From the sublime to the ridiculous: top physics and minimum bias events in the ATLAS detector at the LHC

This thesis is comprised of tw o separate physics themes, both of which in v olv e the A TLAS detector situated at the LHC at CERN. The rst constituent is a study of the top quark sig- nal in the fully-leptonic channel for proton-proton collisions at a centre-of-mass ener gy of 10 T e V . Here an e v ent counting analysis is performed based on Monte Carlo simulation. This is supplemented by a study into one of the sources of systematic error . The second component is forw ard-backw ard correlations in minimum bias e v ents. F or this, there is a Monte Carlo hadron-le v el comparison of the correlation for 900 Ge V centre-of-mass colli- sions, follo wed by a comparison of Monte Carlo predictions to data for 900 Ge V and 7 T e V collisions. T op Ph ysics A measurement of the fully-leptonic t t cross-section in the three decay channels ee , and e is performed on A TLAS produced fully simulated pseudo-e v ent data-samples. Selec- tion rates for signal and background e v ents consistent with A TLAS results are found along with the kinematic distrib utions of selected e v ents. A calculation of the non-hadronic t t cross-section, based on the measured cross-sections, will then return the theoretical v alue of 217 : 06 pb used to generate the original samples, sho wing the closure of the pseudo-analysis process. A more detailed study is made of the systematic uncertainty arising from v ariations in the initial (ISR) and nal (FSR) state sho wering models, based on the Pythia e v ent generator . A f ast simulation of the A TLAS detector is used with similar object and e v ent selection to the fully simulated case. The ef fect of ISR v ariations on the signal is found to be ne gligible as it is w ashed out in the subsequent decays of the t t system. Ho we v er , the ef fect of FSR is found to cause 5% uncertainty in the selected signal e v ents. In addition, in the main background of each of the selection channels the ef fect of FSR is found to produce v ariations of up to 30% in well populated channels. The v ariations in signal and background measurements will then be used to calculate a ne w estimate of the systematics on the measured t t cross-section for each channe