Search for CP violation in single top quark events with the ATLAS detector at LHC

This thesis documents an exciting period in the timeline of the ATLAS experiment where several milestones were achieved. The studies presented summarise part of the ATLAS research program between 2008 and 2013. The first part of this thesis presents some performance studies obtained with the first ATLAS data: global cosmic-ray runs recorded during the commissioning period and first proton-proton collisions runs. The second and main part is dedicated to the measurement of a new observable which is sensitive to $CP$ violation in single top quark decays, using the full 7 TeV dataset collected in 2011 which corresponds to an integrated luminosity of 4.66 $fb^{-1}$. This genuine observable is a forward-backward asymmetry $A_{FB}^N$ which is expected to be zero in the Standard Model (SM) and non-zero values would imply complex phases in the $Wtb$ vertex Lagrangian. The SM provides a purely left-handed vector coupling ($V_L$) in the $Wtb$ vertex, while a more general Lagrangian (an extension of the SM) allows also right-handed vector and left- and right-handed tensor couplings ($V_R$, $g_L$ and $g_R$, respectively). The $W$ helicity fractions measured in top quark pair events have allowed to set limits to the real part of these anomalous couplings. However, they are not sensitive to their complex phases which would imply that the top quark decay has a $CP$-violating component. The $W$ polarisation fractions with respect to a new direction (normal to the plane formed by the top quark spin direction and the $W$ boson momentum) are very sensitive to the imaginary part of one of these couplings, $g_R$. The measurement of this asymmetry involves understanding the process at generator level, the full reconstruction of the top quark, a precise determination of the background processes, applying an unfolding procedure and estimating several systematic uncertainties. The final result is $A_{FB}^N$ = 0.031 $\pm$ 0.065 (stat.) $^{+0.029}_{-0.031}$ (syst.), consistent with the SM prediction given the uncertainty. Assuming a value for the top quark polarisation of P = 0.9, which is the prediction for the t-channel single top production, the first experimental limits on Im(gR) are found to be [-0.20; +0.30] at 95% confidence level.