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Large Hadron Collider experiments at CERN on
track with cosmic rays
Geneva, Switzerland, July 26: The giant CMS particle detector at
CERN
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 square metres 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.
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