Commissioning of the TRT with cosmics rays

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 35 two-dimensional space points with 0.17 mm resolution for charged particle tracks with |η| < 2 and pT > 0.5 GeV. 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. In advance of proton collisions, the TRT has been successfully commissioned with data collected from several million cosmic ray muons, and from beam-halo or beam-splash events from single beams in the LHC. This talk will present the operational status of the detector, the performance in terms of charged particle track reconstruction, the status of the alignment, and contributions to the ATLAS trigger system. As very high momentum muons can also produce transition radiation, the detection of transition radiation will also be presented as well as the expected improvement in electron identifica tion.