Observation of top quark pair production and estimation of W+jets background with ATLAS at the LHC

An analysis has been presented based on an integrated luminosity of 2.9 pb-1 of 7 TeV center-of-mass energy proton-proton collision data in ATLAS at the Large Hadron Collider. The data have been collected since March until September of 2010. Clear signals from W+ jets and Z+ jets events have been seen. The aim of the analysis is to observe top quark pair production in the collision data. Two analysis approaches have been used independently and yielded consistent result. The first approach allowed more multijet background in the data in order to estimate its rate reliably. The other approach introduced a discriminator selection to suppress the multijet background. The dominant background contributions in the signal region are W+jets and multijet production processes. The rates of these backgrounds in the signal region have been estimated with data-driven methods using signal-free, sideband regions as auxiliary measurements. The use of Monte Carlo simulation based normalizations were minimized as much as possible. After applying the signal region event selection cuts 23:9 ± 6:9 background and 35:1 ± 10:3 signal candidate events have been found. Assuming a backgroundonly hypothesis in the data a one sided Gaussian sigma excess of 3.0 over the background has been observed. The size of the excess is consistent with the expectation for top quark pair production in the data. The increasing amount of data at the LHC allows further refinement of the top quark observation analysis. Better understanding of W+jets, Z+jets and QCD multijet events will lead to better background estimations in the top signal region. Observation of top quark pair production, at a level of more than 5 standard deviations, is expected as a result of the analysis of the full proton-proton collision data collected over the year 2010.