Measurement of the Flavour Composition of Dijet Events in Proton-Proton Collisions at $\sqrt{s} = 7\,$TeV with the ATLAS Detector at the LHC

This dissertation presents a measurement of the flavour composition of dijet events produced in proton-proton collisions at a center-of-mass energy $\sqrt{s} = 7\,$TeV. The data is reconstructed with the ATLAS detector at the LHC and the full data sample of 2010 is used. Three types of jet flavours, bottom, charm and light, are distinguished and thus six possible flavour combinations are identified in the dijet events. Kinematic variables, based on the properties of displaced decay vertices and optimised for jet flavour identification, are employed in an event-based likelihood fit. Multidimensional templates derived from Monte Carlo are used to measure the fractions of the six dijet flavour states as functions of the leading jet transverse momentum in the range $40\,$GeV to $500\,$GeV and jet rapidity $|y| < 2.1$. The fit results agree with the predictions of leading- and next-to-leading-order calculations, with the exception of the dijet fraction composed of a bottom and a light flavour jet, which is underestimated by all models at large transverse jet momenta. In addition, the difference between bottom jet production rates in leading and subleading jets is measured and found to be consistent with the next-to-leading-order predictions. The ability to identify jets containing two $b$-hadrons is demonstrated and used to identify a deficiency in the predictions of leading order Monte Carlo for the contribution of these jets to dijet production.