Search for the decays $B_s^0\to {\tau }^{+}{\tau }^{-}$ and $B^0\to {\tau }^{+}{\tau }^{-}$

A search for the rare decays $B_s^0\to {\tau }^{+}{\tau }^{-}$ and $B^0\to {\tau }^{+}{\tau }^{-}$ is performed using proton–proton collision data collected with the LHCb detector. The data sample corresponds to an integrated luminosity of 3fb${}^{-1}$ collected in 2011 and 2012. The $\tau$ leptons are reconstructed through the decay ${\tau }^{-}\to {\pi }^{-}{\pi }^{+}{\pi }^{-}{\nu }_{\tau }$. Assuming no contribution from $B^0\to {\tau }^{+}{\tau }^{-}$ decays, an upper limit is set on the branching fraction $\mathcal{B}(B_s^0\to {\tau }^{+}{\tau }^{-})<6.8\times {10}^{-3}$ at 95% confidence level. If instead no contribution from $B_s^0\to {\tau }^{+}{\tau }^{-}$ decays is assumed, the limit is $\mathcal{B}(B^0\to {\tau }^{+}{\tau }^{-})<2.1\times {10}^{-3}$ at 95% confidence level. These results correspond to the first direct limit on $\mathcal{B}(B_s^0\to {\tau }^{+}{\tau }^{-})$ and the world’s best limit on $\mathcal{B}(B^0\to {\tau }^{+}{\tau }^{-})$.