The subterranean ballet of ALICE
The experiment caverns of the Large Hadron Collider (LHC) are staging a dazzling performance during Long Shutdown 2 (LS2). The resplendent sub-detectors, released from their underground homes, are performing a fascinating ballet. At the end of February, ALICE removed the two trackers, the inner tracker system and the time projection chamber, from the detector. At the very start of the long shutdown, on 3 December 2018, the teams began disconnecting the dozens of sub-detectors. And finally, on 25 February, the two trackers were ready to be removed.
The trackers are located around the collision points and are used to reconstruct the tracks of the particles produced in the collisions. The data they generate are essential for identifying the particles and understanding what happened during the collision. ALICE's inner tracker is a 1.5-metre-long tube, 1 metre in diameter. It will be replaced with a new, much more precise detector closer to the collision point, formed of seven pixel layers and containing a total of 12.5 billion pixels. The current detector is still in the cavern and could spend its retirement as a museum piece in an exhibition above ground.
The time projection chamber is an imposing cylinder, measuring 5.1 metres in length and 5.6 metres in diameter, weighing an enormous 15 tonnes. The huge sub-detector was nonetheless hoisted out in just four hours, to be transferred to a building where it will undergo a complete metamorphosis. The current detector is based on multiwire proportional chamber technology. To increase the detector's acquisition speed by a factor of 100, the readout system will be equipped with much faster components called gas electron multipliers (GEMs), and the electronics will be completely replaced. The teams have started the renovation work, which should take around 11 months.
At present, the removal process is continuing in the cavern. Most of the calorimeters have been removed for refurbishment. Around 50 people are hard at work at the experiment.