Observation of Z+b

Events coming from the associated production of the Z boson with at least one bottom-quark jet are studied using proton-proton collisions delivered by the LHC at a centre of mass energy of 7\,TeV and recorded by the CMS detector in 2010. The selection of a final state with two leptons, electrons or muons, from the Z, and an additional b-tagged jet is described. The event yield and shapes of kinematic variables in 36\,pb$^{-1}$ of data recorded by CMS using single lepton triggers are compared with Monte Carlo predictions from MADGRAPH. In the data sample, 29 (36) events pass the dielectron (dimuon) and high-purity based b-tagging selections, with a measured purity of $0.83 \pm 0.10$ of Z+b events. For the same b-tagging selection, the ratio $\frac{\sigma(Z+b)}{\sigma(Z+j)}$ is found to be $0.054 \pm 0.016$ ($0.046 \pm 0.014$) in the data for the Z$\rightarrow$ ee (Z$\rightarrow \mu\mu$) selection, compared to $0.043 \pm 0.005$ ($0.047 \pm 0.005$) estimated from NLO theory predictions with the same kinematic selections on the leptons and jets than the selections applied to the data.