© Wikimedia Commons, Public Domain. Author: JabberWok2, 30 November 2007.
A pion () decays into two high-energy photons () that deposit their energy (shown in red) in a detector. Predicted by Yukawa in 1949, pions play an important role in explaining the low-energy properties of the strong force that holds atomic nuclei together. The strong force is described further in Unit 2. (Unit: 1)