The ATLAS Muon Trigger ``Slice''

The ATLAS experiment at the LHC will face the challenge of selecting interesting candidate events in pp 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 an interaction rate of 1 GHz the First Level trigger, hardware implemented, will reduce this rate to around ~100 kHz. Then the software based High Level Trigger (HLT), composed by the Second Level and the Event Filter reduces the rate to ~ 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 programme; 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 MonteCarlo simulations and results obtained on real events collected during cosmic data taking runs.