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This illustration depicts how hypothetical particles known as "monopoles" would participate in interactions between protons (p) and antiprotons (p with a horizontal bar on top) in experiments at the Fermi National Accelerator Laboratory in Illinois. This illustration is an example of a Feynman diagram, a pictorial representation of interactions between subatomic particles. At the high energies used at Fermilab, it is often the case that a single quark (q) inside the proton scatters from a single antiquark (q-bar) inside the antiproton. Since these quarks are the chief players in the interaction, the diagram shows the quark q and antiquark q-bar indicated alongside the respective proton p and antiproton p-bar to which they belong. These interactions can, according to theoretical expectation, occasionally happen through the emission of "virtual photons" (photons which exist artificially for only a brief instant and which cannot be directly measured; indicated by the Greek letter gamma) which in turn connect with a virtual monopole loop (M). The net result of this rare interaction scheme would be the release of two "real" photons which could be detected in the laboratory. Physicists at Fermilab are currently searching for such signs of monopoles, whose existence is a subject of great debate. (Illustration by Malcolm Tarlton, AIP.)
Link to Fermilab lay language article on this topic
Link to Fermilab web site