\sqrt{s}=4.600\mathrm{GeV}$. The signal shape of the ${\overline{\mathrm{\Lambda }}}_c^{-}$ is described by the simulated shape convolved with a Gaussian resolution function and the background is modeled with an ARGUS function. The points with error bars represent data. The (red) solid curves indicate the fit results and the (blue) dashed curves describe the background shapes.

Figure 3

(a) The comparison of the $M_{\mathrm{BC}}^{\mathrm{sig}}$ distributions of the candidate events for ${\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}\gamma$ between data and MC simulation. The two red arrows indicate the signal region. (b) The profile log-likelihood ratio curve versus $\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{+}\gamma )$. The intersection of the curve and horizontal line indicates the upper limit of the BF at the 90% C.L., where the black solid curve is the scan result with systematic uncertainties.