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Particle tracks from a proton-proton collision seen by the ATLAS (a toroidal LHC apparatus) detector at CERN (the European particle physics laboratory) near Geneva, Switzerland. Before the collision the ions had been accelerated by the large hadron collider (LHC). The collision produced a kaon meson. The particle was not seen, but is revealed through its decay products; a positive pion and a negative pion (red lines). The lines for the pions curve in opposite directions in the detector's magnetic field due to their opposite charges. (KEYSTONE/SCIENCE PHOTO LIBRARY/ATLAS EXPERIMENT, CERN/SCIENCE PHOTO LIBRARY)