Using a $pp$ collision data sample corresponding to an integrated luminosity of $3.0{\mathrm{fb}}^{-1}$, collected by the LHCb detector, we present the first search for the strangeness-changing weak decay ${\mathrm{\Xi }}_b^{-}\to {\mathrm{\Lambda }}_b^0{\pi }^{-}$. No $b$ hadron decay of this type has been seen before. A signal for this decay, corresponding to a significance of 3.2 standard deviations, is reported. The relative rate is measured to be $$\frac{f_{{\mathrm{\Xi }}_b^{-}}}{f_{{\mathrm{\Lambda }}_b^0}}\mathcal{B}({\mathrm{\Xi }}_b^{-}\to {\mathrm{\Lambda }}_b^0{\pi }^{-})=(5.7\pm 1.8_{-0.9}^{+0.8})\times {10}^{-4},$$where $f_{{\mathrm{\Xi }}_b^{-}}$ and $f_{{\mathrm{\Lambda }}_b^0}$ are the $b\to {\mathrm{\Xi }}_b^{-}$ and $b\to {\mathrm{\Lambda }}_b^0$ fragmentation fractions, and $\mathcal{B}({\mathrm{\Xi }}_b^{-}\to {\mathrm{\Lambda }}_b^0{\pi }^{-})$ is the branching fraction. Assuming $f_{{\mathrm{\Xi }}_b^{-}}/f_{{\mathrm{\Lambda }}_b^0}$ is bounded between 0.1 and 0.3, the branching fraction $\mathcal{B}({\mathrm{\Xi }}_b^{-}\to {\mathrm{\Lambda }}_b^0{\pi }^{-})$ would lie in the range from $(0.57\pm 0.21)\%$ to $(0.19\pm 0.07)\%$.

• Received 13 October 2015

DOI:https://doi.org/10.1103/PhysRevLett.115.241801

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© 2015 CERN, for the LHCb Collaboration