A top quark mass measurement using the stabilized R₃₂ variable with the ATLAS detector

This thesis presents one of the first measurements of the top quark mass at the ATLAS experiment, one of the four large experiments located along the LHC. First discovered in 1995 at the Tevatron experiments CDF and D0, the top quark is the heaviest of the known elementary particles. Due to its very large mass and large Yukawa coupling, it may yet play an unknown role in electroweak symmetry breaking. The large Yukawa coupling also means it has a strong connection to the Higgs field, and thus gives an indirect insight into the mass of the missing Higgs boson. To measure the top quark mass, a kinematic reconstruction is performed using the KLFitter to properly reconstruct the decay products in the lepton + jets channel of a t anti t pair decay. The lepton+jets channel has a significant branching ratio and a moderate background. Using the kinematic information of the event, along with the b-tagging efficiency and rejection, the kinematic likelihood fitter obtains a reconstruction efficiency of about 70 % for each of the separate μ+jets and e+jets channels. From the reconstructed event, a so-called R₃₂ estimator is constructed, which is built from the hadronic hemisphere of the t anti t decay. The R₃₂ is the ratio of the measured hadronic top mass and the reconstructed hadronic W mass. The ratio reduces the significant uncertainty due to the jet energy scale, the largest of the uncertainties on the top mass measurement. A further optimization of the estimator is performed to limit other systematics such as the bJES and initial and final state radiation effects. This is done with the use of the kinematic likelihood and larger p_T cuts. A template fit is performed using a parametrization of the signal and background components of the R₃₂ distribution. The method is cross-checked using ensemble tests. The resulting mass is determined using 1.04 fb^-1 of data collected during the 2011 ATLAS run. A large number of systematics are also quantified. The measurement is performed in both the e+jets and μ+jets channels separately and then combined into a single lepton+jets measurement using the best linear unbiased estimator (BLUE) method. The top mass is measured to be: m^l+jets_top=174.4± 0.9(stat.)±2.5(syst.) GeV/c², with only a 1.5 % relative uncertainty on the top mass, a significant achievement for the early ATLAS data taking.