Search for the lepton number violating process ${\mathrm{\Xi }}^{-}\to {\mathrm{\Sigma }}^{+}{e}^{-}{e}^{-}+c\mathrm{.}c\mathrm{.}$

We present a search for the lepton number violating decay ${\mathrm{\Xi }}^{-}\to {\mathrm{\Sigma }}^{+}{e}^{-}{e}^{-}+c\mathrm{.}c\mathrm{.}$ with $(10087\pm 44)\times {10}^6$ $J\mathrm{/}\psi$ events collected by the BESIII detector at the BEPCII collider. Employing a blind analysis strategy, no significant signal is observed above the expected background yield. The upper limit on the branching fraction is determined to be $\mathrm{B}\mathrm{r}({\mathrm{\Xi }}^{-}\to {\mathrm{\Sigma }}^{+}{e}^{-}{e}^{-}+c\mathrm{.}c\mathrm{.})<2.0\times {10}^{-5}$ at the $90\mathrm{\%}$ confidence level.