Particle physics, study of subatomic particles (those particles that are smaller than atoms), including protons, neutrons, electrons, and a wide variety of much more unstable particles. Physicists now classify subatomic particles into 4 general classes. The smallest of these are the bosons, which have no mass. They include the photon, which is a packet of energy, and 8 types of gluons. The next class, the leptons, has 12 particles: the electron, which carries a negative electromagnetic charge, the positron, which is identical but carries a positive charge, 2 muons (of opposite charge), and a neutrino associated with each of these 3 pairs. The third class of particles is the mesons, which are larger in mass than the leptons. They are nuclear particles that serve to hold the nuclei of atoms together. The most massive of the 4 classes is the baryons, which include the proton, the neutron, and heavier particles called hyperons. Mesons and baryons, unlike the other 2 classes, are governed by the strong nuclear force, which is one of the 4 fundamental forces of nature (the others being gravity, electromagnetism, and the weak nuclear force). For this reason, these 2 classes of particles are sometimes grouped together under the term hadrons. Many particle physicists today subscribe to a theory first put forward in 1964 by Murray Gell-Mann and George Zweig stating that all the hedrons are ultimately composed of still more elementary particles called quarks. Particle physics studies all these various particles and the relationships and interactions among them.