First observation of B¯ s 0 →D s2 *+ Xμ-v¯ decays

Using data collected with the LHCb detector in proton–proton collisions at a centre-of-mass energy of 7 TeV, the semileptonic decays B¯s0→Ds*Xμ-v¯ and B¯s0→D0K+Xμ-v¯ are detected. Two structures are observed in the D0K+ mass spectrum at masses consistent with the known Ds1+(2536) and Ds2*(2573)+ mesons. The measured branching fractions relative to the total B¯s0 semileptonic rate are B(B¯s0→Ds2*+Xμ-v¯)/B(B¯s0→D0→Xμ-v¯)=(3.3±1.0±0.4)%, B(B¯s0→Ds1*+Xμ-v¯)/B(B¯s0→D0→Xμ-v¯)= (5.4±1.2±0.5)%, where the first uncertainty is statistical and the second is systematic. This is the first observation of the Ds2*+ state in B¯s0 decays; we also measure its mass and width.