Studies of quarkonium production and polarisation with early data at ATLAS

This thesis presents a range of studies of the production cross-section and polarisation of quarkonium at the ATLAS experiment at the LHC. As data-taking has not yet begun, these studies are conducted on fully simulated and reconstructed Monte Carlo samples. The main studies of this thesis are an analysis of the production cross-sections of J/psi and Upsilon mesons at ATLAS with 10 pb^-1 of data, along with an analysis of the polarisation of the J/psi and Upsilon that can provide information towards understanding the underlying QCD production mechanism. In addition, we are able to predict the observability of the three chi_c states through its radiative decays to J/psi. I present details of a new method of data-driven muon reconstruction efficiency determination using Inner Detector tracking information in quarkonium decays, and studies into the uses of quarkonia for detector performance, commissioning and data quality monitoring. Using a new method of measuring the polarisation distribution that reduces systematic errors, 10 pb^-1 of data (corresponding to around one week of well-calibrated data-taking) will allow the J/psi polarisation to be measured to an accuracy of 0.02-0.06 (dependent on the level of polarisation itself). This is competitive with the best measurements published from the Tevatron, but with just 1/100th the integrated luminosity and at higher transverse momenta than probed at any previous experiment. Simil ar sensitivities are expected for Upsilon polarisation with 100 pb^-1 of data. The differential production cross-sections at such integrated luminosities are expected to be measured to an accuracy of 1%.