The ATLAS Muon Trigger Slice

At LHC the 40 MHz bunch crossing rate dictates a high selectivity of the ATLAS trigger, which has to keep the full physics potential of the experiment with a limited DAQ and storage capability. A three-level trigger has been designed to reduce the input rate to about 100 Hz. The capability to select events with muons at an early stage of the trigger system is crucial to cope with the expected rates. In this paper we will describe the trigger of the muon system (muon vertical slice). The first level of trigger, implemented in a custom hardware, will use measurements from the trigger chambers of the muon spectrometer to reduce the initial rate to 100 kHz. It selects muons with transverse momentum above programmable thresholds with a coarse evaluation of the muon position (regions of interest, RoIs). The RoIs are passed to the second trigger level in which fast algorithms reconstruct muons with high transverse momentum combining full granularity information from trigger and precision chambers of the muon spectrometer. A third trigger level, the event filter will access the full event to further reduce the rate to about 100 Hz. To accomplish this, components of physics analysis traditionally deferred to offline physics analysis, are embedded within the on-line trigger system. Along with the muon vertical slice implementation and description we will also present the expected performance relative to signal efficiencies, background rejection and execution time.