The ATLAS Inner Detector Trigger

This presents a challenging environment for detailed track reconstruction since the times available at Level 2 and the EventFilter are around 40 ms and 4 s respectively, for the full processing. Within this time, the data from interesting regions of the Inner Detector have to be read out, unpacked, clustered and converted to the global ATLAS coordinates. Only then can the pattern recognition follow to identify the tracks of charged particles which are then finally used in combination with information from the other ATLAS subdetectors to accept or reject events, according to whether they satisfy certain trigger signatures.

The ATLAS Inner Detector Trigger algorithms have been running online since the start of data taking with proton-proton collisions in December 2009. As the LHC is currently being commissioned the number of filled bunches and the luminosity is significantly lower than the full design luminosity. As a result the trigger algorithms have been running parasitically which enables the study of the tracking performance in terms of efficiency and latency to be studied and compared to the results from running the algorithm on simulated events and the results of commissioning the algorithms using data from cosmic ray muons.

As the understanding of the machine and it's performance increases, so the luminosity will increase and the rejection of events will need to be enabled. For this to happen it is necessary to continue to commission and develop the tracking trigger algorithms using real tracks to ensure minimal loss of interesting data. At this conference the results on the performance of the Inner Detector track trigger algorithms in collisions at centre-of-mass energies of 900GeV and 7TeV will be presented for the first time at an international conference and will show that the efficiency for reconstruction is better than 98% for much of the interesting high transverse momentum region. This is particularly encouraging, since without this level of performance the effectiveness of the trigger would be severely hampered which would in turn have a significant impact on the ability of the ATLAS detector to contribute to the physics goals of the LHC program.