Measurement of B hadron angular correlations in association to a Z boson

The angular correlation between B hadrons produced in association with a $Z/\gamma^* \to \ell^+ \ell^-$, where $\ell=\rm{e}$ or $\mu$, is studied using proton-proton collisions recorded by the CMS detector at the LHC center-of-mass energy of 7 TeV. The identification of displaced secondary vertices is made with no use of jets, which allows to study B pair production at small angular separation. The normalized production cross section as function of the angular separation is compared with QCD predictions at tree-level and NLO accuracy. The measurement is performed in the kinematic region defined by two B hadrons with $p_T>15$~GeV and $|\eta|<2$, and two oppositely charged leptons with $p_T>20$~GeV, $|\eta|<2.4$ and dilepton invariant mass $60<\mathrm{M}_{{\ell \ell}}<120$~GeV. This preliminary study provides a test of the standard model prediction and gives further insight into the dynamics of heavy quark production in association with vector bosons.