First observation of Bs→D∗+s2Xμν decays.

Using data collected with the LHCb detector in proton-proton collisions at a centre-of-mass energy of 7 TeV, the semileptonic decays Bs→D+sXμν and Bs→D0K+Xμν 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 Bs semileptonic rate are B(Bs→D∗+s2Xμν)/B(Bs→Xμν)=(3.3±1.0±0.4)%, and B(Bs→D+s1Xμν)/B(Bs−>Xμν)=(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 Bs decays; we also measure its mass and width.