The purpose of this study is the specification, design and development of online selection algorithms, and associated framework, for the second-level trigger of the ATLAS experiment, and their evaluation on a large-scale prototype

Casado, M P

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CERN-OPEN-2002-06129/10/2002Online physics-selection software in the ATLAS experiment at LHCM. Pilar Casado, CERNIntroductionThe purpose of this study is the specification, design and development of online selec-tion algorithms, and associated framework, for the second-level trigger of the ATLASexperiment, and their evaluation on a large-scale prototype. The Atlas experimentATLAS (A Toroidal LHC ApparatuS) is a general-purpose physics detector which willoperate at the Large Hadron Collider (LHC) in 2007. One of its main goals is to investi-gate the mechanism of mass generation of existing particles and to search for new phy-sics beyond the Standard Model (SM). The detector will take data produced in proton-proton collisions at 14 TeV center of mass energy for design luminosity of 1034 cm-2s-1,beam crossings being 25 ns apart. A global view of the experiment is presented in Fig. 1.The detector is 22 meters high, 44 meters long, and the overall weight is about 7000 tons.The Trigger systemThe trigger system is an important part of the ATLAS detector which is the responsibleof reducing the event rate at the LHC (~109 Hz) to a manageable level (~100 Hz) for datastorage. In this reduction it is crucial to keep high efficiency for interesting signals whileundesirable events are rejected. To meet this requirement, the ATLAS trigger system isforeseen as a three-level trigger. The first-level (LVL1) trigger is implemented in customhardware, while the second-level (LVL2) and third-level (LVL3 or Event Filter, EF) trig-gers (together referred to as High Level Triggers, HLT) are implemented as computingfarms interconnected by high-speed networks using commodity components. Both LVL2and the EF will execute complex software selection algorithms based on properties ofinteresting processes. Triggering at LHC is significantly more difficult than at today’s more advancedmachines due to both the event rate and the event size. For the last decade, Research andDevelopment (R&D) has been carried on for the Trigger and Data AcQuisition (T/DAQ)systems at LHC, and as a result of this work a logical architecture has been presented,supported by various implementation studies [1].Many important decisions for the ATLAS T/DAQ project are being taken now using arealistic large-scale prototype, which is flexible enough to evaluate different alternatives.The prototype is built on the work done in previous projects, and moreover the wholesystem is being integrated in subsequent phases for the first time. Initial tests haveFig. 1 Overall view of the ATLAS experiment.already been carried out to study the functionality of the data collection and control soft-ware, and to investigate implementation options. In a subsequent step, HLT selectionsoftware is going to be added and assessed using simulated detector data. The results ofthis study will be a fundamental part of the T/DAQ Technical Design Report, foreseenfor publication in 2003. High Level Trigger physics-selection softwareThe physics-selection software is responsible for selecting interesting signals whilebackground events are discarded. The correct design and implementation of this piece ofthe project is fundamental as it will not be possible to recover a rejected event. There-fore, the selection software, together with many other components of the experiment,determine the physics potential of the ATLAS detector.The HLT selection software is formed by four main elements (see Fig. 2):1. The HLT algorithms are used to process the event and obtain abstract objects such aselectrons, jets and muons, which are the basis to make the trigger decision.2. The Event Data Model covers all data entities in the event and their relationshipswith each other. The data entities span from the direct data taken from the detector tothe most abstract objects used in physics analyses.3. The Steering controls the selection software. It arranges the algorithm processing forthe event analysis in the correct order.4. The Data Manager handles the event data during the trigger processing.The LVL2 and EF algorithms are developed in close collaboration with the Physics andEvent-Selection Architecture (PESA) group of the ATLAS collaboration. In previousstudies, only an heterogeneous set of algorithms was developed. Eventually a completeand coherent set of algorithms with realistic input data will be provided and assessed.After the design and implementation of the selection algorithms in the prototype, theselection efficiency of the interested events will be studied as a function of the triggeroutput rate. Relevant detector properties will be taken into account, as for examplethresholds of readout channels. ConclusionsVery important elements of the ATLAS T/DAQ architecture are being defined now usinga realistic large-scale prototype. Initial tests have already been carried out to study thefunctionality of the data collection and control software. In a subsequent step, HLT selec-tion software will be added and assessed using simulated detector data. The results ofthis study will be a fundamental part of the T/DAQ Technical Design Report.References[1] ATLAS high-level triggers, DAQ and DCS Technical Proposal (CERN-LHCC-2000-17)HLTSSWDataManager HLTAlgorithmsEventDataModelSteeringFig. 2 Components of the HLT Selection Software.

Online physics-selection software in the ATLAS experiment at LHC

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