A Closer Look
Any of the smallest, discrete entities of which the universe is composed, including the quarks, leptons, and gauge bosons, which are not themselves made up of other particles. Most types of elementary particles have mass, though at least one, the photon, does not. Also called fundamental particle
. See also composite particle, subatomic particle.
The smallest known units of matter, or elementary particles, are classified under three distinct groups: the quarks, the leptons, and the bosons. The six types or “flavors” of quarks are the up quark, the down quark, the charm quark, the strange quark, the top quark and the bottom quark. All quarks have mass, electric charge, and a special kind of charge called color, and each is associated with a distinct antiparticle, making twelve quarks in all. The leptons include the electron, the muon, the tau particle, the electron neutrino, the muon neutrino, and the tau neutrino. These particles also have distinct antiparticles; the neutrinos are electrically neutral and, if they do have mass, are extremely light. Each of these elementary particles interacts with other elementary particles through one or more forces: the electromagnetic force (between particles with electric charge), the strong force (between particles with color charge, such as the quarks), the weak force (between all leptons and quarks), and the gravitational force (between all particles). These forces are mediated by yet another set of elementary particles, the gauge bosons: when two particles interact, they exchange one or more gauge bosons. The gauge bosons include the W and Z bosons, which mediate the weak nuclear force, the gluon, which mediates the strong nuclear force, and the photon, which mediates the electromagnetic force. The hypothetical graviton, which would mediate the gravitational force, has not been isolated. A sixth boson, the Higgs boson, is believed to interact with the other elementary particles in such a way as to impart mass to them; it too has not been experimentally isolated. Though these particles are believed to be elementary, they can under certain circumstances change into other elementary particles. In beta decay, for example, an up quark turns into a down quark, emitting an electron and an electron antineutrino in the process. All known forms of matter and energy are made of combinations of and interactions between elementary particles; atoms, for example, are made of electrons orbiting a nucleus composed of quarks bound together into larger particles, the protons and neutrons. Whether these particles might themselves be composed of more fundamental building blocks is an open question, and the construction of a “theory of everything” that would explain the properties of all of the known particles and forces remains the ultimate goal for modern physics.