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CPLEAR experiment. The photograph shows various experiments fed via beam pipes with antiprotons made at the Low Energy Antiproton Ring (LEAR) at CERN, the European centre for particle physics outside Geneva. CPLEAR is the yellow-faced cube at the end of the beam pipe curving to the right. It studied the matter-antimatter asymmetry known as charge-parity or CP violation in the decays of 'strange' particles called neutral kaons. In 1998, CPLEAR discovered a small but clear difference between particle interactions moving forwards and backwards in time. This time-reversal asymmetry means that there is an 'arrow of time' at the level of particle interactions.
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