The ATLAS Muon Trigger "Slice"

The ATLAS experiment at the LHC will face the challenge of selecting interesting candidate events in proton-proton collisions at 14 TeV center of mass energy, while rejecting the enormous number of background events. The trigger system architecture is organized in three levels. From a bunch crossing rate of 40 MHz the First level trigger, hardware implemented, will reduce this rate to around <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">~</sub> 75 kHz. Then the software based high level trigger (HLT), composed by the second level trigger and the event filter reduces the rate to <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">~</sub> 200 Hz. HLT is implemented on commercial CPUs using a framework built on the common ATLAS object oriented software architecture. Inclusive trigger selections are used to collect events for the ATLAS physics program; final states with muons are crucial for electroweak precision measurements as well as Higgs and SUSY searches. In this paper we will present the implementation of the muon slice, signal efficiencies, background rejection rates and system performances (execution time,...) for online muon selection based on Monte Carlo simulations and results obtained on real events collected during cosmic data taking runs.