First evidence for the two-body charmless baryonic decay $B^0 \to p \bar{p}$

The results of a search for the rare two-body charmless baryonic decays $B^0 \to p \bar{p}$ and $B_s^0 \to p \bar{p}$ are reported. The analysis uses a data sample, corresponding to an integrated luminosity of 0.9 fb$^{-1}$, of $pp$ collision data collected by the LHCb experiment at a centre-of-mass energy of 7 TeV. An excess of $B^0 \to p \bar{p}$ candidates with respect to background expectations is seen with a statistical significance of 3.3 standard deviations. This is the first evidence for a two-body charmless baryonic $B^0$ decay. No significant $B_s^0 \to p \bar{p}$ signal is observed, leading to an improvement of three orders of magnitude over previous bounds. If the excess events are interpreted as signal, the 68.3\% confidence level intervals on the branching fractions are \begin{eqnarray} \cal{B}(\rm{B}^0 \to p \bar{p}) & = & ( 1.47 \,^{+0.62}_{-0.51} \,^{+0.35}_{-0.14} ) \times 10^{-8} \,, \nonumber \\ \cal{B}(\rm{B}_s^0 \to p \bar{p}) & = & ( 2.84 \,^{+2.03}_{-1.68} \,^{+0.85}_{-0.18} ) \times 10^{-8} \,, \nonumber \end{eqnarray} where the first uncertainty is statistical and the second is systematic.