Measurement of the B0s → J/ψ anti-K*0 Branching Fraction and Angular Amplitudes

A sample of 114±11 B0s → J/ψ K− 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 B0s → J/ψ anti-K*0 decay, with anti-K*0 → K− π+. The K− π+ mass spectrum of the candidates in the B0s peak is dominated by the anti-K*0 contribution. Subtracting the nonresonant K− π+ component, the branching fraction of B0s → J/ψ anti-K*0 is (4.4 (−0.4)(+0.5) ± 0.8) x 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.