Measurement of BB Angular Correlations based on Secondary Vertex Reconstruction in Proton-Proton Collisions at 7 TeV

In proton-proton collisions at the CERN Large Hadron Collider (LHC), pairs of beauty quarksare abundantly produced through strong interaction. Studies of the b quark production andthe angular correlations between b and b quarks reveal substantial information on the dynamics of the hard scattering subprocesses within perturbative Quantum Chromodynamics(QCD). Beauty quarks are relevant for physics studies with top quarks and for low massHiggs searches and bb production is a main background for many new physics searches.A measurement of the angular correlations between beauty and anti-beauty hadrons (BB)produced in proton-proton collisions at a center-of-mass energy of 7 TeV is presented. Theregion of small angular separation between the two hadrons is probed for the first time. TheB hadrons are identified by the presence of displaced secondary vertices from their decaysand the angle is measured between the directions from the reconstructed primary-interactionvertex to the secondary vertex positions. To reconstruct BB pairs even at small openingangle, a method based on an iterative secondary vertex finder is introduced.The differential BB production cross section is measured as a function of the opening angleusing data collected with the Compact Muon Solenoid (CMS) detector during 2010 andcorresponding to an integrated luminosity of 3.1 pb−1 . The measurement is performed fordifferent event energy scales, characterized by the leading jet transverse momentum. Thedata exhibit a substantial enhancement of the cross section in the collinear region. Therelative amount of BB pairs produced with small opening angles, compared to the pairswith back-to-back topology, increases with the event energy scale. The measurements arecompared to theoretical predictions based on perturbative QCD calculations. It is observedthat in particular in the collinear region the measured data is not described well by any ofthe Monte Carlo predictions.