Study of b quark pair production mechanisms in pp collisions with the CMS experiment at LHC.

摘要

Bottom quarks produced in high energy hadron collisions provide a viable system for testing Quantum ChromoDynamics (QCD), the theory of strong interactions. In next-to-leading order perturbative QCD, bottom-antibottorn (bb¯ ) quark pair production is modeled by three distinct production mechanisms: flavor creation, flavor excitation and gluon splitting, and the fraction in which each mechanism occurs is reflected in the production cross section. These fractions can be estimated experimentally by measuring the angular correlations between jets of particles arising from b and b¯ quarks and used to test the predictions of perturbative QCD.;This dissertation describes expectations for determination of the fraction of production mechanisms in inclusive bb¯ pair production in proton-proton collisions at the upcoming Large Hadron Collider (LHC) at 10 TeV with the Compact Muon Solenoid (CMS) detector. The characteristics of b hadrons, such as long flight time and large muon branching fraction, make jets from b quarks distinguishable from jets arising from gluons and lighter quarks and allow for experimental identification of b jets. The analysis presented in this dissertation is based on an event sample of 8pb-1 integrated luminosity from a large scale Monte Carlo simulation of CMS data. A likelihood ratio based selection method is devised to extract proton-proton collision events containing two b jets identified using b jet discriminators that will be derived from real collision data. From the angular correlations between the two b jets and the PYTHIA Monte Carlo modeling of production mechanisms, the fractions are extracted with an overall relative uncertainties of ∼20% dominated by systematic errors. This result will provide an early test of perturbative QCD in a new and highest ever energy regime when the LHC collision data become available in 2010.