Küçük, H; (2016) Measurement of the inclusive-jet cross-section in proton-proton collisions and study of Quark-Gluon Jet discrimination with the ATLAS experiment at the LHC. Doctoral thesis , UCL (University College London).

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Abstract

Several jet measurements have been performed using data collected during the 2011 and 2012 data taking periods at respectively 7 and 8 TeV proton-proton collision energy by the general-purpose ATLAS experiment at the LHC. These measured are compared to Leading Order and Next Leading Order Monte Carlo predictions. This thesis works focalises in particular on two topics: the measurement of the inclusive double-differential jet cross section, and the separation between jets originated by quarks and those originated by gluons, motivated by its possible use in searches for contact interactions in dijet events. In the first part of thesis, the inclusive double-differential cross section has been measured at √s=8 TeV on low-luminosity runs taken in 2012, as a function of jet transverse momentum in bins of jet rapidity, covering a range of 25 < pT < 134 GeV and |y| < 3.0. Jets are defined using the anti-kt algorithm with two radius parameters of 0.4 and 0.6. The data sample has been collected using two special runs: a no-pile up run, and a low mu run using data-taking periods from A2 an A6. The total luminosity of the no pile-up and the low µ runs is 193.3 nb-1. The reason for using these two runs is to measure the lowpT region for jets, without the very large systematic uncertainties that would be caused by pileup in this kinematic region. This measurement has been merged with the high mu run cross-section measured by Gagik Vardanyan, for the ATLAS publication on the inclusive double-differential cross section measurement at √s=8 TeV. Separating quark-initiated jets from gluon-initiated jets can provide improve the reach of many beyond-the-standard model searches such as compositeness and contact interactions. In order to distinguish quark from gluon jets, a likelihood was built based on the number of charged track in the jet, and on the track width for the analysis of the 2011 dataset. Once jets have been labelled as quark-originated or gluon-originated, dijet events can be labelled as quark-quark, quark-gluon, and gluon-gluon. The detector-level invariant mass spectrum has been measured in bins of y* in 2011 data, and compared to Standard Model QCD or models of contact interactions with different ∧ scales. The tool has been further improved to have a better quark-gluon jet discrimination for the 2012 analysis. Calorimeter-based variables have been added to the track-based ones used in 2011, to extend the rapidity coverage and the discrimination power of the tool. The additional advantage of using 2012 data is the increased luminosity, from the 4.7 ±0.2 f b-1 of 2011 to the 20.3 f b-1 for 2012, allowing the use of data-driven techniques to measure the jet shapes used in the discrimination. As quark and gluon fractions and jet properties are strongly dependent on jet kinematics, all variables are studied in bins of pT and η.

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