Muon Event Filter Software for the ATLAS Experiment at LHC

At LHC the 40 MHz bunch crossing rate dictates a high selectivity of the ATLAS Trigger system, which has to keep the full physics potential of the experiment in spite of a limited storage capability. The level-1 trigger, implemented in a custom hardware, will reduce the initial rate to 75 kHz and is followed by the software based level-2 and Event Filter, usually referred as High Level Triggers (HLT), which further reduce the rate to about 100 Hz. In this paper an overview of the implementation of the offline muon recostruction algortihms MOORE (Muon Object Oriented REconstruction) and MuId (Muon Identification) as Event Filter in the ATLAS online framework is given. The MOORE algorithm performs the reconstruction inside the Muon Spectrometer providing a precise measurement of the muon track parameters outside the calorimeters; MuId combines the measurements of all ATLAS sub-detectors in order to identify muons and provides the best estimate of their momentum at the production vertex. In the HLT implementation the muon reconstruction can be executed in "full scan mode", performing pattern recognition in the whole muon spectrometer, or in the "seeded mode", taking advantage of the results of the earlier trigger levels.An estimate of the execution time will be presented along with the performances in terms of efficiency, momentum resolution and rejection power for muons coming from hadron decays and for fake muon tracks, due to accidental hit correlations in the high background environment of the experiment.