Search for the Rare Decay $B^0\to {\tau }^{+}{\tau }^{-}$ at BABAR

We present the results of a search for the decay $B^0\to {\tau }^{+}{\tau }^{-}$ in a data sample of $(232\pm 3)\times {10}^6$ ${\rm Y}(4S)\to B\overline{B}$ decays using the BABAR detector. Certain extensions of the standard model predict measurable levels of this otherwise rare decay. We reconstruct fully one neutral $B$ meson and seek evidence for the signal decay in the rest of the event. We find no evidence for signal events and obtain $\mathcal{B}(B^0\to {\tau }^{+}{\tau }^{-})<4.1\times {10}^{-3}$ at the 90% confidence level.

Figure 1

Standard model box and penguin processes that can mediate $B^0\to {\ell }^{+}{\ell }^{-}$ ($q=t,c,u$).

Figure 2

Top panel: the $m_{\mathrm{ES}}$ distribution for the hadronic companion $B$ in data (dots) and scaled simulated background (upper histogram) before the signal $B^0\to {\tau }^{+}{\tau }^{-}$ selection is applied; the lower histogram is obtained by subtracting the background from the data. The companion-$B$ yield is $N_{B^0{\overline{B}}^0}=(2.80\pm 0.27)\times {10}^5$. Bottom panel: the $m_{\mathrm{ES}}$ distribution after the signal $B^0\to {\tau }^{+}{\tau }^{-}$ selection. The fitted probability density function (short-dashed line) and its ARGUS component (dashed line) are superimposed on the data (dots). We obtain $N_{\mathrm{obs}}=263\pm 19$ events in the peak.

Figure 3

The $E_{\mathrm{res}}$ distribution in the nominal sample (left panel) and the control sample (right panel) for data (dots), simulated background (solid histogram), and simulated signal (dashed histogram). The simulated signal distribution normalization is arbitrary. All requirements except those for $E_{\mathrm{res}}$ and $NN$ are imposed. The events to the left of the vertical line are selected.