A search for the rare twobody charmless baryonic decay $B^+ \to p \bar\Lambda$ is performed with $pp$ collision data, corresponding to an integrated luminosity of $3 fb^{1}$, collected by the LHCb experiment at centreofmass energies of 7 and 8 TeV. An excess of $B^+ \to p \bar\Lambda$ candidates with respect to background expectations is seen with a statistical significance of 4.1 standard deviations, and constitutes the first evidence for this decay. The branching fraction, measured using the $B^+ \to K^0_{\mathrm S} \pi^+$ decay for normalisation, is \begin{eqnarray} \mathcal{B}(B^+ \to p \bar\Lambda) & = & ( 2.4 ^{+1.0}_{0.8} \pm 0.3 ) \times 10^{7} , \nonumber \end{eqnarray} where the first uncertainty is statistical and the second systematic.
Invariant mass distribution of $ p \overline{\Lambda} $ candidates after full selection. The result of the fit to the data (blue, solid) is shown together with each fit model component, namely the $ B ^+ \rightarrow p \overline{\Lambda} $ signal and the combinatorial background. 
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Invariant mass distribution of $ K ^0_{\mathrm{ \scriptscriptstyle S}} \pi ^+ $ candidates after full selection. The result of the fit to the data (blue, solid) is shown together with each fit model component, namely the $ B ^+ \rightarrow K ^0_{\mathrm{ \scriptscriptstyle S}} \pi ^+ $ signal, the $ B ^0_{{\scriptscriptstyle (} s {\scriptscriptstyle )}} \rightarrow K ^0_{\mathrm{ \scriptscriptstyle S}} h^{+} h^{'}$ partially reconstructed background, and the combinatorial background. The vanishingly small $ B ^+ \rightarrow K ^0_{\mathrm{ \scriptscriptstyle S}} K ^+ $ misidentified crossfeed is not displayed. 
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Animated gif made out of all figures. 
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Summary of systematic uncertainties relative to the measured $ B ^+ \rightarrow p \overline{\Lambda} $ branching fraction. The contributions are split into those that come from the signal decay and those that come from the normalisation decay. The total corresponds to the sum of all contributions added in quadrature. 
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Created on 11 July 2020.