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AccessScience from McGraw-Hill Education Page 1 of 8 www.accessscience.com Elementary particle Contributed by: Frank E. Close Publication year: 2018 Key Concepts • Elementary particles are the most fundamental components of matter and cannot be further subdivided into smaller constituents. • Elementary particles include quarks (the constituents of protons and neutrons), leptons (electrons, muons, taus, and neutrinos), gauge bosons (photons, gluons, and W and Z bosons) and the Higgs boson. • The fundamental forces that act on elementary particles are the electromagnetic force, the strong force, the weak force, and gravity. • Characteristics of elementary particles include mass, spin, and charge. • To each kind of elementary particle there corresponds an antiparticle, or conjugate particle, which has the same mass and spin, but has the opposite value of charge and ∕ or flavor quantum number. • The standard model of particle physics classifies and describes the behavior of all known elementary particles. Fundamental, irreducible units that constitute the matter of the universe and express the forces of nature. The standard model of particle physics, which emerged in the 1970s, classifies and describes the 17 known elementary particles in terms of their properties such as mass, spin and charge (Fig. 1 and table). See also: ELECTRIC CHARGE ; MASS ; SPIN (QUANTUM MECHANICS) ; STANDARD MODEL . Our identification of the elementary particles has evolved with our understanding of matter. The idea that everything is made from a few basic elements originated in ancient Greece. In the nineteenth century, the elementary pieces of matter were believed to be the atoms of the chemical elements, but in the first half of the twentieth century, atoms were found to be made up of electrons, protons, and neutrons. These became known at the time as elementary particles, that is, particles that are not compounds of other particles. Today, however, protons and neutrons are known to be compounds of quarks, which are therefore considered to be true elementary particles. See also: ELECTRON ; MATTER (PHYSICS) ; NEUTRON ; PROTON . AccessScience from McGraw-Hill Education Page 2 of 8 www.accessscience.com StaandardFig 1. The standardmodel of model particle of particlephysics physics expressed graphically, including the six types, or flavors, of quarks; the six leptons; the four gauge bosons; and the Higgs boson. (Credit: Fermilab) Historical overview The first elementary particle discovered was the electron in 1897 by English physicist J.J. Thompson. The discovery of the proton—long presumed an elementary particle—proceeded in the early 20th century, with British physicist Ernest Rutherford naming it in 1920. The attraction of opposite electric charges grips negatively charged electrons around the positively charged protons in an atomic nucleus. The amount of positive charge a proton exerts is equal in magnitude but opposite in sign to that of an electron. This is crucial for the fact that matter in bulk is electrically neutral, with the result that gravity controls the motion of planets and our attraction to the Earth’s surface. However, why these two particles carry such precisely counterbalanced electric charges is a mystery. See also: COULOMB’S LAW ; ELECTRIC CHARGE ; GRAVITY . 2 A proton is some 1836 times as massive as an electron, their masses being, respectively, 938.3 and 0.511 MeV ∕ c, , 2 where c is the speed of light. A neutron has a mass of 939.6 MeV ∕ c, and is very similar to a proton. In the immediate aftermath of its discovery in 1932 by English physicist James Chadwick in 1932, the neutron was thought to be a version of a proton with no electric charge; however, today their relationship is understood to be more profound. See also: ELECTRONVOLT . AccessScience from McGraw-Hill Education Page 3 of 8 www.accessscience.com Basic properties of the elementary particles in the standard model of particle physics, 1 blank Particle , Mass, 2 , Spin , Charge, 3 , Notes , Quarks , blank blank blank blank blank 1 2 blank up , 0.0022 , ∕2 , ∕3 , Discovered 1968; along with the down quark, forms protons and neutrons , 1 2 blank charm , 1.28 , ∕2 , ∕3 , Discovered 1974 , 1 2 blank top , 173 , ∕2 , ∕3 , Discovered 1995 , 1 1 blank down , 0.0047 , ∕2 , - ∕3 , Discovered 1968; along with the up quark, forms protons and neutrons , 1 1 blank strange , 0.096 , ∕2 , - ∕3 , Discovered 1968 , 1 1 blank bottom , 4.18 , ∕2 , - ∕3 , Discovered 1977 , Leptons , blank blank blank blank blank 1 blank electron , 0.000511 , ∕2 , -1 , Discovered 1897; binds with protons to form atoms , 1 blank muon , 0.105 , ∕2 , -1 , Discovered 1936 , 1 blank tau , 1.78 , ∕2 , -1 , Discovered 1975 , < 1 blank electron neutrino , 0.00000022 , ∕2 , 0 , Discovered 1956 , < 1 blank muon neutrino , 0.0017 , ∕2 , 0 , Discovered 1962 , < 1 blank tau neutrino , 0.0155 , ∕2 , 0 , Discovered 2000 , Gauge bosons , blank blank blank blank blank blank photon , 0 , 1 , 0 , Concept proposed by Albert Einstein in 1905; carries electromagnetic force , blank gluon , 0 , 1 , 0 , Discovered 1978; carries strong force , blank W boson , 80.4 , 1 , + ∕ - 1 , Discovered 1983; carries weak force , blank Z boson , 91.2 , 1 , 0 , Discovered 1983; carries weak force , Scalar bosons , blank blank blank blank blank blank Higgs boson , 125 , 0 , 0 , Discovered 2012; imbues particles with mass , ,1 According to: C. Patrignani et al. (Particle Data Group), The Review of Particle Physics , Chin. Phys. C, 40(100001), 2016 and 2017 update. ,2 Expressed in gigaelectron volts (GeV). ,3 Expressed in units of proton charge. An electron or a proton is stable, at least on time scales longer than the age of the universe. When neutrons are in the nuclei of atoms such as iron, they too may survive unchanged for billions of years. However, an isolated neutron is unstable, with a mean life of 886 s. Neutrons can also be unstable when large numbers of them are packed into a nucleus with protons; this leads to natural radioactivity of many elements. Such neutrons undergo beta decay, a result of the weak force, which converts a neutron into a proton and emits an electron and a neutrino. See also: BETA PARTICLES ; RADIOACTIVITY . AccessScience from McGraw-Hill Education Page 4 of 8 www.accessscience.com Neutrinos have no electric charge and masses that still remain too small to measure with present techniques. The 2 neutrino emitted in the neutron beta decay has a mass that is less than 2 eV ∕ c, , that is, less than 1 ∕ 100,000 the mass of the electron. The Austrian-born Swiss physicist Wolfgang Pauli first proposed the existence of the neutrino in 1930. See also: NEUTRINO . In this early era of elementary particle physics, the particles considered elementary were the four just named, plus the photon ( γ ), the quantum particle of electromagnetic radiation. This simple picture began to break down around 1950 with the discoveries of new forms of particles, first in cosmic rays and then in experiments at high-energy particle physics accelerators. With modern hindsight, it is possible to classify the discoveries into two classes: leptons and hadrons. See also: COSMIC RAYS ; PARTICLE ACCELERATOR ; PHOTON . Today, six members of the lepton family are known: three that are electrically charged—the electron ( e ), muon μ τ ν ν ( ), and tau ( )—and three varieties of neutrino known as the electron-neutrino ( ,e ), the muon neutrino ( ,μ ), ν and the tau neutrino ( ,τ ). All of these are fundamental particles. Leptons are unaffected by the strong (nuclear) force but feel the weak force and, if electrically charged (the electron, muon, and tau), the electromagnetic force. See also: LEPTON ; STRONG NUCLEAR INTERACTIONS ; WEAK NUCLEAR INTERACTIONS . Particles that feel the strong force are known as hadrons. In turn, hadrons are composed of elementary particles called quarks, which were first proposed in 1964 by the U.S. physicist Murray Gell-Mann and the Russian-born U.S. physicist George Zweig. There are six flavors of quark. The up (u), charm (c), and top (t) have electric 2 1 charge + ∕3 ; the down (d), strange (s), and bottom (b) have charge − ∕3 . Two up quarks and one down quark combine to make a proton, while a neutron consists of two down quarks and an up quark. As individual quarks cannot appear in isolation, a direct measure of their mass is not possible, but approximate scales of mass have been determined. When they are trapped inside hadrons, the up and down quarks have energies of around 300 MeV; most of this is due to their motion, with their masses being less than 6 MeV ∕ c2. The strange quark is some 90 MeV ∕ c2 more massive, the charm quark mass is around 1.3 GeV ∕ c2 (1300 MeV ∕ c2), the bottom quark mass is around 4.2 GeV ∕ c2, and the top quark mass is around 173 GeV ∕ c2. The reason behind these values, or even their qualitative pattern, is not understood. See also: HADRON ; QUARK . Strange hadrons contain one or more strange quarks. Hadrons containing charm or bottom quarks are known as charm and bottom hadrons (including a special category known as charmonium and bottomonium). Top quarks are so heavy that no hadrons containing them have been identified. It is even possible that top hadrons cannot → ν form, as the top quark is so unstable that it decays (in a form of beta decay, t be ,e ) before it can grip to other quarks to make observable hadrons. The six flavors of quark and six leptons in the standard model of particle physics are fermions, defined by having the same half-integer spin, or intrinsic angular momentum, and conserved quantum numbers. See also: ANGULAR MOMENTUM ; QUANTUM STATISTICS ; SPIN (QUANTUM MECHANICS) ; STANDARD MODEL . AccessScience from McGraw-Hill Education Page 5 of 8 www.accessscience.com Fundamental particles and interactions The fundamental forces that act on the particles are gravity, the electromagnetic force, and the strong and weak forces.
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