First observation of B⁰s→D*s2+Xμν̄- 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 ⁰ _s→D ⁺ _s+Xμν̄ and B ⁰ _s→D ⁰K ⁺Xμν̄ are detected. Two structures are observed in the D0K+ mass spectrum at masses consistent with the known D _s1(2536) ⁺ and D _s2*(2573) ⁺ mesons. The measured branching fractions relative to the total B-s0 semileptonic rate are B(B ⁰ _s→D _s2*+Xμν̄)/B(B ⁰ _s→Xμν̄)=(3.3±1.0±0.4)%, and B(B ⁰ _s→D ⁺ _s1Xμν̄)/B(B ⁰ _s→Xμν̄)=(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 ⁰ _s decays; we also measure its mass and width.