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A cosmic ray shower recorded in the ALICE time projection chamber ALICE's time projection chamber began cosmic ray commissioning in June The path of a cosmic ray recorded in two of the ATLAS tracking detectors

A cosmic challenge for the LHC experiments

Cosmic particles are raining down on CERN. To get ready to analyse particle collisions at the Large Hadron Collider (LHC), which will be commissioned in spring 2008, the new accelerator’s four large experiments are testing their equipment using a natural particle accelerator: space. 

Every second, we are bombarded by particles from outer space, coming from places such as stars, supernovae and possibly even black holes. When one of these high-energy cosmic rays hits the atmosphere, it triggers a succession of collisions, generating a shower of particles that reaches the earth's surface.

The LHC experiments are making use of this natural source of particles to test components of their detectors. The detectors are built either like giant Russian dolls, with sub-detectors inserted one inside the other, or with sub-detectors aligned in a row. Each sub-detector has a different role in detecting a specific type of particle or particle property. The sub-detectors are tested separately using cosmic rays at the time they are made. Once all the components are assembled, cosmic rays are again used to test the whole detector, together with the trigger and data acquisition systems that read out and record information from the detector. This allows the physicists, for the first time, to reconstruct real physics events using all of their equipment.

On 26 July, CMS, one of the four large LHC experiments, began a spectacular cosmic ray test. The detector, consisting of colossal 15 metre-diameter twelve-sided sections housing the various sub-detectors and the experiment’s magnet, was completely closed. The CMS collaboration then powered up the magnet, the largest superconducting solenoid in the world. Measuring 12.5 metres long and weighing 220 tonnes, its task is to bend the trajectories of charged particles.  Cooled to –269°C, the CMS magnet produces a magnetic field of 4 teslas, almost 100 000 times the Earth's magnetic field. Once the magnet was powered up, cosmic ray tests of a complete slice of the detector could start.

This test follows other cosmic challenges for the LHC Collaborations. One year ago, ATLAS, the largest LHC experiment, tested one of its sub-detectors (the hadronic calorimeter) for the first time in situ in the experimental cavern 100 metres below the surface. In June, ATLAS carried out another cosmic ray test on two of its combined trackers. CMS had already detected cosmic rays in its muon chambers last December and in part of its tracker in March. Finally, ALICE, which will study the quark-gluon plasma, a state of matter which is thought to have existed immediately after the Big Bang, commissioned its main tracker, which is the largest time projection chamber in the world, by recording and reconstructing the tracks of cosmic rays. The fourth large experiment, LHCb, has a horizontal configuration with elements lined up one after the other, making cosmic ray tests at this stage redundant.  Nevertheless, cosmic rays were used in the initial operating tests of its components before they were installed.

These are some of the last tests for the LHC detectors before they are commissioned next year, when they will face an even greater challenge. By unravelling the millions of collisions that the LHC will produce every second, the four large experiments will be expected to deliver the answers to a number of fundamental questions about the Universe. Those questions include, how do particle acquire their mass? What accounts for the missing mass in the Universe? Why is there more matter than anti-matter? Or even, how did matter evolve immediately after the Big Bang, the explosion which gave birth to the Universe...?

August 2006