AccessScience from McGraw-Hill Education Page 1 of 21 www.accessscience.com Elementary particle Contributed by: Frank E. Close Publication year: 2015 Key Concepts • Elementary particles—including protons, neutrons, and electrons—were conventionally considered to be fundamental particles of matter, although some of these are now known to be compounds of other particles. • Other elementary particles include neutrinos (which result from the decay of unstable neutrons), quarks (fundamental constituents of protons and neutrons), photons and other bosons, muons and taus (types of leptons), and mesons (a type of hadron). • Fundamental forces that act on elementary particles are gravity, the electromagnetic force, the strong force, and the weak force, all of which act through the exchange of gauge bosons. • Interactions of elementary particles may cause scattering and transformations (decays and reactions) of particles. • Characteristics of elementary particles include mass, spin, charge, strangeness, charm, and bottomness. • The standard model explains the mass of elementary particles as being due to their interactions with the Higgs field, as manifested by Higgs bosons. A label usually applied to protons, neutrons, electrons, and related subatomic particles that were previously considered to be fundamental, although some of these are now known to be compounds of other particles. 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 as elementary particles, that is, particles that are not compounds of other particles. Today, however, many such particles (including the proton and neutron) are known to be compounds of quarks. All of these are conventionally, if illogically, known as elementary particles. See also: ELECTRON ; NEUTRON ; PROTON . AccessScience from McGraw-Hill Education Page 2 of 21 www.accessscience.com Historical overview The electrical attraction of opposite electric charges is what grips negatively charged electrons around the positively charged atomic nucleus; it is the protons that give a nucleus its charge. The amount of positive charge on a proton is equal in magnitude but opposite in sign to that on 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 ; GRAVITATION . 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, 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. 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 . Neutrinos have no electric charge and masses that are 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. See also: NEUTRINO . In the 1930s, the known elementary particles were the four just named and the photon ( γ ), the quantum particle of electromagnetic radiation. The electron and the neutrino are known as leptons and are still recognized as elementary in the sense that they are not composed of more fundamental constituents, whereas the proton and the neutron are built from quarks. Two varieties, or flavors, of quark are required to make protons and neutrons. 2 They are the up quark (denoted u ), with an electric charge that is a fraction + ∕3 of a proton’s charge, and the 1 down quark ( d ), whose fractional charge is − ∕3 . The combination uud is sufficient to make a proton, and ddu makes a neutron. See also: LEPTON ; PHOTON ; QUARKS . The up and down quarks, together with the electron and the neutrino, form a basic family of what is known as the standard model of fundamental particles. These particles are fermions, defined by having an intrinsic angular ,1 momentum or spin of ∕, 2 in units of Planck’s quantum. The photon is a boson, with an integer spin of 1. The exchange of photons between electrically charged particles transmits the electromagnetic force. See also: ANGULAR MOMENTUM ; ELECTROMAGNETISM ; QUANTUM STATISTICS ; SPIN (QUANTUM MECHANICS) ; STANDARD MODEL . AccessScience from McGraw-Hill Education Page 3 of 21 www.accessscience.com 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 . The muon is a lepton with the same spin and electric charge as an electron, but some 207 times more massive at 2 105.7 MeV ∕ c , . Today, six members of the lepton family are known: three that are electrically charged—the 2 electron ( e ), muon ( μ), and tau ( τ) [mass 1777 MeV ∕ c , ]—and three varieties of neutrino known as the ν ν ν electron-neutrino ( ,e ), the muon neutrino ( ,μ ), and the tau neutrino ( ,τ ) [ Table 1 ]. The masses of the neutrinos have not yet been directly measured, but it is known from neutrino oscillations that at least two neutrinos have nonzero masses. 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: STRONG NUCLEAR INTERACTIONS ; WEAK NUCLEAR INTERACTIONS . Particles that feel the strong force are known as hadrons. The cosmic rays revealed the existence of unstable hadrons, some of which became known as strange particles, such as the K meson and the lambda hyperon ( Λ). Experiments at particle accelerators enabled such particles to be produced when the kinetic energy of particles colliding with atomic nuclei was converted into ephemeral particles, which were revealed by their trails in bubble chambers (or nowadays by electronic detectors). Among these were short-lived heavier versions of the −23 proton and the neutron, known as resonances, whose lifetimes of the order of 10, s are similar to the time it takes a beam of light to traverse a proton. All of these hadrons are composed of quarks. See also: HADRON ; PARTICLE DETECTOR . 2 There are six flavors of quark. The up ( u ), charm ( c ), and top ( t ) have electric charge + ∕3 ; the down ( d ), strange 1 ( s ), and bottom ( b ) have charge − ∕3 ( Table 1 ). 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, 2 2 their masses being at most 6 MeV ∕ c , . The strange quark is some 100 MeV ∕ c , more massive, the charm quark 2 2 2 mass is around 1.3 GeV ∕ c , (1300 MeV ∕ c , ), the bottom quark mass is around 4.5 GeV ∕ c , , and the top quark 2 mass is around 175 GeV ∕ c , . The reason behind these values, or even their qualitative pattern, is not understood. 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. AccessScience from McGraw-Hill Education Page 4 of 21 www.accessscience.com WIDTH:D Fundamental particles and interactions The fundamental forces that act on the particles are gravity, the electromagnetic force, and the strong and weak forces. These forces act through the exchange of particles known as gauge bosons ( Table 1 ). For the electromagnetic, strong, and weak forces, these have been identified, respectively, as the photon, gluon, and weak bosons; the graviton, the quantum of the gravitational force, is firmly predicted by theory, but the prospect of direct observation is exceedingly remote. The gravitational force between individual particles is so feeble that it can be ignored for all practical purposes. See also: FUNDAMENTAL INTERACTIONS ; GLUONS ; GRAVITON . The interactions of particles are responsible for their scattering and transformations (decays and reactions). Because of interactions, an isolated particle may decay into other particles. Two particles passing near each other may be transformed, perhaps into the same particles but with changed momenta (elastic scattering) or perhaps AccessScience from McGraw-Hill Education Page 5 of 21 www.accessscience.com into other particles (inelastic scattering).