Animated particle tracks showing the production of photons from a matter-antimatter collision in the L3 detector at CERN, the particle physics laboratory outside Geneva. The detector is represented schematically in white. An electron and a positron are accelerated to high energy in CERN's LEP collider, and have entered unseen from left and right along the beam pipe that runs through the detector. They collide and annihilate at centre. Their energy materialises in the form of a new electron and positron (blue tracks and 'towers') and three photons. Being neutral, the photons leave no track, but are detected when they deposit energy (red towers) in L3's electromagnetic calorimeter.