The ATLAS trigger - high-level trigger commissioning and operation during early data taking

The ATLAS experiment is one of the two general-purpose experiments due to start operation soon at the Large Hadron Collider (LHC). The LHC will collide protons at a centre of mass energy of 14~TeV, with a bunch-crossing rate of 40~MHz. The ATLAS three-level trigger will reduce this input rate to match the foreseen offline storage capability of 100-200~Hz. This paper gives an overview of the ATLAS High Level Trigger focusing on the system design and its innovative features. We then present the ATLAS trigger strategy for the initial phase of LHC exploitation. Finally, we report on the valuable experience acquired through in-situ commissioning of the system where simulated events were used to exercise the trigger chain. In particular we show critical quantities such as event processing times, measured in a large-scale HLT farm using a complex trigger menu

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

Event reconstruction algorithms for the ATLAS trigger.

The ATLAS experiment under construction at CERN is due to begin operation at the end of 2007. The detector will record the results of proton-proton collisions at a center-of-mass energy of 14 TeV. The trigger is a three-tier system designed to identify in real-time potentially interesting events that are then saved for detailed offline analysis. The trigger system will select approximately 200 Hz of potentially interesting events out of the 40 MHz bunch-crossing rate (with 109 interactions per second at the nominal luminosity). Algorithms used in the trigger system to identify different event features of interest will be described, as well as their expected performance in terms of selection efficiency, background rejection and computation time per event. The talk will concentrate on recent improvements and on performance studies, using a very detailed simulation of the ATLAS detector and electronics chain that emulates the raw data as it will appear at the input to the trigger system

The ATLAS trigger : commissioning with cosmic rays.

The ATLAS detector at CERN's LHC will be exposed to proton-proton collisions from beams crossing at 40 MHz. At the design luminosity there are roughly 23 collisions per bunch crossing. ATLAS has designed a three-level trigger system to select potentially interesting events. The first-level trigger, implemented in custom-built electronics, reduces the incoming rate to less than 100 kHz with a total latency of less than 2.5μs. The next two trigger levels run in software on commercial PC farms. They reduce the output rate to 100-200 Hz. In preparation for collision data-taking which is scheduled to commence in May 2008, several cosmic-ray commissioning runs have been performed. Among the first sub-detectors available for commissioning runs are parts of the barrel muon detector including the RPC detectors that are used in the first-level trigger. Data have been taken with a full slice of the muon trigger and readout chain, from the detectors in one sector of the RPC system, to the second-level trigger algorithms and the data-acquisition system. The system is being prepared to include the inner-tracking detector in the readout and second-level trigger. We will present the status and results of these cosmic-ray based commissioning activities. This work will prove to be invaluable not only during the commissioning phase but also for cosmic-ray data-taking during the normal running for detector performance studies

Integration of the trigger and data acquisition systems in ATLAS

During 2006 and the first half of 2007, the installation, integration and commissioning of trigger and data acquisition (TDAQ) equipment in the ATLAS experimental area have progressed. There have been a series of technical runs using the final components of the system already installed in the experimental area. Various tests have been run including ones where level 1 preselected simulated proton-proton events have been processed in a loop mode through the trigger and dataflow chains. The system included the readout buffers containing the events, event building, level 2 and event filter trigger algorithms. The scalability of the system with respect to the number of event building nodes used has been studied and quantities critical for the final system, such as trigger rates and event processing times, have been measured using different trigger algorithms as well as different TDAQ components. This paper presents the TDAQ architecture, the current status of the installation and commissioning and highlights the main test results that validate the system