Meson

Then separately, you could take one constituent and one anticonstituent, a matching piece of antimatter, and you build the lighter class, the mesons.

If you sat down and worked out every possible way to combine three flavors, three at a time for baryons, and one and one for mesons, the combinations fell automatically into the exact octets and decuplets Gell-Mann had already mapped out a couple years earlier.

He gave his paper the title A Schematic Model of Baryons and Mesons.

But notice the word schematic in the title.

He was far from sure that these things were real particles you could find in a lab.