First observation of B ¯ s 0 → D s 2 ⁎ + X μ − ν ¯ 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¯ s 0 →D s * Xμ-v¯ and B¯ s 0 →D 0 K + Xμ-v¯ are detected. Two structures are observed in the D0K+ mass spectrum at masses consistent with the known Ds1+(2536) and D s2 * (2573) + mesons. The measured branching fractions relative to the total B¯ s 0 semileptonic rate are B(B¯ s 0 →D s2 *+ Xμ-v¯)/B(B¯ s 0 →D 0 →Xμ-v¯)=(3.3±1.0±0.4)%, B(B¯ s 0 →D s1 *+ Xμ-v¯)/B(B¯ s 0 →D 0 →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 D s2 *+ state in B¯ s 0 decays; we also measure its mass and width.