COMPASS reflects the way to identifying particles

1 March 2002

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COMPASS - COmmon Muon Proton Apparatus for Structure and Spectroscopy - is a new experiment to probe deep with nucleons, in other words, protons and neutrons. Located at CERN, Europe's main research centre for particle physics near Geneva, its main aim is to discover more about how the spin of the nucleons is shared among the more elementary particles they contain - in particular among the gluons that bind the quarks together via the strong force. The experiment fires 'heavy electrons" (particles called muons) at a "polarized"  target in which the spins of the protons and neutrons are aligned in a magnetic field.  Various detectors intercept the particles that emerge from the target.

This image shows the inside of a Ring Imaging Cherenkov (RICH) detector. When particles travel faster than light through gas inside this detector they emit cones of Cherenkov radiation which form faint rings of light on a surface. The size of the rings reveal the velocity, and therefore generally the type of the particle. These mirrors - 116 in this case - reflect the light onto photosensitive detectors in what forms the world's largest surface of cesium iodide. 

Credit: CERN Photo Please contact person or institution named for information about permission for public or commercial use.