Measurement of the Absolute Branching Fraction for ${\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }{e}^{+}{\nu }_e$

We report the first measurement of the absolute branching fraction for ${\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }{e}^{+}{\nu }_e$. This measurement is based on $567{\mathrm{pb}}^{-1}$ of $e^{+}{e}^{-}$ annihilation data produced at $\sqrt{s}=4.599\mathrm{GeV}$, which is just above the ${\mathrm{\Lambda }}_c^{+}{\overline{\mathrm{\Lambda }}}_c^{-}$ threshold. The data were collected with the BESIII detector at the BEPCII storage rings. The branching fraction is determined to be $\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }{e}^{+}{\nu }_e)=[3.63\pm 0.38(\text{stat})\pm 0.20(\text{syst})]\%$, representing a significant improvement in precision over the current indirect determination. As the branching fraction for ${\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }{e}^{+}{\nu }_e$ is the benchmark for those of other ${\mathrm{\Lambda }}_c^{+}$ semileptonic channels, our result provides a unique test of different theoretical models, which is the most stringent to date.

Figure 1

Fits to the $M_{\mathrm{BC}}$ distributions for different ST modes. The points with error bars are data, the (red) solid curves show the total fits, and the (blue) dashed curves are the background shapes.

Figure 2

(a) Scatter plot of $M_{p{\pi }^{-}}$ versus $U_{\text{miss}}$ for the ${\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }{e}^{+}{\nu }_e$ candidates. The area between the dashed lines denotes the $\mathrm{\Lambda }$ signal region and the hatched areas indicate the $\mathrm{\Lambda }$ sideband regions. (b) Fit to the $U_{\text{miss}}$ distribution within the $\mathrm{\Lambda }$ signal region. The points with error bars are data, the (red) solid curve shows the total fit, and the (blue) dashed curve is the background shape.