Trigger and Data Acquisition

As shown in Fig. 3.9, the rate of interesting physics events occurring at the LHC design luminosity (L = 10³⁴cm⁻²s⁻¹) is many orders of magnitude lower than the 40 MHz design collision rate. The extraction of interesting physics events by the ATLAS trigger and data-acquisition (TDAQ) system relies on three subsequent decision levels where the selection quality is refined after each step. The decision system design of the TDAQ is such that the rate of collision events stored permanently amounts to about 400 Hz, that must be selected from the initial collision rate. A block-diagram visualisation of the ATLAS trigger system is given in Fig. 3.10. The three layers of the event selection are Level-1 (L1), Level-2 (L2), and event filter (EF). The L2 and event filter together form the High-Level Trigger (HLT).

Level 1

The L1 trigger acts at hardware level, exploiting ATLAS customised electronics, while the HLT is almost entirely software based, and exploits the networking hardware systems. The

Figure 3.9.: The predicted cross sections for typical SM known processes and exemplary Higgs productions as a function of the pp centre-of-mass energy. The LHC centre-of-mass energy working point of 7 TeV is drawn as solid line. The expected event rate for the design luminosity value is shown on the scale of the right side of the plot. At the centre-of-mass energy of 7 TeV the production rate of top quarks (σt) is visible, below the overwhelming production of W andZ bosons and high-energy jets events. [Cat00]

Figure 3.10.:Block diagram of the Trigger/DAQ system. The orders of magnitude at the different trigger levels are shown, starting from an interaction rate of about 1 GHz in which an average value µof multiple interactions per bunch crossing is considered. The average size of the events accepted by the trigger amounts to about 1.3 MB. The actual rate of accepted events is about 400 Hz, leading at a rate of about 550 MB/s saved on disk [A⁺08].

L1 trigger uses reduced-granularity information from the RPCs and TGCs searching for high-p_t muon signatures, and the from the calorimeter sub-systems to identify events with important electromagnetic clusters, jets, τ-leptons, E_T^miss and large total transverse energy.

If accepted by the L1 trigger the event data is moved through the Readout Driver (ROD) into the Readout Buffer (ROB) for temporary storage. The detector readout systems can withstand a maximum L1 output rate of 75 kHz, , upgradeable to 100 kHz; the L1 decision must be taken within 2.5 µs after the associated bunch crossing.

Level 2

For each event passing the L1 decision, Regions-of-Interest (RoI) are created, that are regions of the detector where the L1 trigger has identified possible trigger objects within the event.

The RoI information, which includes energy, position and signature type, is used by the L2 trigger. At this level the full granularity in the RoI can be exploited. The typical size of a RoI amounts to about 2% of the event data, and the average event processing time of approximately 40 ms. At this point the event rate is reduced to less than 3.5 kHz, which is passed over to the EF. The selection criteria of both L1 and L2 are primarily inclusive, such as high-E_t objects above defined thresholds.

Event Filter

The event filter acts offline on fully-built events processed by the event builder (EB) to further select events down to a rate at which they can be recorded for subsequent offline analysis.

At the end of the trigger chain, the event rate is reduced to approximately 400 Hz, with an average event processing time of about four seconds, which is realised by a parallel processing of the events. The average size of the accepted events is about 1.3 MB. The HLT algorithms use the full granularity and precision of calorimeter and muon chamber data, as well as the tracking information from the inner detector, that help to refine the trigger selections.

Typically the same algorithms of the later offline reconstruction are used. Better information on energy deposition improves the threshold cuts, while track reconstruction in the inner detector significantly enhances the particle identification (for example distinguishing between electrons and photons).

According to the EF trigger items associated, recorded events are written into datasets that are named after the trigger stream they belong to (e. g.. “Muons”, “Egamma”, “TauJetEt-Miss”). Under such general assumptions, events containing two or more objects of different physical nature that have simultaneously fired their dedicated trigger can be duplicated by being stored in more than one stream dataset².

Data Acquisition

The DAQ system collects and buffers the event data from the detector-specific readout electronics at the L1 trigger rate. It transmits to the L2 trigger any data requested by the trigger (typically the data corresponding to RoI’s) and, for those events fulfilling the L2 selection criteria, event-building is performed. The assembled events are then moved by the data acquisition system to the event filter, and the events selected are moved to permanent event storage. In addition to controlling movement of data down the trigger selection chain, the data acquisition system is responsible for the configuration, control and monitoring of

2For example an event where both an electron and a muon have fired the trigger can be stored in the

“Egamma” and in the “Muons” trigger stream simultaneously.

the ATLAS detector during data-taking. Supervision of the detector hardware (gas systems, power-supply voltages, etc.) is provided by the Detector Control System (DCS).