Search for decays of $B^0$ mesons into pairs of leptons: $B^0\to e^{+}{e}^{-},$ $B^0\to {\mu }^{+}{\mu }^{-},$ and $B^0\to e^{\pm }{\mu }^{\mp }$

We search for the decay of the $B^0$ meson into a pair of leptons in the suppressed channels $B^0\to e^{+}{e}^{-},$ $B^0\to {\mu }^{+}{\mu }^{-}$ and in the lepton number violating channel $B^0\to e^{\pm }{\mu }^{\mp }$ in a sample of $9.7\times {10}^6\hspace{0.5em}B\overline{B}$ pairs recorded by CLEO detector. No signal is found, and the following upper limits on the branching fractions are established: $\mathcal{B}{(B}^0\to e^{+}{e}^{-})<\mathrm{}8.3\mathrm{}\times {10}^{-7},$ $\mathcal{B}{(B}^0\to {\mu }^{+}{\mu }^{-})<\mathrm{}6.1\mathrm{}\times {10}^{-7},$ $\mathcal{B}{(B}^0\to e^{\pm }{\mu }^{\mp })<\mathrm{}15\mathrm{}\times {10}^{-7}$ at 90% confidence level. A new lower limit on the Pati-Salam leptoquark mass $M_{\mathrm{LQ}}>27\mathrm{}\hspace{0.5em}\mathrm{TeV}$ is established at 90% confidence level.