In the 1950s new particles with masses in between those of the electron and the proton were found. They were given the name “mesotrons” where “meso” meant intermediate. This was soon shortened to meson. The most commonly produced meson in particle accelerators is the pi-meson or pion. Pions can be positively or negatively charged or uncharged.

In the quark model mesons are composed of a quark and an anti-quark. The quarks that make up the three pions are shown below.

It is also possible to create particles with different combinations of three up and down quarks. These particles have higher masses than protons and neutrons and can only be created by colliding protons and neutrons together with very high energies. Examples are the combinations uuu with charge 2 and the ddd with charge -1. These particles decay via the strong interaction. The uuu decays when its extra energy creates a ddpair. The d quark replaces one u quark in the uuu particle, resulting in a proton and the 5 combines with the extra u quark to form a T .