| Over the last 20 years the ATLAS detector has gone from an idea to an operational detector. In that time thousands of people have contributed by way of research, development, installation, calibration, maintenance, and analysis. This thesis describes my contribution to that effort.;This thesis also examines the expected performance of the ATLAS detector to a tt¯ cross-section measurement using the dilepton decay channel, with either electrons or muons in the final state along with jets and missing transverse energy. The choice of generator and parton shower algorithm used for Monte Carlo generation does affect the final event kinematics, with differences between next-to-leading order and leading order generators having the largest discrepancy. If a transverse momentum cut of 20 GeV is applied to the reconstructed electrons and muons, the trigger efficiency for single electrons is found to be 98.7%, and for single muons 82.1%. Using the single lepton triggers the signal-to-background ratio was found to be 2.6 for the ee channel, 2.7 for the mumu channel, and 5.3 for the emu channel; similar results were found with the dilepton triggers, motivating the desire of having these triggers in the ATLAS trigger menu. A measurement of the tt¯ cross-section using the dilepton channels will be possible with 200 pb-1 of data with spp = 7 TeV, but in the early stages of ATLAS data taking this measurement will be limited by the luminosity uncertainty.;A test beam took place in 2003 with the ATLAS forward calorimeter. With this data the electromagnetic response of FCal1 was confirmed to be within +/- 1.5% from unity in the range of energies from 10 GeV to 200 GeV. The intrinsic energy resolution of the FCal was found to be (27.0 +/- 1.0)%/ E ⊕ (3.5 +/- 0.1)% for electrons and (86.5 +/- 3.6)%/ E ⊕ (7.2 +/- 1.1)% for hadrons (with a simple hadronic weighting scheme applied). Variations in parameters of the topological clustering algorithm were studied, and a suggestion is made that the energy cut parameter of the algorithm be increased from the default 500 MeV to about 1 GeV. A study with electrons and hadrons confirms that using calorimeter shower shape information, a likelihood can be built and used to help identify these two types of showers. Monte Carlo and data comparisons find that the electromagnetic energy scale of the Monte Carlo agrees with the data within +/-2% for 10 GeV to 200 GeV hadrons. However, the Monte Carlo showers tend to be more dense, more narrow, and shorter than the data for high energy hadrons (with energies ≥ 60 GeV). Using the test beam data and Monte Carlo, a study was made on the characteristics of energy deposition at large values of |eta| near the LHC beam pipe. A correction for the energy loss is suggested in order to improve the response by 13% and the energy resolution by 12% for hadrons striking the FCal front face at 4.3 < |eta| < 4.9. |
