Direct measurement of the top-quark decay width with the ATLAS detector

The top quark is the heaviest known elementary particle. Due to its large mass, the top quark decays before it forms bound states. This makes the top quark a unique particle in the Standard Model. Precise measurements of its properties could be used as tests of the consistency of the Standard Model and potential deviations could point to physics Beyond the Standard Model. This thesis deals with the direct measurement of the top-quark decay width using data collected in proton-proton collisions at centre-of-mass energies of 8 and $13$ TeV with the ATLAS detector at the Large Hadron Collider. The thesis focuses on the more recent measurement at $13$ TeV while the most important highlights of the $8$ TeV measurement are summarised. The decay width of the top quark is extracted from the data using a likelihood fit of distributions of variables sensitive to the top-quark decay width in $t\bar{t}$ pair production. The measurement is performed in a direct way, thus it is less model-dependent compared to indirect methods.