Search for the rare decays $B^0_{(s)}\to\tau^+\tau^-$

A search for the rare decays $B^0_{(s)}\to\tau^+\tau^-$ is performed using $pp$ collision data, corresponding to an integrated luminosity of 3.0 fb$^{-1}$ collected by the LHCb detector in 2011 and 2012 at centre-of-mass energies of 7 and 8 TeV, respectively. The $\tau$ leptons are reconstructed through the hadronic decay $\tau^+\to\pi^+\pi^-\pi^+\overline\nu_{\tau}$. No signal is observed and a limit is set for the first time on the branching fraction $\mathcal{B}(B_s^0\to\tau^+\tau^-) < 2.4\,(3.0)\times 10^{-3}$ at $90~(95)\%$ confidence level, assuming that the signal is fully dominated by $B^0_S$ decays. If instead the signal is assumed to be fully dominated by $B^0$ decays, the limit is $\mathcal{B}(B^0\to\tau^+\tau^-)<1.0\,(1.3) \times 10^{-3}$ at $90~(95)\%$ confidence level.