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A combined mass and particle identification fit is used to make the first observation of the decay B̄s0→Ds±K□ and measure the branching fraction of B̄s0→Ds±K□ relative to B̄s0→Ds+π-. This analysis uses 1.2fb-1 integrated luminosity of pp̄ collisions at s=1.96TeV collected with the CDF II detector at the Fermilab Tevatron collider. We observe a B̄s0→Ds±K□ signal with a statistical significance of 8.1σ and measure B(B̄s0→ Ds±K□)/B(B̄s0→Ds+π-)=0.097±0.018(stat)±0. 009(syst). © 2009 The American Physical Society.

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Physical Review Letters