© 2015 CERN, for the LHCb Collaboration. Published by the American Physical Society under the terms of the »http://creativecommons.org/licenses/by/3.0/» Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Using a pp collision data sample corresponding to an integrated luminosity of 3.0 fb-1, collected by the LHCb detector, we present the first search for the strangeness-changing weak decay Ξb-→Λb0π-. 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 fΞb-fΛb0B(Ξb-→Λb0π-)=(5.7±1.8-0.9+0.8)×10-4,where fΞb- and fΛb0 are the b→Ξb- and b→Λb0 fragmentation fractions, and B(Ξb-→Λb0π-) is the branching fraction. Assuming fΞb-/fΛb0 is bounded between 0.1 and 0.3, the branching fraction B(Ξb-→Λb0π-) would lie in the range from (0.57±0.21)% to (0.19±0.07)%.
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