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Production and decay of photons in the L3 detector at CERN, the particle physics laboratory outside Geneva. In this computer display, the detector is represented schematically in white. An electron and positron, accelerated to high energy in CERN's LEP collider, have entered unseen from left and right along the beam pipe that runs through the detector and annihilated at centre. Their energy materialises in the form of a new electron and positron (blue tracks and 'towers') and 3 photons. Being neutral, the photons leave no track, but are detected when they deposit energy (red towers) in L3's electro- magnetic calorimeter. The photon is the carrier particle of the electromagnetic force. (KEYSTONE/SCIENCE PHOTO LIBRARY/CERN) === ===