Testsing and Implementing Address Decoders for the LATOME Firmware

The ATLAS (A Toroidal LHC ApparatuS) Experiment is one of two multipurpose collider-detectors at CERN’s Large Hadron Collider (LHC). As many other collider-detectors before it, ATLAS has an ’onion-like’ structure with a vertex detector, tracking chamber, electromagnetic and hadronic calorimeters and muon chambers, along with a forward calorimeter and endcap. This project revolves around the firmware of ATLAS’ electromagnetic-, endcap- and forward calorimeter, the so-called Liquid
Argon (LAr) Calorimeter (LArCal).

One of the primary goals of ATLAS were to discover the Higgs boson via the channels H → γγ or H → 4l, which it did in 2012. This requires an effective and precise EM calorimeter and a triggering system looking for high energy deposits in the EM calorimeter. The LArCal is structured as an accordian with interleaving layers of lead absorbers, liquid argon and electrodes. A relativistic charged particle going through the calorimeter will initiate particle showers in the absorber, the particles of which will then ionize the argon, and the free electrons will drift to the electrodes and generate an electrical signal. The electrical signal is sent to an Analog-to-Digital-Converter (ADC) which digitizes it and sends it out of the experimental cavern and to the backend electronics in an adjacent cavern. The shape of the ADC signal is seen in Fig. 1.

From the proton-proton collisions delivered by the LHC every 25ns ATLAS generates on the order of ∼ 60 terabytes of data pr. second. Most of the data comes from low-energy strong interactions, which are uninteresting to most physics analyses. Therefore, the amount of data needs to be severely reduced in multiple steps. The first of these data-reduction steps is the Level 1 triggering system, which is designed to save or discard an event with a latency of only 2.5μs. The level 1 triggering system related to the LArCal triggers on high transverse energy, ET. A specially designed piece of hardware electronics, called a LATOME (LAr Trigger prOcessing MEzzanines), calculates the transverse energy at a very high rate using FPGAs (Field Programmable Gate Array). Originally, the LArCal was segmented into 0.1∆η × 0.1∆φ ’trigger-towers’, but with the increase in the LHC’s luminosity
in Run-3 a finer granularity of the triggering system was needed[1]. For this purpose the LArCal is segmented into ∼ 0.025∆η × 0.1∆φ so-called Super-Cells (SC). As such, one LATOME board, ie. one FPGA, is designed to calculate and transmit energies of up to 320 SCs with an estimated latency of 350ns.