LHC experiments at CERN on track with cosmic rays

The gigantic CMS detector will be closed for the first time. (Image: CERN)

Geneva, 26 July 2006. The giant CMS particle detector at CERN1 has been sealed and switched on to collect data for an important series of tests using cosmic ray particles. The CMS ‘cosmic challenge’ will be carried out with segments of the full set of sub-detectors including a tracking detector composed of 2 m2 of silicon sensors. This is larger than any used in CERN’s previous generation of experiments, but only about 1% of the final detector that will be installed in CMS when CERN’s new flagship particle accelerator, the Large Hadron Collider (LHC), starts up next year.

The LHC is a discovery machine, designed to answer fundamental questions about the Universe. Four major experiments, ALICE, ATLAS, CMS and LHCb, will observe high-energy particle collisions produced by the LHC, looking for answers to questions such as what gives matter its mass, what the invisible 96% of the Universe is made of, why nature prefers matter to antimatter and how matter evolved from the first instants of the Universe’s existence.

Progress with the LHC accelerator passed an important milestone on 12 July, with installation of the main superconducting dipole magnets reaching the halfway mark when the 616th dipole out of a total of 1232 was installed at 3 am. The dipoles are the LHC’s key elements, and will steer the machine’s high-energy beams around their 27 km orbit.

Installation of all four detectors is also proceeding well. After having detected its first cosmic rays in situ in one of its sub-detectors (the hadron calorimeter) a year ago, ATLAS began running two of its particle tracking detectors together in June, recording cosmic rays. These detectors are extraordinarily complex devices, providing millions of channels of information so that particles can be identified and measured with great accuracy. ATLAS also recorded an important milestone in May, with the first operation underground of a superconducting magnet for an LHC experiment, and the cool down of part of the experiment’s energy-measuring calorimeter system.

The CMS experiment registered cosmic rays for the first time in a complete sector of four muon chamber measuring stations last December, and in March, cosmic rays were recorded in part of its tracker. In the CMS cosmic challenge, muon chambers and tracker will be joined by calorimeters to form a slice of the full CMS detector.

In June, the ALICE experiment registered cosmic rays in its main tracking device, a time projection chamber (TPC). With a diameter of 5 m and length of 5 m, the ALICE TPC is the largest of its kind worldwide. Nearing completion, the TPC now has all read-out chambers installed with the custom electronics complete for its approximately 560 000 read-out channels.

The LHCb detector’s specific geometry rules out tests of the complete detector with cosmic rays, but certain components have been tested with cosmic particles prior to installation. In addition, LHCb has just passed an important stage in installing a huge piece of its detector, a support structure called the ‘bridge’ because of its shape. This piece, weighing 10 t and measuring 18 m in length, will support the detector’s particle tracking system.

Running with cosmic rays, as well as in test beams, is important for the experiments because it allows the physicists to check that everything is working as it should before the detectors are made ready for start-up in their experimental caverns. These are the latest of several recent milestones in preparation for first beams from the LHC in 2007.


1. CERN, the European Organization for Nuclear Research, is the world's leading laboratory for particle physics. It has its headquarters in Geneva. At present, its Member States are Austria, Belgium, Bulgaria, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Italy, Netherlands, Norway, Poland, Portugal, Slovakia, Spain, Sweden, Switzerland and the United Kingdom. India, Israel, Japan, the Russian Federation, the United States of America, Turkey, the European Commission and UNESCO have Observer status.

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