The ATLAS Trigger Menu: Design and Performance

The ATLAS trigger is a three-tiered system designed to select events of interest for the diverse ATLAS physics program such as Higgs Boson decays. At the same time the rate of events has to be reduced in order to stay within the limitations of available resources such as the output bandwidth, processing power and recording rate. At design capacity, the LHC has a bunch-crossing rate of 40 MHz whereas ATLAS detector has an average recording rate of about 300Hz. The decision to record an event is based on physics signatures found in the event such as energetic jets, leptons or large missing energy. The ATLAS trigger menu consists of several hundred trigger chains which are used during data taking. Each chain defines the selection criteria at each of the three trigger levels for a single physics signature. Additionally, the trigger menu specifies, depending on the physics purpose of the trigger, at which given rate the trigger is running. The continuously increasing luminosities together with optimisations of algorithms and selection definitions also need to be taken into consideration. We give a description of the trigger menu design for the data taking conditions of 2012 based on previous experience gained for various LHC luminosities that spanned many orders of magnitude in the years 2010 and 2011. We describe how the rates of the various triggers are determined and furthermore discuss the testing and validation of the trigger menu before being used for data taking. The trigger monitoring which can be roughly divided into online and offline monitoring. We describe the various tools used both online and offline to monitor the trigger performance and behaviour, so that quick action can be taken in case of problems.