The electromagnetic and hadronic calorimeters of the LHC's ATLAS experiment play a key role in identifying the products of proton-proton collisions from among countless combinations. Segmented into concentric layers, the calorimeter cells selectively measure the more penetrating electromagnetic (electrons or photons) and hadronic energy deposits (protons, pions, quark-bound jets, low-energy gluons, etc.).
The triggering system of the electromagnetic calorimeter must be capable of very rapid sorting in order to consider only data from the regions where interactions likely to be of interest take place (i.e. high-energy photons or electrons, energetic jets, missing transverse energy, etc.).
Until now, the triggering system used the analog sum of signals produced by groups of adjacent cells, without any information on the penetration depth of the electromagnetic jets.
The new system, whose outgoing data stream is purely digital (~200 Gbit/s per board), allows the analysis of the jets with a finer granularity (10 times more signals used for triggering) by including an energy and time measurement for each detection layer. The energy measurement notably allows the triggering system to distinguish between the electromagnetic and hadronic contributions. Moreover, digitizing the data very early in the acquisition chain enables the calculation of observables that are more informative than the analog sums.
Relying on their experience acquired during the development of the previous analog boards, Irfu physicists and engineers took part in the design, development, manufacturing and testing of 74 (out of 150 total) new boards used in triggering the electromagnetic calorimeter.
Each of those state-of-the-art board contains around 13,000 radiation-resistant components, including 130 ASICs (Application Specific Integrated Circuits), on a very large 24-layer (41 × 49 cm²) PCB cooled by water circulation. Furthermore, each one handles 320 channels of the triggering system and is connected to the main system by 40 optical links at 5.2 Gbps. The whole ensemble generates a total data stream of 29 terabits/s (1 terabit = 1012 bits).