First collisions recorded after the LHC restart

ALICE ITS upgrade detector and subsystems
ALICE ITS upgrade detector and subsystems, © 2020-2022 CERN

On July 5, after a break of more than three years, Run 3 started at the CERN Large Hadron Collider (LHC). Run 3 is a four year period of data collection, starting with proton-proton collisions at a center-of-mass energy of 13.6 TeV, the highest ever reached.

The Large Hadron Collider in a nutshell

The Large Hadron Collider (LHC) is the world’s largest and most powerful particle accelerator. It consists of a 27-kilometre ring of superconducting magnets with several accelerating structures to boost the energy of the particles along the way. The beams inside the LHC are made to collide at four locations around the accelerator ring where particle detectors are located, of which one is the ALICE detector. The Large Hadron Collider (LHC) has restarted operations on 22 April this year after a break of more than three years in which maintenance, consolidation and major upgrades have taken place. Since the restart in April, the LHC has been prepared towards the physics-data collection that has started earlier this week.

The ALICE detector upgrade

During the Long Shutdown (LS2), ALICE has completed major upgrades of the detector subsystems, where one of the main ones was the upgrade of the inner silicon tracker. The GRASP/Nikhef ALICE group has played a major role in the construction, installation and commissioning of this new cutting-edge sub-detector. The new ALICE tracker is the core of the ALICE experiment, and it is the largest tracker ever built based on Monolithic Active Pixel Sensors (MAPS), a technology not different from the sensors everyone has in the camera of their cellphone. The structure features about 24-thousand sensors each with half a million pixels of about 0.002 cm in size. Those sensors have been assembled to create a cylindrical structure with seven concentrical layers capable to reconstruct the trajectory of sub-atomic particles with a precision of the order of few millionths of a meter. In the picture above, the outer tracker layers during assembly is shown while on the picture below, it is possible to see the tracker layers being installed in the core of the ALICE detector.


ALICE installation
Installation of tracker layers in the core of the ALICE detector © 2020-2022 CERN