Commissioning of the ATLAS Transition Radiation Tracker with Cosmic Rays and single LHC beams

The ATLAS Transition Radiation Tracker (TRT) is the outermost of the three subsystems 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 35 two-dimensional space points with 0.17 mm resolution for charged particle tracks with pT > 0.5 GeV within |η| < 2. Transition radiation X-rays, generated by particles with γ > 1000 in the special material between the straws, are absorbed in the Xenon based gas mixture and give rise to large signal amplitudes. 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. The talk will report on the commissioning and first operational experience of the TRT detector and its sophisticated off-detector systems for cooling, active gas, low- and high- voltage, data acquisition and controls. Initial performance studies, based on the reconstruction and analysis of several million cosmic ray tracks, and from beam-halo or beam-splash events from single beams in LHC, will be described.