Measurement of Multi-Jet Production in Proton-Proton Collisions at 7 TeV Center-of-Mass Energy and Hadronic Calibration Studies with the ATLAS Detector at CERN

Abstract: The ATLAS experiment – situated at the Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN) in Geneva – took its first collsion data in 2010. Physics topics include finding the Higgs boson, heavy quark physics, and looking for extensions of the Standard Model of Particle Physics such as supersymmetry. In this thesis, inclusive multi-jet production has been studied with the ATLAS detector in proton-proton collisions at a center-of-mass energy of 7 TeV, using an integrated luminosity of 17 nb−1. The anti-kt algorithm with distance parameter ℝ = 0.6 is used to identify jets. The inclusive multi-jet cross section is measured, as well as the ratio ofcross sections for inclusive production of n − 1 and n jets for n ≤ 6. The differential cross sections of the first, second, third and fourth leading jets as a function of transverse momentum, and the differential cross section as a function of the scalar sum of the pT of selected jets, HT, for different jet multiplicities are presented. The ratio of the differential cross section as a function of HT for 3-jet and 2-jet events is also measured. The results are compared to expectations based on leading order QCD, which agree with the data. In addition, a new method for calibrating the hadron response of a segmented calorim-ter is developed and successfully applied to 2004 ATLAS combined beam test data. It is based on a principal component analysis of the calorimeter layer energy deposits, exploiting longitudinal shower development information to improve the measured energyresolution. For pion beams with energies between 20 and 180 GeV, the particle energy is reconstructed within 3% and the energy resolution is improved by 11% to 25% comparedto the response at the electromagnetic scale. Multi-mode optical readout cables for the ATLAS liquid argon calorimeters, about one hundred meters in length, were installed between the main ATLAS cavern and the counting room in the USA15 cavern. Patch cables were spliced onto the ribbons and the fiber attenuation was measured. For 1296 fiber pairs in 54 cables, the average attenuation was 0.69 dB. Only five fibers were found to have losses exceeding 4 dB, resulting in a failure rate of less than 2 per mill. In the ATLAS liquid argon barrel presampler, short circuits consisting of small pieces of dust, metal, etc. can be burned away in situ by discharging a capacitor over the high voltage lines. In a burning campaign in November 2006, seventeen existing short circuits were successfully removed. An investigation on how to implement saturation effects in liquid argon due to high ionization densities resulted in the implementation of the effect in the ATLAS Monte Carlo code, improving agreement with beam test data. The timing structure of hadronic showers was investigated using a Geant4 Monte Carlo. The expected behavior as described in the literature was reproduced, with the exception that some sets of physics models gave unphysical gamma energies from nuclear neutron capture.