Improved sensitivity in a search for the rare decay ${\mathit{K}}_{\mathit{L}}^0$→${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$

In a search for the decay ${\mathit{K}}_{\mathit{L}}^0$→${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$, no candidates have been observed. We determine the sensitivity from the detected number of CP-violating ${\mathit{K}}_{\mathit{L}}^0$→${\mathrm{\pi }}^{+}$${\mathrm{\pi }}^{\mathrm{-}}$ decays and place a 90% confidence level upper limit on the branching ratio of B(${\mathit{K}}_{\mathit{L}}^0$→${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$,‖${\mathit{M}}_{\mathit{K}}$-${\mathit{M}}_{\mathit{e}\mathit{e}}$‖<6 MeV/${\mathit{c}}^2$)≤4.1×${10}^{\mathrm{-}11}$. This result is a significant improvement over previous measurements, although still above the standard model prediction of 3×${10}^{\mathrm{-}12}$.