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Research project The ATLAS experiment at CERN

The ATLAS experiment is one of four large experiments at the Large Hadron Collider (LHC) facility at CERN, and one of two general-purpose detectors designed to explore a wide range of questions about the fundamental constituents of matter and the forces of nature.

Stockholm University has been a strong participant in ATLAS from the early development phase, with important instrumentation contributions in two areas: The Level-1 Calorimeter Trigger and the front-end data-taking and readout system for the hadronic Tile Calorimeter.

Project description

Part of the ATLAS Level-1 Calorimeter Trigger - Photo CERN
Part of the ATLAS Level-1 Calorimeter Trigger - Photo CERN

The ATLAS Level-1 Calorimeter Trigger

To exploit the full physics potential of LHC, the accelerator has to produce as many collisions as possible. Bunches of protons meet head-on in the detectors 40 million times each second, with multiple collisions every crossing (up to ~40 in 2012). The total data volume generated by the millions of detector sensors is far greater than can be read out and stored. ATLAS is designed with a three-level trigger and data acquisition system that selects only ~200 "interesting" events per second out of the 40 million to be stored for later analysis.

Stockholm University has had an integral role in the design and upgrade of the Level-1 Calorimeter Trigger (L1-Calo), a custom-built electronics system that performs a coarse-resolution analysis of the deposited particle energies from all 40 million collisions each second. The results of the various selection algorithms are used to select fewer than 75 thousand of these events to be read out and further analyzed in more detail by large CPU farms, while the remainder are discarded. The challenge for L1-Calo is to achieve such a large rate reduction without losing valuable physics.
Stockholm university participation in L1Calo

Stockholm University has been a part of the ATLAS L1-Calo collaboration from the early design and development stages, and made important contributions to the overall system architecture. In the original system, we designed and produced a custom backplane and crate infrastructure that forms the backbone of both the electron/tau and jet/energy-sum digital algorithm processor subsystems. Stockholm also designed the digital jet-finding algorithm and the jet counting and readout firmware.

Stockholm also had a coordinating role in the 2014-2015 upgrade of the L1Calo system, which added improved digital signal processing for the incoming calorimeter signals, as well as topology-based trigger algorithm capabilities to help L1Calo keep readout rates at manageable levels as LHC collision rates increase, without sacrificing valuable physics.

Tile Calorimeter front-end readout electronics

The ATLAS hadronic Tile Calorimeter (TileCal) is a so-called sampling calorimeter, constructed of interleaving plates of iron and scintillating plastic, with around 10 thousand individual readout channels. It is designed to primarily measure the energy and direction of hadrons produced in the proton-proton collisions. TileCal plays a crucial role in both particle identification and energy/momentum measurements, as well as measuring imbalances in an event's transverse energy that can indicate the presence of "invisible" particles (neutrinos, or possibly other yet-hypothetical particles)

Stockholm-designed electronics for the upgraded Tile Calorimeter in the ATLAS experiment
Stockholm-designed electronics for the upgraded Tile Calorimeter in the ATLAS experiment

Stockholm participation in TileCal

Stockholm University designed and produced the current front-end "digitizer" system for TileCal, which performs analog/digital conversion for all 10 thousand detector channels, stores the digital samples in pipeline delays, and reads out data from triggered events for further analysis off the detector.

A full replacement of the front-end TileCal electronics is planned for the mid-2020s, that will provide full readout of all TileCal data off detector, allowing improved calorimeter trigger capabilities at high-luminosity LHC. Stockholm is designing the high-speed control and data readout board for this upgrade (pictured above).

Project members

Project managers

Samuel Silverstein


Department of Physics
Samuel Silverstein