The pi-meson, the messenger of the strong force that keeps the atomic nucleus together, has an unusual property.    From Wordnik.com.  [The Nobel Prize in Physics 2008 - Illustrated Presentation] Reference

Nambu's theory also claimed that the pi-meson was a composite particle – today we know that it is composed of quarks.    From Wordnik.com.  [The Nobel Prize in Physics 2008 - Illustrated Presentation] Reference

G stable pi-meson.    From Wordnik.com.  [Accelerators and Nobel Laureates] Reference