An unstable lepton that is common in the cosmic radiation near the earth's surface, has a mass 203.77 times the mass of the electron an average lifetime of 2.20X10-6 second, and exists in negative and positive forms related as particle and antiparticle.

U 介子(一种不稳定的轻子，在地球表面的宇宙辐射中常有发现，质量为电子的206.77倍，平均寿命2.20x10-6 秒，在粒子与反粒子中以负及正的形式存在)。