Noun
a meson involved in holding the nucleus together; produced as the result of high-energy particle collision
Source: WordNetpi meson
As more types of mesons were discovered in accelerator experiments later, it was eventually found that the mu meson significantly differed not only from the pi meson (of about the same mass), but also from all other types of mesons. Source: Internet
Later, the discovery of the pi meson showed it to have the properties of Yukawa's particle. Source: Internet
Newer mesons also showed evidence of behaving like the pi meson in nuclear interactions, but not like the mu meson. Source: Internet