They usually decay into a muon and a muon neutrino and are considerably lighter than the kaon.  
They were discovered in 1947 by an experiment, in which special photographic plates called nuclear emulsions were placed on top of a mountain, conducted by Brazilian physicist Cesar Lattes.  The lightest of the quark-antiquark pairs, this particle's positive version is more stable and more common than its neutral version.  They usually decay into a muon and a muon neutrino and are considerably lighter than the kaon.  FTP, name this meson which usually consists of an up and an anti-down quark.
They were discovered in 1947 by an experiment, in which special photographic plates called nuclear emulsions were placed on top of a mountain, conducted by Brazilian physicist Cesar Lattes.  
Pion or Pi meson