Identification of double b-hadron jets from gluon-splitting with the ATLAS Detector

This thesis presents a study of the substructure of jets containing bhadrons with the purpose of distinguishing between “single” b-jets, where the b-quark originates at the matrix-element level of a physical process (e.g. top, W or Higgs decay) and “merged” b-jets, produced in the parton shower QCD splitting of a gluon into a collimated b quark-antiquark pair. The ability to reject b-jets from gluon splitting is important to reduce the QCD background in Standard Model analyses and in new physics searches that rely on b-quarks in the final state. A separation technique has been designed that exploits the kinematic and topological differences between both kinds of b-jets using track-based jet shape and jet substructure variables combined in a multivariate likelihood analysis. In simulated events, the algorithm rejects 95% (50%) of merged b-jets while retaining 50% (90%) of the single b-jets, although the exact values depend on pT , the jet transverse momentum. The method developed is applied to measure the fraction of double b-hadron jets as a function of jet pT , using 4.7 fb−1 of pp collision data at √s = 7 TeV collected by the ATLAS experiment at the Large Hadron Collider in 2011.