Simulation, Software and First ATLAS Physics.

Updates to the ATLAS fast simulation software are presented which improve the flexibility of its calorimeter and reconstructor objects allowing the easy implementation of new and unforeseen detector effects. Both hot and dead cell types are studied as initial examples and can be seen to be working as expected. A suite of jet finding algorithms known as ‘FastJet’ is introduced and linked into the ATLAS code framework to help unify the description of jets between fast and full simulations as well as in the reconstruction of data. Preparations for an early-data supersymmetry search in the 0-lepton, jets and $E^{miss}_T$ channel are presented which, in the absence of a study of the background systematics, show the validity of the channel in detecting the ‘SU3’ mSUGRA benchmark point above the combined standard model background. The most up-to-date publication observes good agreement between the simulated SM background and data up to values of $E^{miss}_T$ ∼ 100 GeV and $M_{eff}$ ∼ 1500 GeV showing a good understanding of both detector and physics simulation and that the real ATLAS detector is performing as expected. The first ATLAS inclusive measurement of charged particle multiplicities in events with $n_{ch}$ ≥ 1 within the kinematic range pT > 500 GeV and |η| < 2.5 is discussed. With a measured charged particle multiplicity per event and per unit of pseudorapidity at η = 0 of 1.333 ± 0.003(stat.) ±0.040(syst.) being some 5-15% higher than predicted, clear differences are evident between the Monte Carlo predictions and what is observed in the data. Contributions from the author including trigger efficiency studies, a ‘Rivet’ analysis routine, a simple simulation of the MBTS as well as the creation of a fast trigger simulation of the MBTS triggers L1 MBTS 1, L1_BTS_2 and L1_MBTS_1_1 are detailed.