The online muon identification with the ATLAS experiment at the LHC

The Large Hadron Collider (LHC) at CERN is a proton-proton collider providing the highest energy and the highest instantaneous luminosity ever achieved at a hadron collider. During 2012 runs, bunch crossings occurred every 50 ns. The online event selection system should reduce the event recording rate down to a few 100 Hz in the harsh conditions with many overlapping pp collisions occurring in one bunch crossing. Muons provide a clear signature for many physics processes and often play an important role for example in the 2012 discovery of a Higgs boson. The ATLAS experiment deploys a three-level online processing scheme. The Level 1 (L1) muon trigger system gets its input from fast muon trigger detectors. Fast sector logic boards select muon candidates, which are passed via an interface board to the central trigger processor and then to the High Level Trigger (HLT). The muon HLT is purely software-based and encompasses a Level 2 (L2) and an Event Filter (EF) trigger for a staged trigger approach. It has access to the data of the precision muon detectors and other detector elements to refine the muon hypothesis. At L2, trigger-specific algorithms are used to increase processing speed by making use of look-up tables and simple algorithms. The EF muon triggers benefit from offline reconstruction software to obtain a precise determination of the track parameters. Algorithms based on two different approaches, namely inside-out and outside-in tracking, are used with a conditional OR to obtain maximal efficiency with minimal processing time. A full overview of the ATLAS muon trigger system is given, the three years of running experience are summarised and a report about online performance such as processing time, trigger rates, efficiency and resolution is given.