Unanswered Questions

Beyond the Proton, Neutron, and Electron

How are the results of collisions in accelerators detected?

A collision between two protons or a proton and anti-proton can produce a large number of particles of different kinds. The task of a detector is to find the direction in which each particle is going, find its charge, momentum, and energy, and identify it. Detectors are huge devices. The CDF (Collider Detector at Fermilab) weighs 5,000 tons and is 12 meters (39 feet) in each dimension. It is operated by a collaboration of about 600 physicists from 60 universities and organizations. ATLAS (A Toroidal LHC ApparatuS) at LHC is even larger. It weighs 7,000 tons, is 44 meters (144 feet) long and 25 meters (82 feet) in diameter; 2,000 physicists are involved in building and operating this experiment.

In both detectors position detectors locate the particles close to where they are produced in the beam line. Somewhat farther away from the beam line a strong magnet deflects the paths of charged particles. Outside the magnet are additional position detectors and finally detectors that measure the energy of the particles. The curved paths charged particles take in the magnet are used to find their charge and momentum.

Each collision can produce billions of pieces of data, only a small number of which are interesting. Electronic circuits and fast computers use data from the position detectors to decide whether the set of data are interesting enough to be recorded. ATLAS produces 100 megabytes of data each second. The entire LHC produces 4 gigabytes of data each second, enough to fill a DVD disk.


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