Tracking and b-tagging in the ATLAS trigger system

The ATLAS detector will operate in a highly challenging environment that requires a sophisticated trigger strategy. The reconstruction of charged particle trajectories is an essential ingredient to achieve the design goal of reducing the 40 MHz bunch crossing rate to about 200 Hz for physics analysis, selecting potentially interesting events with good efficiency. The online b-jet selection is another important part of the trigger which improves its flexibility and extends the physics programme. More specifically, b-jet acceptance is increased and background reduced by lowering jet transverse energy thresholds at the first trigger level and then applying b-tagging at subsequent levels. This work refers to both track reconstruction and b-jet selection in the on-line system. The readiness for data taking is studied mainly for SiTrack, a tracking algorithm exploiting a combinatorial approach with data coming from the silicon detectors and designed to be deployed at the second trigger level. Performance studies are presented and discussed, profiting from simulated and real data from both cosmic rays and first proton collisions, and the present status of the art is detailed. Then the on-line b-jet selection is presented and its performance is characterized on single jets. The entire chain of algorithms is described as well as the set of triggers to enhance the physics potential and to commission the b-tagging which is inserted in the menu for the LHC running in 2010 and 2011. Improvements in the b-jet selection are discussed, together with a study to reveal the sensitivity of these triggers versus both the beam width and possible displacements in its determination. The gain in acceptance is studied for the tt hadronic channel and the ttH hadronic channel when b-jet triggers are requested to fire, demonstrating a good selection efficiency for these two benchmark signals together with a final output rate manageable for both initial and higher luminosities.