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The LHC is on a Collision Course With Matter

Looking for Higgs' Boson

Assembly of the outer shell of the CMS magnet.

The largest experimental facilities in the history of particle physics is being built at CERN, near Geneva. This very large-scale scientific and technical undertaking, which has mobilised partners from well beyond Europe, will be operational by 2007.

Unveiling the mysteries of matter at the subatomic level has led physicists over the last half century to build constantly bigger machines. Particle accelerators and colliders of ever greater power and performance are needed to penetrate matter and the forces that hold it together, operating at infinitely small scales. In a few years time, a new superhero of particle colliding will arrive on the scene, the Large Hadron Collider (LHC), currently under construction at CERN1 near Geneva and due for completion in 2007.

The LHC will accelerate two proton beams in opposite directions around its 27 kilometer circular tunnel, until they reach nearly the speed of light. At the intersecting points of the two beams, protons will collide with extreme particle violence, releasing up to 14 tera-electron volts2 for proton-proton collisions, which is seven times more than the world's currently most powerful collider, the American Tevatron facility, has generated out of proton-antiproton collisions. The released energy will take material form as a basket of particles that will need to be counted, identified and measured as to energy, speed and the like. The picture that will emerge from this analysis will likely lead to revamping current particle theory. In particular physicists hope to capture a particle known in theory for three decades but so far elusive to any detection attempts: the Higgs boson.

The Higgs boson is the only missing element in the famous Standard Model of particle physics, which takes into account three of the four forces of the universe (the weak force, the strong force, and electromagnetic force). Since the Higgs' role in the Model is to confer mass to the other particles, its discovery is one of the LHC's most critical missions. Physicists are convinced that the Higgs boson has characteristics susceptible of being detected, but even if this turns out to be wrong, they are banking on the  LHC to produce a bouquet of new particles revealing new phenomena such as the "super symmetry " some theorists see lying beyond the Standard Model. New particles will mean new discoveries.

If the LHC represents big new steps for physical theory, getting it built poses no less of a challenge in technological and industrial terms. Detecting the  smallest of particles require two gigantic complexes one of which, ATLAS, consists of 100 million detection elements of several different types, each capable of furnishing 40 million readings per second and located inside a cylinder 22 meters long and 26 meters in diameter. Some 2,000 scientists and engineers have been working for ten years on ATLAS, which will cost 350 million euros, of a total price for the LHC of two billion euros.

Getting the LHC up and running requires worldwide collaboration including significant contributions from Russia, Japan, Canada and the United States alongside the twenty European member States that make up CERN. Like its predecessors, the LHC will generate for participating countries a great deal of technological fallout for use in other fields such as superconductivity, medical equipment, computor grids for calculation, and other vital areas. Inevitably, a project of this scope and daring encounters obstacles both budgetary and technological, and the LHC has been forced at one or two points to revise its calendar. ATLAS and CMS (the other detecting complex) are more than half way completed, and fabrication of the 1,300 dipolar magnets needed for the LHC is proceeding.  The latest projections call for the LHC to be operational in 2007.



à lire

C. Llewellyn Smith, « Le Grand Collisionneur de hadrons ». Pour la Science, septembre 2000, pp. 74-81.


Étienne Augé
Laboratoire de l'accélérateur linéaire (LAL)
CNRS-Université Paris XI

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