1960 Muonium

Possible varieties of matter far exceed the ordinary. If exotic atoms had greater lifetimes they would seem more ordinary, and more things would be possible.

Quark, baryon, meson, hadron, gluon, lepton, boson, fermion, photon; these are categories of physical particles that make up our world. Since most of us are familiar only with the photon, here is a brief outline:

Quark

Most matter is made of elementary particles known as quarks. Quarks have spins of ½.

Hadrons

All particles made of two or more quarks held together by the strong force. Protons and neutrons, which are combinations of three quarks and form atomic nuclei, are hadrons. Hadrons undergo strong interactions.

Baryon

A combination of an odd number of quarks, usually three. Baryons are hadrons.

Meson

A combination of an even number of quarks, usually two, a quark and an antiquark, bound by the strong interaction. Mesons are hadrons and are unstable. They carry the nuclear force that holds atomic nuclei together. Charged mesons decay into electrons and neutrinos; uncharged mesons may decay into photons.

Gluon

An elementary particle (not made of quarks), a vector boson that mediates the strong interaction between quarks. Gluons carry qualities between quarks.

Lepton

An elementary particle (also not made of quarks) having a half-integer spin, a charge of -1, and not undergoing strong interactions. Electron, muon, tau, electron neutrino, muon neutrino, and tau neutrino are leptons.

Fermion

Particles with half-integer spins. Two fermions cannot occupy the same quantum state; they retain their own identity. Leptons and composite particles made up of an odd number of quarks including baryons are fermions.

Boson

Particles with integer spins. Two bosons can occupy the same quantum state at the same time. They don’t retain their own identity. Some bosons are elementary particles (gluons, photons), and some are composites (mesons). A gauge boson carries a force.

Photon

An elementary particle, a vector boson, that mediates the electromagnetic interaction. A photon does not carry a charge. A photon is a vector boson and, like a gluon, has a spin of 1.

Maybe someone can think of a way to diagram these differences clearly.

See also in The book of science:

• 1897—Electron—Joseph John Thomson
• 1932—Antimatter—Paul Dirac, Carl David Anderson
• 1932—Phonon—Igor Tamm
• 1932-1951—Positronium—Carl David Anderson, Stjepan Mohorovičić, Martin Deutsch
• 1934,1947—Meson—Hideki Yukawa, Cecil Powell, César Lattes, Giuseppe Occhialini
• 1935—Majorana fermion—Ettore Majorana
• 1936—Muon—Carl David Anderson, Seth Neddermeyer
• 1956—Neutrino—Wolfgang Pauli, Frederick Reines, Clyde Cowan
• 1971-1977—Tau—Yung-su Tsai, Martin Lewis Perl

Readings in wikipedia:

• “Vernon W. Hughes”
• “Muonium”
• “True muonium”
• “Exotic atom”
• “Lepton”
• “Particle physics”