First Observation of anti-B0s → D∗+_s2 X μ− anti-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 anti-B0s → Ds+ X μ− anti-ν and anti-B0s → D0 K+ X μ− anti-ν are detected. Two structures are observed in the D0 K+ mass spectrum at masses consistent with the known D_s1(2536)+ and D∗_s2(2573)+ mesons. The measured branching fractions relative to the total anti-B0s semileptonic rate are B(anti-B0s → D∗+_s2 X μ− anti-ν)/B(anti-B0s → X μ− anti-ν) = (3.3 ± 1.0 ± 0.4)%, and B(anti-B0s → D+_s1 X μ− anti-ν)/B(B0s → X μ− anti-ν) = (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 anti-B0s decays; we also measure its mass and width.