Commissioning and performance of the ATLAS Transition Radiation Tracker with first high energy pp and Pb-Pb collisions at LHC

The ATLAS Transition Radiation Tracker (TRT) is the outermost of the three sub-systems of the ATLAS Inner Detector at the Large Hadron Collider at CERN. It consists of close to 300000 thin-wall drift tubes (straws) providing on average 30 two-dimensional space points with 0.12-0.15 mm resolution for charged particle tracks with |η| <; 2 and p_T >; 0.5 GeV. Along with continuous tracking, it provides particle identification capability through the detection of transition radiation X-ray photons generated by high velocity particles in the many polymer fibers or films that fill the spaces between the straws. Custom-built analog and digital electronics is optimized to operate as luminosity increases to the LHC design. In this article, a review of the commissioning and first operational experience of the TRT detector will be presented. Emphasis will be given to performance studies based on the reconstruction and analysis of LHC collisions. The first studies of the TRT detector response to the extremely high track density conditions during the November 2010 heavy ion LHC running period will be presented. These studies give interesting insight to the expected performance of the TRT in future high luminosity LHC proton-proton runs.