Boson

Mesons are particles made of a quark and an antiquark, both of which are fermions. Mesons, however, are bosons. Physicists have a rule for determining whether a composite particle is a boson or a fermion. If a particle contains an odd number of fermions, it is a fermion. If it contains an even number of fermions, it is a boson. Mesons contain a quark and an antiquark—that is, two fermions—so they are bosons. This rule extends to larger particles also. For example, a nucleus of light helium contains two protons and one neutron. Both protons and neutrons are fermions (because they contain an odd number of quarks), so the light helium atom is also a fermion. A nucleus of ordinary helium contains two neutrons and two protons, so it is a boson.

There are six quarks and six antiquarks, so 36 possible mesons exist. The first meson discovered, the pion (p), plays an important part in keeping the nucleus of an atom together. It carries the strong force between protons and neutrons, just as gluons carry the strong force between quarks. Other important types of mesons include kaons and D particles.

Einstein and German physicist Max Planck suggested the existence of photons in 1905. In the early 1920s several physicists performed experiments that confirmed the existence of the photon. In the mid-1920s Pauli developed the exclusion principle—the rule that separates bosons from fermions. Around the same time, Bose collaborated with Einstein to develop a set of rules for photons and other particles that do not obey the exclusion principle.

The photon was the only known boson until the 1930s. In 1934 Japanese physicist Yukawa Hideki predicted the existence of a particle that held particles together in the nuclei of atoms. This particle is now known as the pion. British physicist Cecil Powell discovered the pion—the first meson to be isolated—in 1947. In 1964 American physicists Murray Gell-Mann and George Zweig proposed that protons and neutrons were made up of smaller particles. This allowed the introduction of gluons and quarks. The first quark was detected in 1964. In the same year British physicist Peter Higgs proposed the existence of the Higgs boson.

Evidence for the gluon was discovered in experiments at the Stanford Linear Accelerator Center (SLAC) in California in 1968. Linear accelerators are machines that boost particles to high speeds. The beams of accelerated particles shoot at a target, and physicists examine the results of the collisions to learn more about particles and interactions (see Particle Accelerators). Italian physicist Carlo Rubbia and Dutch physicist Simon Van der Meer discovered evidence for the W and Z bosons in 1983. Searches continue for the graviton and other bosons, including the Higgs boson.