New Small Wheel (NSW) descent into ATLAS'
The 100 tonnes wheel is lowered 80 metres underground in the ATLAS cavern at the LHC level. The NSW consists in a set of new precision tracking detectors based on the Micromegas technology and new trigger detectors Small Strip Thin Gap Chambers. The production of these detectors involves 9 countries worldwide. (Image: CERN)

Long Shutdown 2

Upgrading the experiment for LHC Run 3

After several years of intense operation, the ATLAS Experiment entered its second maintenance period in December 2018 (called "Long Shutdown 2" or LS2). Over the course of 3.5 years, members of the Collaboration installed critical upgrades to the experiment and carried out maintenance work on its systems. Significant upgrades were also made to the LHC and its accelerator complex.

This incredibly productive period ran alongside continued analysis of LHC Run 2 data. Highlights of the activities carried out can be found below.

General LS2 resources:

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Explanations of the different detector parts of the ATLAS experiment during the LS2 (Long Shutdown 2) LHC upgrade. (Image: CERN)

New Small Wheel Installation

The most iconic new additions to the ATLAS experiment are the New Small Wheel Detectors (NSW). Named in comparison to ATLAS’ 25-metre diamter “big wheel” detectors, they now sit on the inner layer of the forward muon spectrometer and outer layer of the ATLAS experiment, where they will be tracking muon particles for the next 20 years.

These new wheels aren’t only an upgrade for Run 3 of the LHC, they are also the first major installation for the High-Luminosity upgrade of the LHC (HL-LHC), scheduled to begin operation in 2029. Their installation follows nearly a decade of dedicated efforts by ATLAS members, who designed, constructed and assembled this high-tech muon detector from scratch.

​​​​​​The NSW detectors are at the forefront of design, using two innovative gaseous detector technologies: micromegas (MM) and small-strip thin-gap chambers (sTGC). These provide both fast and precise muon-tracking capabilities. The improved spatial resolution allowed by the NSW will be especially critical for the ATLAS “trigger”, the system that decides which collision events to store and which to discard. The trigger will rely on the NSW’s excellent resolution to confirm whether a particle originated from the interaction point, thus reducing our chances of saving data from unwanted background events.

Key Numbers

  • 16 sectors per wheel
  • 2 million MM readout channels
  • 350,000 sTGC electronic readout channels

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ATLAS new small wheel C lowering (Image: CERN)

Liquid Argon Calorimeter Upgrades

The Liquid Argon (LAr) Calorimeter lies at the heart of the ATLAS experiment, measuring the energy of charged and neutral particles across a wide range of energies (from about 50 MeV (MIP) in a single cell to about 3 TeV). It plays a critical role in ATLAS’ online event-selection system – also known as the “trigger” – as it quickly provides the energy measurements used to select which collision events should be saved and studied.

New digital electronics will now improve trigger selection, providing a higher resolution and more granularity to the electromagnetic calorimeter’s trigger. The new front-end electronics increases tenfold the information available at the trigger level, improving the ability to reject jets while preserving electrons and photons.

Members of the LAr team label the hundreds of optical fibre cables coming from the experimental cavern. The very noisy environment due to cooling ventilation required them to wear hearing protection. Image: C. Camincher (Image: CERN)

The calorimeter uses a new, finer-granularity scheme called “Super Cells”, which provides information from each calorimeter layer. Since this upgrading to this scheme required a major – and potentially very disruptive – intervention in the data-taking chain, ATLAS researchers decided to incorporate these new elements while keeping the old “legacy” trigger system fully operational, and thus also revalidated the calorimeter’s original boards during LS2.

Key numbers

  • 5000 new optical fibres
  • 1524 Front-End readout Boards refurbished
  • 124 new LAr Trigger Digitizer readout Boards installed
  • 23.6 Tbps (Tera-bits per second) of data from the Super Cells

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New Muon Chambers Installation

New muon chambers – including 8 small-diameter Monitored Drift Tube (sMDT) modules and 16 next-generation Resistive Plate Chambers (RPC) – have been installed inside the experiment. These new detectors are capable of withstanding the higher rates and provide a robust standalone muon confirmation. Their installation into the ATLAS experiment improves the overall muon trigger coverage of ATLAS, which will be essential for HL-LHC operation

The new muon chambers are compact and are a first test for the future phase-2 installations for HL-LHC. These are the first RPC chambers in the inner layer of the muon spectrometer and sit in a region called BIS78. They will allow physicists to improve the precision of the momentum measurements of muons passing in these regions.

The new sMDT chambers feature smaller diameter tubes (15mm compared to 30mm of the original MDTs), providing an order of magnitude higher rate capability. This will be vital for the intense environment of HL-LHC.

Key numbers

  • 8 new stations on Side A
  • 15 mm diameter tubes in sMDTs

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Detectors,Muon Spectrometer,Technology,ATLAS,Upgrade Activities
On 17 September 2020, the ATLAS collaboration installed the new BIS78 station inside the experiment. (Image: CERN)

ATLAS Trigger Upgrades

Collisions among proton bunches occur inside the ATLAS experiment up to 40 million times a second. Only a fraction of the collision events are valuable for research and the ATLAS detector must decide which events to store for analysis. This decision-making is done courtesy of the sophisticated Trigger and Data Acquisition System (TDAQ), which has upgraded hardware and software to spot a wider range of collision events while maintaining the same acceptance rate.

Member of the ATLAS trigger group (Silvia Franchino) in front of the trigger racks located in a service hall adjacent to the ATLAS experiment cavern. (Image: N. Caraban Gonzalez/CERN)

The significant improvements to the LAr calorimeter mentioned above increase the granularity of information used by the trigger. With better resolution, the trigger will be better able to reject background events. The calorimeter trigger now has modern large-scale FPGAs, allowing physicists to run more sophisticated algorithms to identify physics objects, and to calculate missing energy in the event with higher precision.

The New Small Wheel system and the BIS78 installations also mentioned above cover the boundary region between the barrel and end-cap part of the ATLAS detector. This improves the trigger’s ability to correctly identify muons and will reduce the rate by a factor of 2.

The software-based High Level Trigger (HLT) was completely rewritten in order to execute trigger algorithms within the multi-threaded software framework AthenaMT, sharing common features between the trigger software and the software used for physics analyses. These improvements will be especially important for Run 3 of the LHC, which will require significant HLT resources to be dedicated to computationally-intense particle tracking.

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ATLAS Forward Proton (AFP) Detector Upgrades

The redesigned ATLAS Forward Proton (AFP) spectrometer sits on either side of the main ATLAS cavern, just over 200 metres downstream from the collision point. Its detectors are based on high-resolution tracking 3D pixel silicon technology and high-precision Time-of-Flight (ToF) quartz-Cherenkov detectors, which reach directly into the LHC beam pipe to only two millimetres from the proton beam itself.

In many LHC collisions, the colliding protons are not destroyed, but remain intact, still emitting some energy that will produce particles in the central ATLAS detector. The two protons will have lost only a few percent of their energy in the collision, and will be deflected by the LHC magnet more than the other non-interacting ones in the beam. They will be measured by the AFP spectrometer, giving the full information of the collision together with the measurements from the central detector.

The AFP ToF detector was redesigned during LS2, allowing it to be inserted into the LHC beamline while keeping the “time-of-flight” AFP photon-measuring devices (MCP-PMT) outside the Roman Pot vacuum. This improved design provides an easier environment for the AFP to operate in and gives physicists easier access to its electronics.

Key numbers

  • 200 metres downstream from collision point
  • 2 mm from LHC beam

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Metrology of AFP detector package just before installation in tunnel. Quartz bars of Time-of-FLight system are visible on the left whereas a set of four 3-D silicon trackers (SiT) is on the right. (Marko Milovanovic) (Image: CERN)

More LS2 Multimedia

Experiments and Collaborations,ATLAS,Detector Construction,Muon chambers
Construction of Micromegas at Building 899 (BB5) and Small Strip Thin-gap Chambers at B180 for the ATLAS New Small Wheel (NSW) (Image: CERN)
ATLAS team members install the final sector, one of the last of the eight 'slices' or 'wedges', onto the ATLAS New Small Wheel muon detector (NSW). The New Small Wheel upgrade is the most challenging and complex one of the ATLAS phase-I upgrade projects. (Image: CERN)

Maintenance work on the ATLAS Big Wheels. (Image: CERN)
Detectors,Muon Spectrometer,Technology,ATLAS,Best
Views from above while the ATLAS New Small Wheel is being lowered down to be placed on side A of the ATLAS detector underground. (Image: CERN)