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ALICE: the journey of a cosmopolitan detector

Detector pieces have travelled across the globe to build ALICE’s new inner detector at CERN


ALICE ITS detector upgrade - Building 167
The various layers of ALICE’s inner tracker being assembled (Image: Maximilien Brice/CERN)

Passengers aboard United flights 577 and 956 from San Francisco to Geneva, via Newark, were in for a surprise when they discovered their discreet, yet unusual, travel companion: one of the airplane seats was occupied not by a human, but by a piece of a detector. Since it was too fragile to be placed in the hold, the part had its own passenger seat, next to that of its guardian angel, a researcher at the Lawrence Berkeley National Laboratory (LBNL). The part is one of the pieces of the ALICE experiment’s new inner detector that have made their way across the globe to reach CERN. Thirty-five institutes from all over the world are involved in the construction, testing and prototyping of the various parts of the new detector called ALICE’s Inner Tracking System

ALICE (A Large Ion Collider Experiment) is one of the four main experiments at the Large Hadron Collider (LHC). The experiment studies matter as it was just after the Big Bang, when the components of atomic nuclei were not bound together. 

With the increase in instantaneous luminosity of heavy-ion collisions planned for the CERN accelerators’ third run, the ALICE detector will boost its read-out rate. This increase in the read-out rate, combined with an online data reconstruction system, means that ALICE will be able to collect 100 times more events than before.  

In order to achieve the desired physics goals, numerous upgrades are being carried out during the current Long Shutdown. One of the worksites concerns the Inner Tracking System, which surrounds the beam pipe. This detector reconstructs the tracks of charged particles and its measurement precision is crucial for physics analyses. The upgrade will therefore result in improved precision, especially in the detection of short-lived particles. 

The current inner tracker will be replaced by a system of seven all-pixel cylindrical layers. The previous system had six layers, of which only the two innermost layers were made up of pixels, while the four external layers were composed of silicon sensors with a much lower granularity. The new Inner Tracking System will be composed of 12.5 billion pixels installed on an area of around 10 m2. These pixels have been specifically developed to cope with a higher collision rate and to reach a fine granularity. The chips thus incorporate both the pixel sensor and the read-out system, enabling the mass of the detector to be reduced by a factor of four compared with its predecessor. Reducing the mass allows for improved precision and efficiency when reconstructing particle trajectories.  

The construction of all the mechanical structures was completed last year. The various pieces of the detector have arrived at CERN, and the CERN teams are now starting to assemble them in two half-barrel structures. The fact that the detector components were available quickly has made it possible to begin the commissioning process. The final installation of the detector in the experimental cavern is planned for next summer.