Commissioning and Performance of the ATLAS Transition Radiation Tracker with high energy 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 0.13 mm resolution for charged particle tracks with |eta|< 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 velocity particles in the many polymer fibers or films that fill the spaces between the straws. The front-end electronics implements two thresholds to discriminate the signals: a low threshold (~300 eV) for registering the passage of minimum ionizing particles, and a high threshold (~6 keV) to flag the absorption of transition radiation X-rays. In this talk, 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 proton-proton collisions. A comparison of the TRT response to two very different center of mass energy collisions (900 GeV and 7000 GeV) will be presented here for the first time.