UK involvement in the upgrade of ATLAS at the LHC

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STFC’s Technology Department and Particle Physics Department have been instrumental in the development of the eFEX modules, a subsystem of the ATLAS experiment.

The Science and Technology Facilities Council’s (STFC) Technology Department and STFC’s Particle Physics Department (PPD) were involved in the development of the Electron Feature Extractor (eFEX). This is a subsystem of the ATLAS experiment (ATLAS) at the Large Hadron Collider (LHC), at CERN in Switzerland.

ATLAS is the largest particle accelerator experiment at the LHC. ATLAS and the Compact Muon Solenoid, another particle accelerator experiment at CERN, are the two detectors that scientists famously used to confirm the existence of the Higgs boson in 2012.

Long Shutdown 2

After years of astounding experimentation and operation, the ATLAS experiment entered a maintenance period in December 2018 called ‘Long Shutdown 2’ for three and a half years. ATLAS would undergo many major upgrades, in the so-called phase-one upgrade:

  • addition of the Muon New Small Wheels (NSW)
  • new readout system for the NSWs
  • digital back-end electronics for the Liquid Argon Calorimeter
  • augmentation of the Trigger and Data Acquisition System
  • new muon chambers in the centre of ATLAS
  • addition of the ATLAS Forward Proton detector

ATLAS was turned on once more for its four-year long run in spring 2022, and by 5 July 2022, ATLAS began operation for physics research.

Special eFEX

When the LHC is in operation, over 40 million proton-proton collisions occur within the ATLAS detector every second, producing far more data than can be recorded or analysed fully. However, only a small fraction of these collisions corresponds to events of interest in the search for new physics. A multi-level ‘trigger’ system is used to identify and select such events for full analysis and storage.

The Level-1 Calorimeter Trigger (L1Calo), together with the Muon and Central Trigger systems, implements the first stage of this selection process. L1Calo receives data on energy deposits from across the ATLAS Liquid Argon Calorimeter and looks for patterns suggestive of interesting interactions. On average, the Level-1 Trigger must reject 399 out of every 400 events, while being highly efficient at identifying events worthy of further analysis by the next stage of the trigger.

The eFEX is the most important element of the Liquid Argon Calorimeter upgrade. The eFEX system is the fundamental element of the ATLAS trigger chain that will allow us to maintain low energy thresholds with high efficiency and good rejection on the primary electron or gamma selection.

STFC’s Technology Department and PPD have played a pivotal role in the impressive upgrades to ATLAS’ L1Calo Trigger – particularly with the integral eFEX project.

The eFEX is a new subsystem of the trigger on the ATLAS and the most important part of the L1Calo upgrade project. Its purpose is to look for patterns of energy deposits in the calorimeter that are characteristic of isolated electron and tau particles. Compared to previous trigger modules, it does this by examining data of finer granularity and running algorithms of greater complexity.

In this way, it will increase the discriminatory power of the trigger, allowing the experiment to maintain good physics selectivity despite the increase the increased collision rate that the LHC can now achieve.

Playing a vital role

Ian Brawn, of STFC’s Technology Department, says:

As the collision rate of the LHC increases beyond that for which the ATLAS experiment was originally designed, the eFEX will play a vital role in helping ATLAS maintain its sensitivity to key physics processes.

It will process data from the ATLAS calorimeters in real-time, looking for energy deposits characteristic of interesting events, helping ATLAS to distinguish these from the overwhelming amount of background data.

The eFEX subsystem comprises of 24 eFEX modules, which were designed at STFC’s Rutherford Appleton Laboratory (RAL), at the Harwell Campus in Didcot, Oxfordshire. Installation of the 24 eFEX modules began at CERN in February 2022 and was completed by September 2022.

An international effort

STFC’s Technology Department’s Electronic System Design Group (ESDG) were involved in the development of the eFEX modules by designing the hardware and firmware and led in project management. Also, STFC’s RAL PPD and the University of Birmingham’s School of Physics and Astronomy were contributors to the firmware design of the modules as well as testing and controlling the software.

Other contributors in the UK include the University of Cambridge, and Queen Mary’s College, London.

The L1Calo upgrade project was an international effort with collaborators around the globe. In the US, this includes the:

  • University of Chicago, Illinois
  • University of Indiana, Indiana
  • University of Michigan State, Michigan
  • University of Oregon, Oregon
  • University of Pittsburgh, Pennsylvania
  • Brookhaven National Lab, New York

Also collaborating internationally in the L1Calo upgrade project are Heidelberg University and the University of Mainz Germany, and Stockholm University, Sweden.

Supporting the eFEX in ATLAS

Ian Brawn, of STFC’s Technology Department adds:

The eFEX hardware has been designed by the RAL Technology department. However, we have worked in close collaboration with our colleagues in the PPD Department, and the School of Physics and Astronomy at the University of Birmingham.

Together, we specified the module, developed the firmware that implements the core functionality, developed the supporting software, and ran an extensive test programme.

We have also worked closely with the Department of Physics at the University of Cambridge and the Department of Physics and Astronomy at Queen Mary College, London, who have provided hardware and software essential to support the eFEX in ATLAS.

We have also benefitted from a close working relationship with Incap Electronics UK, who showed perseverance and resilience in assembling the eFEX modules in unexpectedly challenging circumstances due to COVID-19 and supply problems.

I am very pleased that the eFEX is now fully installed and working in ATLAS.

Top image:  Credit: dani3315, iStock, Getty Images Plus via Getty Images

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