ALICE gets green light for new subdetectors

Two detector upgrades to ALICE, the heavy ion physics experiment at the Large Hadron Collider (LHC), have recently been approved for installation during the LHC’s next long stop, which will take place from 2026 to 2028. The first is an upgrade of the three innermost layers of the Inner Tracking System (ITS3) and the second is a new direct calorimeter (FoCal), optimized for photon detection in the forward direction of the ALICE detector.

High-energy collisions of heavy ions like lead nuclei at the LHC recreate quark-gluon plasma – the hottest and densest fluid ever studied in a laboratory. In addition to studying the properties of the quark-gluon plasma, the ALICE program covers a wide range of topics involving the strong interaction, such as determining the structure of nuclei and interactions between unstable particles, as presented in the article “A journey through the quark-gluon”. plasma and beyond.”

Internal Tracking System (ITS3)

ALICE’s current internal tracking system, installed for the ongoing LHC run, is the world’s largest pixel detector to date, at 10 m² of active silicon area and nearly 13,000 million pixels. The new indoor tracking system, ITS3, builds on the successful use of monolithic active pixel sensors and takes this concept to the next level.

“ALICE is like a high-resolution camera, capturing intricate details of particle interactions. ITS3 is poised to increase the pointing resolution of tracks by a factor of 2 compared to the current ITS detector,” van say Alex Kluge and Magnus Mager, the ITS3 project manager. “This will greatly improve measurements of the thermal radiation emitted by the quark-gluon plasma and provide insights into the interactions of charm and beauty quarks as they propagate through the plasma.”

ITS3 sensors are 50 µm thick and up to 26 × 10 cm². To achieve this, a new stitching technology was used to connect individual sensors together into a large structure. These sensors can now be bent around the beampipe into a truly cylindrical shape. The first layer will be placed only 2 mm from the beampipe and 19 mm from the interaction point. It can now be cooled by air instead of water and has a much lighter support structure, significantly reducing the materials and their effect on the particle trajectories seen in the detector.

Direct Calorimeter (FoCal)

The FoCal detector consists of an electromagnetic calorimeter (FoCal-E) and a hadronic calorimeter (FoCal-H). FoCal-E is a very granular calorimeter consisting of 18 layers of silicon pad sensors, each as small as 1 × 1 cm²and two additional special layers with 30×30 μm pixels². FoCal-H is made of copper capillary tubes and shiny fibers.

“By measuring inclusive photons and their correlations with neutral mesons, and the production of jets and charmonia, FoCal offers a unique possibility for a systematic survey of QCD at small Bjorken-x. FoCal extends the scope of ALICE by adding new capabilities to explore the small-x-parton structure of nucleons and nuclei,” said Constantin Loizides, FoCal project leader in the ALICE collaboration.

Newly built FoCal prototypes have recently been tested with beams at the CERN Accelerator Complex, Proton Synchrotron and Superproton Synchrotron, demonstrating their performance in line with expectations from detector simulations.

The ITS3 and FoCal projects have reached the important milestone of completing their technical design reports, which were approved by the CERN review committees in March 2024. The construction phase of ITS3 and FoCal now begins, with the detectors that will be installed in early 2028 to be ready for data acquisition in 2029.