In a year full of surprises in high‐energy physics, an experiment at the SPEAR colliding‐electron‐beam facility has uncovered yet another unexpected phenomenon: A significant number of events have been seen in which an electron and positron collide and produce one electron and one muon plus some missing energy. The experimenters can find no conventional explanation for this anomalous production of leptons and postulate that this reaction could proceed through the production of a pair of new particles. Each of the U particles, as they have been tentatively called, would then decay into an electron or a muon plus one or two neutrinos.

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