A sample of 114±11 Bs0→J/ψK-π+ signal events obtained with 0.37 fb-1 of pp collisions at √s=7 TeV collected by the LHCb experiment is used to measure the branching fraction and polarization amplitudes of the Bs0→J/ψK̅ *0 decay, with K̅ *0→K-π+. The K-π+ mass spectrum of the candidates in the Bs0 peak is dominated by the K̅ *0 contribution. Subtracting the nonresonant K-π+ component, the branching fraction of Bs0→J/ψK̅ *0 is (4.4-0.4+0.5±0.8)×10-5, where the first uncertainty is statistical and the second is systematic. A fit to the angular distribution of the decay products yields the K*0 polarization fractions fL=0.50±0.08±0.02 and f∥=0.19-0.08+0.10±0.02.
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