Direct top quark decay width measurement in the $t \bar{t}$ lepton+jets channel at $\sqrt {s}$ = 8 TeV with the ATLAS experiment

The top quark $t$ plays an essential role in the field of elementary particle physics - in particular due to its exceptional properties comprising a large mass, which approximately equals the mass of a tungsten atom, and an enormously short lifetime. This thesis is devoted to the study of a fundamental property of the top quark - its decay width. Its value is predicted by the established Standard Model of particle physics and deviations may hint at yet unknown physics beyond this model. A direct measurement of the decay width of the top quark is presented. The analysis is based on $t \bar{t}$ events in the lepton+jets decay channel using data taken in proton-proton collisions at a centre-of-mass energy of $\sqrt {s}$ = 8 TeV. The dataset was recorded in 2012 with the ATLAS detector at the Large Hadron Collider at CERN and corresponds to an integrated luminosity of 20.2 fb$^{-1}$. The decay width of the top quark is extracted by utilising a template fit to one-dimensional distributions of kinematic quantities, performed simultaneously in the hadronic and the leptonic decay branch of $t \bar{t}$ events. Since the measurement is a direct measurement of the top quark decay width, it is less model-dependent in comparison to indirect approaches. This enables the measurement to probe a broad class of Standard Model extensions.