Trigger and data acquisition system

The 40 MHz proton-proton collision rate of LHC produces a huge amount of read-out signals in ATLAS detector: a trigger system organized on three distinct levels (Level-1, Level-2 and Event filter) has been implemented to select potentially in-teresting events (see figure2.17).

The first level trigger

The first level (L1) trigger (schematized in figure2.18) is implemented on detector with custom made electronics board and it uses data from calorimeters and muon trigger chambers.

The calorimeter trigger uses reduced granularity informat ions from all the calorime-ters and searches electrons, photons, jets with high E_T or events in which there is a large E^miss_T and a large total transverse energy. The trigger algorithm is based on the multiplicity of hits from clusters found in the calorimeters and from global energy deposition.

The level 1 muon trigger uses signals of muon trigger chambers RPC and TGC and searches for coincidence of hits in trigger station consistent with high-p_T muons coming from interaction point. There are six independently programmable p_T thresholds. Information from all muon trigger sectors are combined by the Muon Central Trigger Processor Interface (MUCTPI).

Informations from calorimeter and muon triggers are combined by the Central Trigger Processor (CTP) which makes the overall L1 trigger accept decision. The detector read-out system can handle a maximum L1 accept rate of about 100 kHz.

Figure 2.17: General view of three levels of the ATLAS trigger system.

The level-1 trigger must operate with a maximum latency of 2.5 µs and has to identify without ambiguity the bunch crossing of interest. Data of all detectors channels are retained in pipeline memories (located on or near the detectors) while the trigger decision is being formed. If an event is accepted by level-1 trigger, the region of interest (RoI), i.e. the information about the geometry location of trigger object is delivered to level-2 trigger.

Figure 2.18: Block diagram of the level-1 trigger. Red, blue and black lines are, respec-tively, the output path to detector front-ends, L2 trigger, and data acquisition system.

High level trigger and data acquisition system

The second level of the trigger uses informations from all the subdetectors with full granularity and precision, in the regions of interest defined by level-1 trigger (in this way the amount of data analyzed is about 2% of the total). The level-2 trigger has an average event processing time of about 40 ms and reduces the trigger rate to approximately 3.5 kHz.

After that, all event data (associated with a given event) are collected and as-sembled in a formatted structure by the sub farm input (SFI) application. Built events are processed by the event filter processing farm. In this step, unlike the L2 trigger, the standard ATLAS analysis and reconstruction program is used. In this final state, the event rate is reduced to roughly 200 Hz and selection proce-dure has an average event processing time of about four seconds. Data of events which passed the event filter selection criteria are received by the event filter out-put nodes (SFO) and are written on files located on CERN central data recording facility. Data are separated on various streams and written on different files de-pending of the trigger signature (e.g. muons stream, minimum bias stream, etc.).

Special streams are the calibration stream and the express stream. The calibra-tion stream is not recorded at the end of the full trigger chain but at level-2 step and it is used for detector calibration. The express stream contains a subset of the events selected by event filter (in fixed percentage for every streams) and it is reconstructed and analysed promptly as soon as SFO closes the data files on disk. This allows to have a quickly feedback of the quality of data taken and the detector status.

ATLAS RPC trigger chambers 3

3.1 Resistive Plate Chambers

Resistive Plate Chambers (RPC) have been developed in 1981 by R. Santonico and R. Cardarelli [30]. They are gaseous parallel plate detectors with a time resolution of∼ 1 ns, consequently attractive for triggering and Time-Of-Flight applications.

Their main advantages, compared to other technologies, consist in their ro-bustness, construction simplicity and relatively low cost of the industrial produc-tion. They are ideal to cover large areas up to few thousand square meters.

RPCs were originally used in streamer mode operation [31], then providing large electrical signals, requiring low gain read-out electronics and not stringent gap uniformity. However, high rate applications and detector ageing issues made the operation in avalanche mode absolutely necessary. This was possible thanks to the use of new highly quenching C₂H₂F₄-based gas mixture instead of the tradi-tional Ar-based mixture and to the development of high gain read-out electronics.

RPC, similarly to Spark Chambers and Parallel Plate Avalanche Chambers, consist of two parallel plate electrodes made with high resistivity material, typi-cally glass or bakelite.

The fundamental processes underlying RPCs are well known. A charged parti-cle produces free charge carriers in the gas, which drift towards the anode and are multiplied in a uniform electric field induced by an external high voltage applied to the electrode plates. The propagation of the growing number of charges induces an electric signal on the read-out strips, which is amplified and discriminated by the front-end electronics.

The chargeQ0reaching the electrode surface is locally removed from the elec-trode itself following an exponential law:

Q(t) = Q₀e^−t/τ withτ = ρε₀ε_r (3.1) whereρ is the electrode volume resistivity and εrandε0are the relative permittiv-ity of the resistive material and of the vacuum respectively.τ is defined as the time needed by the electrode to get charged again thus recovering the initial voltage in

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the gap and varies fromτ ≈ 1 s for glass resistive plates (for which the volume resistivity isρ ≈ 10¹²Ω cm) to τ ≈ 10 ms for plastic-laminated plates (for which ρ ≈ 10¹⁰Ω cm).