Though they have a rest-frame lifetime of about 2.2 microseconds, the effects of special relativity increase this time greatly in Earth's frame, and they are the most numerous energetic charged particles at sea level.  
They can be used to catalyze room temperature fusion, but their instability causes the process to fall far short of the break-even energy.  Soon after their discovery, they were shown not to be Yukawa's pion, as negative ones were captured in "atomic-like" orbits around nuclei and were not quickly absorbed into these nuclei.  Though they have a rest-frame lifetime of about 2.2 microseconds, the effects of special relativity increase this time greatly in Earth's frame, and they are the most numerous energetic charged particles at sea level.  Discovered by Neddermeyer and Anderson, this is, FTP, what lepton with a mass roughly 207 times that of an electron?
They can be used to catalyze room temperature fusion, but their instability causes the process to fall far short of the break-even energy.  
muon (or mu lepton, or mu particle)