Performance of the ATLAS Transition Radiation Tracker with Cosmic Rays and First High Energy Collisions at LHC

The ATLAS Transition Radiation Tracker (TRT) is the outermost of the three subsystems of the ATLAS Inner Detector at the Large Hadron Collider (LHC) at CERN. It consists of close to 300000 thin-wall drift tubes (straws) providing on average 30 two-dimensional space points with 130 μm resolution for charged particle tracks with |η| < 2 and pT > 0.5 GeV. The TRT is immersed in a 2 T magnetic field generated by the central solenoid, significantly contributing together with the other two Inner Detector sub-systems to the particle momentum reconstruction. 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. As expected from the production of transition radiation (TR), the first collision results show the increase in the number of detected TR-photons for particles with a gamma-factor above 1000, thus enabling the TRT to separate electrons from pions over a momentum range between 1 and 150 GeV. 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 co mmissioned 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 talk will describe the operational experience 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.