The top quark is unique as compared to all other quarks because it decays before hadronized and
is one of the massive known elementary particles. The top quark is an intriguing particle,
massive and fleeting in which continues to provide a broad range of interesting research topic.
This thesis has four parts, and the first chapter contains the literature review which includes the
brief overview of standard model, basic terminology and some experimental parameters
regarding particle physics. The second one includes the detail description of the different subsystem of the ATLAS detector
Chapter three focuses on the different production modes of top quark like single top quark and
top quark produced in pair.
Chapter four discuss the samples, event selection and tt̄ cross-section measurement at their
respective energies of 7,8 and 12 TeV. The measured cross-section and uncertainties are
presented and compared to theoretical prediction based on standerd model.