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 ∼30 measurements with position resolution of about 120 μm for charged particle tracks with |η| < 2 and pT > 0.5 GeV. Along with continuous tracking, it provides particle identification capability through the detection of transition radiation, X-ray photons generated by high momentum particles in the many polymer fibers or films that fill the spaces between the straws. Custom-built analog and digital electronic read-out are optimized to operate at the LHC design luminosity. 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 proton-proton collision data collected at the LHC. In addition, the response of the TRT detector to the extremely high track density conditions encountered during the first heavy ion LHC collisons will be presented.