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Elementary Particle

Elementary Particle

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Elementary

Contributed by: Frank E. Close

Publication year: 2018

Key Concepts

• Elementary are the most fundamental components of and cannot be further subdivided

into smaller constituents.

• Elementary particles include (the constituents of and ), (,

, taus, and ), gauge (, , and ) and the Higgs .

• The fundamental forces that act on elementary particles are the electromagnetic force, the strong force,

the weak force, and .

• Characteristics of elementary particles include , , and .

• To each kind of there corresponds an , or conjugate particle, which has

the same mass and spin, but has the opposite value of charge and ∕ or flavor number.

• The of classifies and describes the behavior of all known elementary

particles.

Fundamental, irreducible units that constitute the matter of the and express the forces of . 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: ; MASS ; SPIN () ; 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 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: ; MATTER (PHYSICS) ; ; . 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 . (Credit: )

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 , 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 . 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 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 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 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: . 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 , 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 , 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 , 0.105 , ∕2 , -1 , Discovered 1936 ,

1 blank , 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 , 0 , 1 , 0 , Concept proposed by in 1905; carries electromagnetic force ,

blank , 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 ), 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 , 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 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- particle physics accelerators. With modern hindsight, it is possible to classify the discoveries into two classes: leptons and . See also: COSMIC RAYS ; PARTICLE ACCELERATOR ; PHOTON .

Today, six members of the 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 ( ,μ ), ν and the ( ,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 ; 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 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 of around 300

MeV; most of this is due to their motion, with their masses being less than 6 MeV ∕ c2. The is some

90 MeV ∕ c2 more massive, the mass is around 1.3 GeV ∕ c2 (1300 MeV ∕ c2), the mass is around 4.2 GeV ∕ c2, and the mass is around 173 GeV ∕ c2. The reason behind these values, or even their qualitative pattern, is not understood. See also: ; 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 hadrons.

The six flavors of quark and six leptons in the standard model of particle physics are , defined by having the same half-integer spin, or intrinsic angular , 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. These forces act through the exchange of particles known as gauge bosons, also part of the standard model. For the electromagnetic, strong, and weak forces, these have been identified, respectively, as the photon, gluon, and weak bosons; the , 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 ; STRONG NUCLEAR INTERACTIONS ; WEAK NUCLEAR INTERACTIONS .

The interactions of particles are responsible for their 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 () or perhaps into other particles (inelastic scattering). The rates or cross sections of these transformations, and so also the 21 23 −1 interactions responsible for them, fall into three groups: strong (typical decay rates of 10, –10, s, ), 16 19 −1 15 −1 electromagnetic (10, –10, s, ), and weak ( < 10, s, ).

The photon, gluon, and weak bosons have spin 1. The photon and gluon have no mass, whereas weak bosons are + − 2 massive. The weak bosons may be electrically charged (the W, and W, ), with masses of 80.4 GeV ∕ c, , or neutral 0 2 (the Z, ), with mass of 91.2 GeV ∕ c, . The fundamental couplings are known as gauge couplings, in which a is absorbed or emitted by a fundamental particle ( or gauge boson). See also: ;

INTERMEDIATE .

A massless photon with little or no energy can be emitted and absorbed by electrons in atoms or within the normal laws of energy and momentum conservation. However, when a neutron undergoes beta decay, − it converts to a proton by emitting a “virtual” W, , violating energy conservation by some 80 GeV. The Heisenberg of quantum mechanics states that such a violation can occur, but only for a limited time, − within which time the W, converts into an electron and a neutrino, which are the real end products of beta decay. The energy violation of 80 GeV is so extreme that the ensuing time scale, and in consequence the distance − through which the W, can travel, is exceedingly short. This limits the effects of the forces associated with W exchange, leading it to appear “weak.” See also: ENERGY ; UNCERTAINTY PRINCIPLE .

0 Data show that the fundamental couplings of a photon or of a W or Z, to a lepton or quark are similar in magnitude, so that the electromagnetic and weak forces are now regarded as being two manifestations of a single electroweak force. The apparent difference in their strengths is because the weak bosons are massive, whereas the photon is massless. These differences in masses, and the consequent differences in the effective forces, are examples of symmetry breaking. The source of this broken symmetry is a spinless particle known as the Higgs boson, long expected theoretically through the latter half of the twentieth century and at last experimentally confirmed at the Large Hadron (LHC) at CERN in 2012. It is the between the Higgs boson and AccessScience from McGraw-Hill Education Page 6 of 8 www.accessscience.com

the fundamental fermions and bosons that gives them their masses. The mass of the Higgs boson is around 126 2 GeV ∕ c, . See also: ; HIGGS BOSON ; HIGGS BOSON DETECTION AT THE LHC ;

(LHC) ; SYMMETRY BREAKING .

In addition to their electric charges and flavors, quarks carry a property called color, which is the source of the strong force between them. As photons couple to electric charge and mediate the electromagnetic force, so massless gluons transmit the color force. There are three varieties of color for quarks. The quantum theory of the color force is called (QCD). See also: COLOR (QUANTUM MECHANICS) .

At high enough energies, such as those during the , the strong, weak and electromagnetic forces should combine into a single force, as proposed by grand unification theories. See also: BIG BANG THEORY ; GRAND

UNIFICATION THEORIES .

Most particles are unstable and decay into particles with lower masses. Massless particles, such as the photon, are stable. Neutrinos, which were long thought to be massless and stable, are now known to have small masses; the lightest neutrino is the lightest fermion and is stable, as its decay would be into bosons, which could not conserve . The present view is that the only massive particles that are strictly stable are the electron and the lightest neutrino or neutrinos. The electron is the lightest ; its decay would be into neutral particles and could not conserve charge. The proton may be stable, although there is no fundamental 34 reason why it need be; for all practical purposes, it is stable with a half-life exceeding 10, years.

Unstable elementary particles must be studied within a short time after their creation, which occurs during the collision of a fast (high-energy) particle with another particle. Such high-energy particles occur naturally in the cosmic rays, but their flux is small; thus, most elementary particle research is based on high-energy particle accelerators.

Antiparticles

To each kind of elementary particle there corresponds an antiparticle, or conjugate particle, which has the same mass and spin, but the opposite value of charge and ∕ or flavor . In quarks, the flavor quantum numbers are termed strangeness, charm, bottomness, or topness, and correspond to the quarks with these associated flavor names, while leptons similarly have lepton numbers. A simple example of an particle is the positively charged , which is otherwise identical to the negatively charged electron. See also:

ANTIMATTER .

Mesons are examples of hadrons that have integer spins and are made of an equal number of quarks and their antimatter equivalents, antiquarks. Scores of are known, including four-particle consisting of two quarks and two antiquarks. It is theoretically possible to form mesons even made entirely from gluons, with no quarks, known as . See also: ; . AccessScience from McGraw-Hill Education Page 7 of 8 www.accessscience.com

Beyond the standard model

Efforts are continuing theoretically and experimentally to go beyond the standard model of particle physics, with the aim of introducing new elementary particles to explain phenomena such as . One particular set of theories is called . These theories propose that there is a further symmetry between force and matter particles, such that the known fermions partner with new bosons, and the known bosons with new fermions, transmitting novel forces. As a result, each known particle would have a sparticle, or , with names such as selectron for the electron, a for the top quark, and for the gluon.

Experimental data has yet to bear out the validity of supersymmetry. See also: SUPERSYMMETRY .

Meanwhile, searches continue for non-standard model, elementary particles at the LHC and in dark matter detection experiments worldwide. If history is any guide, the modern notion of “elementary particle” could very well break down into still-more fundamental units, or find that the standard model is but a portion of a much larger, encompassing, particle-based reality. See also: DARK MATTER .

Frank E. Close

Keywords ; boson; electromagnetism; electron; fermion; fundamental interactions; fundamental particles; grand unified theory; neutrino; particle accelerator; quark; supersymmetry;

Test Your Understanding

1. Compare and contrast leptons and hadrons, and give examples of each type of particle.

2. Compare and contrast photons, gluons, and weak bosons.

3. What is an antiparticle? How do know that antiparticles exist?

4. Critical Thinking: What combination of up and down quarks make up protons and neutrons?

5. Critical Thinking: Which elementary particles have the strongest interactions with the Higgs field, and

why?

Bibliography

G. Aad et al. (The ATLAS Collaboration), Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC, Phys. Lett. B. , 716(1):1–29, 2012

DOI: http://doi.org/10.1016/j.physletb.2012.08.020

F. Abe et al. (CDF Collaboration), Observation of Top Quark Production in p p Collisions with the Collider

Detector at Fermilab, Phys. Rev. Lett. , 74(14):2626, 1995 DOI: http://doi.org/10.1103/PhysRevLett.74.2626 AccessScience from McGraw-Hill Education Page 8 of 8 www.accessscience.com

S.W. Herb et al., Observation of a Dimuon Resonance at 9.5 GeV in 400-GeV Proton-Nucleus Collisions, Phys.

Rev. Lett. , 39(5):252, 1977 DOI: http://doi.org/10.1103/PhysRevLett.39.252

K. Patrignani et al. (), The Review of Particle Physics, Chin. Phys. C , 40:100001, 2016 and

2017 update DOI: http://doi.org/10.1088/1674-1137/40/10/100001

Additional Readings

S. Braibant, G. Giacomelli, and M. Spurio, Particles and Fundamental Interactions , Springer, 2012

F. Close, The New Cosmic Onion , Taylor and Francis, 2007

Y. Nagashima, Elementary Particle Physics , 2 vols., Wiley-VCH, 2013

P. G. Ratcliffe, An Introduction to Elementary Particle Phenomenology , IOP Publishing, 2014

CERN

Fermilab

Particle Data Group: The Particle Adventure

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