The ATLAS New Small Wheels (NSW) detectors
The ATLAS New Small Wheels (NSW) being assembled in building 191. At the time of the photo, work is still in progress to complete the wheel with eight 'wedges' or 'slices' before it is installed in the ATLAS cavern. The small wheel detectors are vital in tracking muon particles. (Image: CERN)

The ATLAS experiment

Final LS2 upgrade work to the ATLAS detector in January 2022. (Image: CERN)

ATLAS is the largest particle detector at the Large Hadron Collider (LHC) and is designed to explore the widest possible range of physics phenomena. It investigates some of the unanswered questions about our Universe: from the origin of mass with the Higgs boson, to possible extra dimensions, to searching for new particles that could make up dark matter.

At 46 m long, 25 m in diameter and 7000 tonnes, the construction of the ATLAS detector pushed the limits of existing technology.

More than 5500 scientists from 245 institutes in 42 countries work on the ATLAS experiment.

Graphics,backgrounder,LS2 upgrades,LHC experiments,Experiments and Tracks
(Image: CERN)


LS2 upgrades

1. Muon new small wheels

The ATLAS new small wheel (NSW) system is made up of two wheel-shaped detectors, sitting at opposite ends of the experiment. It is an integral part of the ATLAS muon spectrometer, designed to select and track particles called muons that pass with little disturbance through the inner parts of the detector. If you imagine the detector as an onion, the muon spectrometer is the outer skin.

Named in comparison to ATLAS’s 25-metre “big wheel” detectors, each new small wheel weighs more than 100 tonnes and is nearly 10 metres in diameter.

2. New readout system for the new small wheels

Each new small wheel consists of 16 wedges, or sectors, covered with layers of detector chambers.The readout capabilities of the overall system are staggering: two million micromegas (MM) readout channels and 350 000 small-strip thin-gap chamber (sTGC) electronic readout channels. Both MMs and sTGCs have excellent precision tracking capabilities, at the level of 100 micrometres, and the fast response time needed to uniquely identify the collision time.

The new small wheels will improve ATLAS’s triggering capabilities and will be able to cope with the higher muon rates expected from the High-Luminosity LHC (HL-LHC).

3. Liquid Argon Calorimeter (LAr)

The LAr Calorimeter lies at the heart of the ATLAS experiment, measuring charged and neutral particles across a wide range of energies (from about 50 MeV to about 3 TeV). It plays a critical role in ATLAS’s 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 electronics will now improve trigger selection, critical to the operation at the future HL-LHC, which requires a higher resolution of the electromagnetic calorimeter’s trigger. Replacing some components of the front-end electronics increases fourfold the level of segmentation available at the trigger level, improving the ability to reject jets while preserving electrons and photons. 



4. Trigger and Data Acquisition System (TDAQ)

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. 

5. New muon chambers in the centre of ATLAS

New muon chambers – including 8 small Monitored Drift Tube (sMDT) modules and 16 next-generation Resistive Plate Chambers (RPCs) – have been installed inside the experiment. These new detectors will improve the overall muon trigger coverage of ATLAS, in preparation for HL-LHC.

6. ATLAS Forward Proton (AFP) spectrometer

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. If a scattered proton emits a photon and loses a few percent of its energy, the LHC magnets deflect the proton into the AFP spectrometer. These scattered protons are among the highest-energy particles measured at the LHC.

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