Early in their training, many physics students come across the idea of spherical cows. Within a few years, they began to accumulate experimental evidence for “gluons,” the gauge particles that keep quarks bound inside protons, neutrons, and other nuclear particles. Unlike the gauge particles themselves, the “ghosts” (as their name implies) were not supposed to represent genuine particles. Feynman zoo: Applying Feynman diagrams to nuclear calculations results in a dizzying array of possibilities. Kaiser, D. Drawing Theories Apart: The Dispersion of Feynman Diagrams in Postwar Physics University of Chicago Press (2005).