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 $300,000$ thin-walled drift tubes (straws) providing on average $30$ two-dimensional space points with $130$~$mu$m resolution for charged particle tracks with $|eta| < 2$ 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. The custom-made radiation-hard 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. 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 and allowed to time-in other ATLAS sub-detectors and level 1 triggers. The very good timing properties of the TRT detector helped to unambiguously identify the first proton-proton collisions in the LHC. 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.