The SINDRUM II experiment looks for the one-particle decay of this particle, and Louis Michel sometimes names the standard decay mode of this particle.  
The SINDRUM II experiment looks for the one-particle decay of this particle, and Louis Michel sometimes names the standard decay mode of this particle.  This particle was discovered by Street and Stevenson in a Wilson Cloud Chamber, and was at first conflated with the carrier of the residual strong force proposed by Yukawa.  The long lifetime of this particle, two point two micro-seconds, is an important test of time dilation because this particle makes up most of the final decay product of cosmic rays that reach sea level.  The bound system of a positive form of this particle and an electron behaves like a hydrogen atom with one ninth the mass. For 10 points, name this lepton approximately two hundred times heavier than the electron.
The SINDRUM II experiment looks for the one-particle decay of this particle, and Louis Michel sometimes names the standard decay mode of this particle.  
Muon [accept Mu Meson from anyone who looks old enough to have been around when they still called it that]