Search for $\cal{CP}$ violation in single top quark events in $pp$ collisions at $\sqrt{s}$ = 7 TeV with the ATLAS detector

This note presents a search for $\cal{CP}$ violation in the decay of single top quarks produced in the $t$-channel where the top quarks are predicted to be highly polarised, using the lepton+jets final state. The data analysed are from proton-proton collisions at $\sqrt{s}$ = 7 TeV recorded by the ATLAS detector at the LHC and consisting of an integrated luminosity of 4.66 fb$^{-1}$. In the Standard Model the couplings of the $Wtb$ vertex are reduced to $V_{\text{L}} \simeq 1$ and $V_{\text{R}} = g_{\text{L,R}} = 0$ at leading order. A forward-backward asymmetry with respect the normal to the plane defined by the $W$ momentum and the top quark polarization, A$^{N}_{FB}$, is used to probe the complex phase of $g_{\text{R}}$. A non-zero value of this asymmetry signals a $\cal{CP}$-violating contribution to the $Wtb$ vertex not accounted for in the Standard Model. The measured value of the asymmetry is A$^{N}_{FB}$ = 0.031 $\pm$ 0.065 (stat.) $^{+0.029}_{-0.031}$ (syst.). Taking this measurement together with the theoretical prediction of the top quark polarisation, the first experimental limits on Im$(g_{\text{R}})$ are determined to be [-0.20, 0.30] at 95% confidence level. Both the measurement and the limit are in good agreement with the Standard Model.