Performance of the ATLAS Transition Radiation Tracker with cosmic rays and first high energy collisions at the 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 (LHC) at CERN. It consists of close to 300000 thin-walled drift tubes (straws) providing on average 30 two-dimensional space points with 130 μm resolution for charged particle tracks with |η| <; 2 and p_T >; 0.5 GeV. Along with continuous tracking, it provides particle identification 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 custom-designed radiation-hard front-end electronics implements two thresholds to discriminate the signals: a low threshold (~300 eV) for registering the passage of charged particles, and a high threshold (~6 keV) to flag the absorption of transition radiation x-rays. The TRT was successfully commissioned with data collected from several million cosmic ray muons. A special Fast-OR signal from the front-end ASICs was utilized to build a level 1 trigger for cosmic rays traversing the ATLAS Inner Detector which allowed other ATLAS sub-detectors and level 1 triggers to align their timing. The very good timing properties of the TRT detector helped discriminate proton-proton collisions from beam background. This proceeding will describe the operational experiences gained with the ATLAS TRT detector during the commissioning with cosmic rays, and will highlight the excellent performance for charged particle tracking and electron identification based on transition radiation obtained from the first proton-proton collision data at 900 GeV and 7 TeV center-of-mass energy.