First Observation of a Muon

The fotograph above is the very first evidence of the existence of muons and was obtained by Paul Kunze in a Wilson cloud chamber while exposed to cosmic rays. In it, there are clearly two ionizing paths produced by two diferent particles. The lower one was produced by an electron and the higher path was of unknown nature. The mysterious path has a smaller curvature and was produced by a considerably stronger but unknown particle since it ionized too much for an electron but too little for a proton.

Kunze suggested that it was a product of a nuclear explosion but didn't claim it as a discovery of a new elementary particle. In his own words he described it as "Double track as a result of a probable nuclear explosion. (...) The nature of the upper positive particle is unknown."

The definitive discovery of the muon only occurred three years later by the hands of Anderson and Neddermeyer, again using a Wilson cloud chamber.