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 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.

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

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.

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.