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lepton(lĕp`tŏn') [Gr.,=light (i.e., lightweight)], class of elementary particleselementary particles,
the most basic physical constituents of the universe. Basic Constituents of Matter
Molecules are built up from the atom, which is the basic unit of any chemical element. The atom in turn is made from the proton, neutron, and electron.
..... Click the link for more information. that includes the electronelectron,
elementary particle carrying a unit charge of negative electricity. Ordinary electric current is the flow of electrons through a wire conductor (see electricity). The electron is one of the basic constituents of matter.
..... Click the link for more information. and its antiparticleantiparticle,
elementary particle corresponding to an ordinary particle such as the proton, neutron, or electron, but having the opposite electrical charge and magnetic moment.
..... Click the link for more information. , the muonmuon
, elementary particle heavier than an electron but lighter than other particles having nonzero rest mass. The name muon is derived from mu meson, the former name of the particle. The muon was first observed in cosmic rays by Carl D.
..... Click the link for more information. and its antiparticle, the tau and its antiparticle, and the neutrinoneutrino
[Ital.,=little neutral (particle)], elementary particle with no electric charge and a very small mass emitted during the decay of certain other particles. The neutrino was first postulated in 1930 by Wolfgang Pauli in order to maintain the law of conservation of energy
..... Click the link for more information. and antineutrino associated with each of these particles. Leptons are the lightest class of particles having nonzero rest mass. From a technical point of view, they are defined by their behavior, being weakly interacting fermions, i.e., leptons can result from the slow decay of nuclear particles such as the neutron but do not experience a strong attraction toward the nuclear particles; they are described by the Fermi-Dirac statistics, which apply to all particles restricted by the Pauli exclusion principleexclusion principle,
physical principle enunciated by Wolfgang Pauli in 1925 stating that no two electrons in an atom can occupy the same energy state simultaneously. The energy states, or levels, in an atom are described in the quantum theory by various values of four different
..... Click the link for more information. . This means that two identical leptons cannot occupy the same quantum state. However, one muon and one electron are allowed to occupy the same state. The muon was originally classed as a mesonmeson
[Gr.,=middle (i.e., middleweight)], class of elementary particles whose masses are generally between those of the lepton class of lighter particles and those of the baryon class of heavier particles. From a technical point of view mesons are strongly interacting bosons; i.
..... Click the link for more information. because of its mass, about 200 times that of the electron, but the subsequent reclassification of particles on the basis of their behavior placed it with the electron in the lepton category. The electron and the muon are almost twins, except for their large mass difference; each is negatively charged, has a positively charged antiparticle, and has an associated neutrino and antineutrino. Separate laws govern the conservation of electron family number and of muon family number, the number being +1 for ordinary particles of either family and −1 for antiparticles (see conservation lawsconservation laws,
in physics, basic laws that together determine which processes can or cannot occur in nature; each law maintains that the total value of the quantity governed by that law, e.g., mass or energy, remains unchanged during physical processes.
..... Click the link for more information. , in physics).
An elementary particle having no internal constituents which interacts through the electromagnetic, weak, and gravitational forces, but does not interact through the strong (nuclear) force. Leptons are very small, less than 10-18 m in size. This is less than 10-3 the size of a nucleus and less than 10-8 the size of an atom. Indeed, existing measurements are consistent with leptons being point particles.
These properties of the lepton family of particles are to be contrasted with the properties of the quark family of particles. Quarks interact through the strong force as well as through the electromagnetic, weak, and gravitational forces. By means of the strong force, quark-antiquark pairs bind together to form hadrons such as the &pgr; meson, and the quarks bind together to form hadrons such as the proton. In contrast, leptons act as individual particles and can be studied as isolated particles whereas, as far as is known, quarks are always inside hadrons and cannot be studied as isolated particles. See Fundamental interactions, Hadron, Quarks
Six leptons are known. There are three known charged leptons: the electron (e), muon (μ), and tau (&tgr;). Associated with each charged lepton is a neutral lepton called a neutrino. A charged lepton and its associated neutrino is said to form a lepton generation. Thus there are three known lepton generations. See Electron, Neutrino