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With the 25 ns time between bunch crossings delivered by the LHC, ATLAS will take data at a rate of 40 MHz. The trigger system is designed to filter out most of the uninteresting (minimum bias) events, while being very efficient in keeping those events that ATLAS was designed to investigate. Uninteresting here is a subjective term, that may change over the years of running ATLAS, when understanding of the physics at the LHC increases. Thus a flexible system is needed, which can accommodate the unexpected. The trigger system is divided in three levels. The first level (L1) is a hardware based trigger system. It is designed to bring the event rate down to 75 kHz. Level 2 and the Event Filter consist of computer farms, and bring the total event rate down to about 2.5 kHz and then to the final rate 200Hz, with an average event size of 1.3 MB.

The data acquisition (DAQ) system consists of all the elements that transport the data of the detector, keep data from those events that pass the trigger requirements, collect all subdetector data from the same bunch crossing and store it on tape.

The Detector Control System (DCS) allows operators to monitor the state of the detector and turn on/off the different subsystems or change operating parameters. The DCS allows safe operation of the detector. Furthermore, it facilitates the communication with the LHC machine.

2.6.1

L1 Trigger

The L1 trigger is based only on information from the calorimeter and muon spectrometer, as can be seen in Figure 2.17. The Central Trigger Processor (CTP) takes information from all calorimeter systems, (EM, Tile, HEC, FCAL), but at a coarser granularity compared to the offline reconstruction. With this information it looks for indications of jets, electrons, photons or hadronic taus with large transverse energy ET (= E sin θ), for events with a large total ET summed over all calorimeter cells and for events with a large ETmiss, the vector sum of all transverse energy measured in the detector. For the jets, there is also the possibility to cut on a highP ET over all jets. An isolation requirement (minimum distance to other high- ET objects) can be placed on the electrons, photons and taus. The CTP cuts on the number of (programmable) threshold crossings per object per bunch crossing.

The L1 muon trigger gets its information from dedicated trigger chambers (The TGCs in the endcap and the RPCs in barrel) in the muon spectrometer. The trigger is based on coincidence in stations within a road. The road is defined as the path of a muon through the spectrometer. The pT threshold is implemented as the allowed width of the path: high pT muon tracks are bent less by the magnetic field, and the allowed path is narrow. Low pT tracks are bent more, and a wider path is allowed for the low pT threshold. The information from the trigger chamber is sent to dedicated hardware that serves as an interface between

2.6. TRIGGER, DATA ACQUISITION AND DETECTOR CONTROL 53

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Detector front-ends L2 trigger Central trigger

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Timing, trigger and control distribution

Calorimeters Muon detectors

DAQ L1 trigger

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Figure 8.2: Block diagram of the L1 trigger. The overall L1 accept decision is made by the central trigger processor, taking input from calorimeter and muon trigger results. The paths to the detector front-ends, L2 trigger, and data acquisition system are shown from left to right in red, blue and black, respectively.

8.2 The L1 trigger

The flow of the L1 trigger is shown in figure8.2. It performs the initial event selection based on information from the calorimeters and muon detectors. The calorimeter selection is based on in- formation from all the calorimeters (electromagnetic and hadronic; barrel, end-cap and forward). The L1 Calorimeter Trigger (L1Calo) aims to identify high-ETobjects such as electrons and pho- tons, jets, andτ-leptons decaying into hadrons, as well as events with large Emiss

T and large total transverse energy. A trigger on the scalar sum of jet transverse energies is also available. For the electron/photon andτ triggers, isolation can be required. Isolation implies that the energetic particle must have a minimum angular separation from any significant energy deposit in the same trigger. The information for each bunch-crossing used in the L1 trigger decision is the multiplicity of hits for 4 to 16 programmable ET thresholds per object type.

The L1 muon trigger is based on signals in the muon trigger chambers: RPC’s in the barrel and TGC’s in the end-caps. The trigger searches for patterns of hits consistent with high-pTmuons originating from the interaction region. The logic provides six independently-programmable pT thresholds. The information for each bunch-crossing used in the L1 trigger decision is the multiplicity of muons for each of the pT thresholds. Muons are not double-counted across the different thresholds.

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Figure 2.17: Block diagram of the L1 Trigger system

the muon system and the CTP. This interface hardware combines the number of threshold crossings in barrel and endcap, taking care to remove double counting by overlapping trigger chamber. The interface hardware sends the number of threshold crossings on to the CTP. Six distinct thresholds for the muon pT can be programmed, three low pT (6-9 GeV) and three high pT (9-25 GeV). The coincidences between the hits in the different layers within a path are determined by programmable custom electronics on (in the barrel and endcap) and near (endcap) the detector systems.

The L1 trigger decision is based solely on the number of times a certain threshold has been crossed. But the L1 trigger system also passes on the location in η and φ where this occurred. This information is used to built Regions of Interest (RoIs), which are used as seeds for the L2 trigger processing, where the full detector information is available, within the RoIs. This drastically reduces the amount of data that is needed for the L2 processing.

The CTP uses the information of the number of threshold crossings in each trigger condi- tion, plus the flags for events exceeding a ΣET or ETmiss threshold. The list of trigger items can be up to 256 items long. These items also contain extra triggers, such as beam pick-up and random triggers. The CTP also receives the LHC clock signal, and through the Trigger Timing and Control chip distributes this clock to the all the detector systems.

2.6.2

High Level Trigger and Data Acquisition

The Data Acquisition system is responsible for getting the data selected by the L1 trigger from the detector through the High Level Trigger (HLT) to storage. A schematic overview of the DAQ/HLT is shown in Figure 2.18. When a signal is found in a sub detector, it is stored in buffers which are located on the front end electronics boards on or near the detector. Data

from specific detectors is sent to the CTP as described above. When a L1 accept signal is received, the data is transferred from the detector front end to the Read Out Drivers (ROD), which package and format the data in a detector specific manner such that it can be stored in the Read Out Buffers (ROB), which are located in the ROS (Read Out System) PCs. In the meantime, the L1 RoI information is sent to the RoI builder. The output RoI structure is sent to the L2 supervisor and used by the L2 trigger as seed for the selection algorithms. The L2 trigger requests data through the supervisor from specific ROSs, typically containing data within an RoI, to apply the selection. This is done in a smart way, such that when it becomes clear that an RoI will not contribute to passing the L2, processing of that RoI is stopped.

Figure 2.18: Schematic overview of the DAQ/HLT system

From here on, control is handed over to the dataflow manager (DFM), which takes the information from the L2 decision. When the L2 discards the event, the DFM sends a signal to all ROSs to expunge the data for that event. When an event is accepted, it requests all data fragments for this event from the ROSs, and sends it to the event builder, where it is combined in a single data structure. When the event is built and sent to the event filter (EF) for the final trigger selection, the DFM sends a message to all ROSs to expunge the data.

The EF consists of a computer farm, which uses offline reconstruction algorithms using information from the entire event. To reduce the time spent on each event, the EF is seeded on the results of the L2 trigger. When the EF accepts the event, it is sent to an output node of the DAQ, where the event is stored on a local disk. On this local storage, the events are divided in streams, based on the trigger selection criteria that were passed. One event may be placed in multiple streams. From these output nodes the events can be picked up and transferred to mass storage.