The Two Families of Fundamental Particles Out of Which All Ordinary Matter Is Made Are

Particle Physics and Cosmology

81 Introduction to Particle Physics

Learning Objectives

By the end of this section, you will be able to:

Describe the four fundamental forces and what particles participate in them
Identify and depict fermions and bosons
Identify and describe the quark and lepton families
Distinguish between particles and antiparticles, and depict their interactions

Elementary particle physics is the report of fundamental particles and their interactions in nature. Those who study simple particle physics—the particle physicists—differ from other physicists in the scale of the systems that they report. A particle physicist is not content to report the microscopic world of cells, molecules, atoms, or even atomic nuclei. They are interested in concrete processes that occur at scales even smaller than diminutive nuclei. At the same time, they appoint the most profound mysteries in nature: How did the universe begin? What explains the pattern of masses in the universe? Why is in that location more matter than antimatter in the universe? Why are free energy and momentum conserved? How will the universe evolve?

Four Primal Forces

An important step to answering these questions is to understand particles and their interactions. Particle interactions are expressed in terms of 4 fundamental force southward. In club of decreasing strength, these forces are the strong nuclear strength, the electromagnetic force, the weak nuclear force, and the gravitational force.

Strong nuclear force. The strong nuclear strength is a very strong attractive force that acts only over very curt distances (about ${10}^{-15}\phantom{\rule{0.2em}{0ex}}\text{m}$ ). The strong nuclear strength is responsible for bounden protons and neutrons together in atomic nuclei. Not all particles participate in the potent nuclear force; for instance, electrons and neutrinos are not affected past it. As the name suggests, this force is much stronger than the other forces.
Electromagnetic force. The electromagnetic force tin can act over very large distances (it has an space range) but is but ane/100 the strength of the potent nuclear force. Particles that interact through this force are said to have "charge." In the classical theory of static electricity (Coulomb's law), the electric force varies as the product of the charges of the interacting particles, and every bit the inverse square of the distances between them. In contrast to the strong force, the electromagnetic force can be attractive or repulsive (opposite charges attract and similar charges repel). The magnetic force depends in a more complicated way on the charges and their motions. The unification of the electric and magnetic force into a single electromagnetic force (an achievement of James Clerk Maxwell) stands as one of the greatest intellectual achievements of the nineteenth century. This strength is central to scientific models of atomic structure and molecular bonding.
Weak nuclear strength. The weak nuclear force acts over very brusk distances $\left({10}^{-15}\phantom{\rule{0.2em}{0ex}}\text{m}\right)$ and, as its name suggest, is very weak. Information technology is roughly ${10}^{-6}$ the strength of the stiff nuclear force. This force is manifested almost notably in decays of elementary particles and neutrino interactions. For example, the neutron can disuse to a proton, electron, and electron neutrino through the weak force. The weak forcefulness is vitally important because it is essential for understanding stellar nucleosynthesis—the process that creates new diminutive nuclei in the cores of stars.
Gravitational force. Like the electromagnetic forcefulness, the gravitational strength can act over infinitely large distances; however, it is only ${10}^{-38}$ as strong equally the strong nuclear force. In Newton's classical theory of gravity, the force of gravity varies as the product of the masses of the interacting particles and every bit the inverse square of the altitude between them. This force is an bonny forcefulness that acts between all particles with mass. In modernistic theories of gravity, this strength behavior is considered a special case for low-energy macroscopic interactions. Compared with the other forces of nature, gravity is by far the weakest.

The central forces may not be truly "fundamental" but may actually exist dissimilar aspects of the same forcefulness. Just as the electric and magnetic forces were unified into an electromagnetic force, physicists in the 1970s unified the electromagnetic forcefulness with the weak nuclear forcefulness into an electroweak force. Any scientific theory that attempts to unify the electroweak force and strong nuclear force is chosen a grand unified theory, and any theory that attempts to unify all 4 forces is called a theory of everything. Nosotros volition return to the concept of unification later in this affiliate.

Classifications of Unproblematic Particles

A big number of subatomic particles exist in nature. These particles tin be classified in two means: the holding of spin and participation in the four central forces. Recall that the spin of a particle is analogous to the rotation of a macroscopic object nearly its own axis. These types of nomenclature are described separately beneath.

Nomenclature past spin

Particles of matter tin be divided into fermion s and boson s. Fermions have half-integral spin $\left(\frac{1}{2}\hslash ,\phantom{\rule{0.2em}{0ex}}\frac{3}{2}\hslash ,\phantom{\rule{0.2em}{0ex}}\text{…}\right)$ and bosons accept integral spin $\left(0\hslash ,\phantom{\rule{0.2em}{0ex}}1\hslash ,2\hslash ,\text{…}\right).$ Familiar examples of fermions are electrons, protons, and neutrons. A familiar example of a boson is a photon. Fermions and bosons behave very differently in groups. For case, when electrons are confined to a small region of infinite, Pauli's exclusion principle states that no two electrons can occupy the same quantum-mechanical land. All the same, when photons are confined to a modest region of space, in that location is no such limitation.

The behavior of fermions and bosons in groups can be understood in terms of the belongings of indistinguishability. Particles are said to be "indistinguishable" if they are identical to one some other. For example, electrons are duplicate because every electron in the universe has exactly the same mass and spin as all other electrons—"when you've seen ane electron, yous've seen them all." If you switch two duplicate particles in the aforementioned small region of space, the square of the wave office that describes this system and can exist measured $\left(|\psi {|}^{2}\right)$ is unchanged. If this were non the instance, we could tell whether or not the particles had been switched and the particle would not exist truly indistinguishable. Fermions and bosons differ by whether the sign of the wave function ( $\psi$ )— not directly observable—flips:

$\begin{array}{c}\psi \to \text{−}\psi \phantom{\rule{0.2em}{0ex}}\text{(indistinguishable fermions),}\hfill \\ \psi \to +\psi \phantom{\rule{0.2em}{0ex}}\text{(indistinguishable bosons).}\hfill \end{array}$

Fermions are said to be "antisymmetric on exchange" and bosons are "symmetric on substitution." Pauli'southward exclusion principle is a upshot of exchange symmetry of fermions—a connection adult in a more than advanced form in mod physics. The electronic construction of atoms is predicated on Pauli's exclusion principle and is therefore directly related to the indistinguishability of electrons.

Classification past force interactions

Fermions can be further divided into quark south and lepton s. The main difference between these two types of particles is that quarks collaborate via the strong force and leptons exercise not. Quarks and leptons (every bit well as bosons to exist discussed later) are organized in (Effigy). The upper ii rows (first three columns in royal) comprise six quarks. These quarks are arranged into two particle families: up, amuse, and top (u, c, t), and down, foreign, and bottom (d, s, b). Members of the same particle family share the same properties merely differ in mass (given in $\text{MeV/}{c}^{2}$ ). For case, the mass of the top quark is much greater than the charm quark, and the mass of the amuse quark is much greater than the up quark. All quarks interact with one some other through the stiff nuclear force.

The families of subatomic particles, categorized past the types of forces with which they interact. (credit: modification of work past "MissMJ"/Wikimedia Commons)

Ordinary thing consists of two types of quarks: the up quark (uncomplicated charge, $q=+2\text{/}3$ ) and the down quark $\left(q=-1\text{/}3\right).$ Heavier quarks are unstable and chop-chop decay to lighter ones via the weak force. Quarks bind together in groups of twos and threes chosen hadron s via the strong force. Hadrons that consist of two quarks are called mesons, and those that consist of iii quarks are called baryons. Examples of mesons include the pion and kaon, and examples of baryons include the familiar proton and neutron. A proton is two up quarks and a downwards quark $\left(\text{p}=uud,\phantom{\rule{0.2em}{0ex}}q=\text{+1}\right)$ and a neutron is one up quark and ii down quarks ( $\text{n}=udd$ , $q=0$ ). Backdrop of sample mesons and baryons are given in (Effigy). Quarks participate in all four cardinal forces: potent, weak, electromagnetic, and gravitational.

The lower ii rows in the figure (in green) contain six leptons arranged into two particle families: electron, muon, and tau ( $e,\mu ,\tau$ ), and electron neutrino, muon neutrino, and tau neutrino ( ${\upsilon }_{e},{\upsilon }_{\mu },{\upsilon }_{\tau }$ ). The muon is over 200 times heavier than an electron, but is otherwise similar to the electron. The tau is about 3500 times heavier than the electron, just is otherwise similar to the muon and electron. Once created, the muon and tau rapidly decay to lighter particles via the weak force. Leptons do non participate in the potent force. Quarks and leptons will be discussed later in this affiliate. Leptons participate in the weak, electromagnetic, and gravitational forces, simply do not participate in the strong forcefulness.

Bosons (shown in red) are the forcefulness carriers of the fermions. In this model, leptons and quarks collaborate with each other by sending and receiving bosons. For example, Coulombic interaction occurs when two positively charged particles send and receive (substitution) photons. The photons are said to "carry" the forcefulness between charged particles. Likewise, allure betwixt ii quarks in an atomic nucleus occurs when two quarks transport and receive gluon s. Additional examples include W and Z boson s (which deport weak nuclear force) and gravitons (which carry gravitational forcefulness). The Higgs boson is a special particle: When it interacts with other particles, it endows them non with forcefulness only with mass. In other words, the Higgs boson helps to explains why particles have mass. These assertions are part of a tentative but very productive scientific model (the Standard Model) discussed later.

Particles and Antiparticles

In the late 1920s, the special theory of relativity and breakthrough mechanics were combined into a relativistic quantum theory of the electron. A surprising event of this theory was the prediction of two free energy states for each electron: Ane is associated with the electron, and the other is associated with some other particle with the same mass of an electron but with a charge of ${\text{e}}^{+}$ . This particle is chosen the antielectron or positron. The positron was discovered experimentally in the 1930s.

Soon it was discovered that for every particle in nature, there is a respective antiparticle. An antiparticle has the aforementioned mass and lifetime as its associated particle, and the contrary sign of electric charge. These particles are produced in high-energy reactions. Examples of high-energy particles include the antimuon ( ${\mu }^{+}$ ), anti-upward quark ( $\stackrel{\text{−}}{u}$ ), and anti-down quark $\left(\stackrel{\text{−}}{d}\right).$ (Note that antiparticles for quarks are designated with an over-bar.) Many mesons and baryons incorporate antiparticles. For example, the antiproton ( $\stackrel{\text{−}}{\text{p}}$ ) is $\stackrel{\text{−}}{u}\stackrel{\text{−}}{u}\stackrel{\text{−}}{d}$ and the positively charged pion $\left({\pi }^{+}\right)$ is $u\stackrel{\text{−}}{d}$ . Some neutral particles, such as the photon and the ${\pi }^{0}$ meson, are their own antiparticles. Sample particles, antiparticles, and their properties are listed in (Figure).

Particles and their Properties
	Particle name	Symbol	Antiparticle	Mass $\left(\text{MeV/}{c}^{2}\right)$	Boilerplate lifetime (s)
Leptons
	Electron	${\text{e}}^{\text{−}}$	${\text{e}}^{+}$	0.511	Stable
	Electron neutrino	${\upsilon }_{e}$	$\stackrel{\text{—}}{{\upsilon }_{e}}$	$\approx 0$	Stable
	Muon	${\mu }^{\text{−}}$	${\mu }^{+}$	105.7	$2.20\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{-6}$
	Muon neutrino	${\upsilon }_{\mu }$	$\stackrel{\text{—}}{{\upsilon }_{\mu }}$	$\approx 0$	Stable
	Tau	${\tau }^{\text{−}}$	${\tau }^{+}$	1784	$<4\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{-13}$
	Tau neutrino	${\upsilon }_{\tau }$	$\stackrel{\text{—}}{{\upsilon }_{\tau }}$	$\approx 0$	Stable
Hadrons
Baryons	Proton	p	$\stackrel{\text{−}}{\text{p}}$	938.3	Stable
	Neutron	n	$\stackrel{\text{−}}{\text{n}}$	939.6	920
	Lambda	${\text{Λ}}^{0}$	$\stackrel{\text{—}}{{\text{Λ}}^{0}}$	1115.6	$2.6\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{-10}$
	Sigma	${\text{Σ}}^{+}$	${\text{Σ}}^{\text{−}}$	1189.four	$0.80\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{-10}$
	Xi	${\text{Ξ}}^{+}$	${\text{Ξ}}^{\text{−}}$	1315	$2.9\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{-10}$
	Omega	${\text{Ω}}^{+}$	${\text{Ω}}^{\text{−}}$	1672	$0.82\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{-10}$
Mesons	Pion	${\pi }^{+}$	${\pi }^{\text{−}}$	139.6	$2.60\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{-8}$
	$\pi$ -Nix	${\pi }^{0}$	${\pi }^{0}$	135.0	$0.83\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{-16}$
	Kaon	${\text{K}}^{+}$	${\text{K}}^{\text{−}}$	493.seven	$1.24\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{-8}$
	1000-Short	${\text{K}}_{S}^{0}$	$\stackrel{\text{—}}{{\text{K}}_{S}^{0}}$	497.7	$0.89\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{-10}$
	k-Long	${\text{K}}_{L}^{0}$	$\stackrel{—}{{\text{K}}_{L}^{0}}$	497.0	$5.2\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{-8}$
	J/ $\psi$	J/ $\psi$	J/ $\psi$	3100	$7.1\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{-21}$
	Upsilon	$\text{Υ}$	$\text{Υ}$	9460	$1.2\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{-20}$

The same forces that hold ordinary matter together also hold antimatter together. Under the right weather condition, it is possible to create antiatoms such every bit antihydrogen, antioxygen, and even antiwater. In antiatoms, positrons orbit a negatively charged nucleus of antiprotons and antineutrons. (Figure) compares atoms and antiatoms.

A comparing of the simplest atoms of matter and antimatter. (a) In the Bohr model, an antihydrogen atom consists of a positron that orbits an antiproton. (b) An antihelium cantlet consists of two positrons that orbit a nucleus of two antiprotons and two antineutrons.

Antimatter cannot exist for long in nature considering particles and antiparticles demolish each other to produce high-energy radiation. A common example is electron-positron annihilation. This process proceeds past the reaction

${\text{e}}^{\text{−}}+{\text{e}}^{+}\to 2\gamma .$

The electron and positron vanish completely and ii photons are produced in their identify. (Information technology turns out that the production of a single photon would violate conservation of free energy and momentum.) This reaction tin can also proceed in the reverse direction: Two photons can annihilate each other to produce an electron and positron pair. Or, a single photon can produce an electron-positron pair in the field of a nucleus, a process called pair product. Reactions of this kind are measured routinely in modern particle detectors. The being of antiparticles in nature is non science fiction.

Watch this video to learn more near matter and antimatter particles.

Summary

The four key forces of nature are, in order of strength: stiff nuclear, electromagnetic, weak nuclear, and gravitational. Quarks interact via the stiff force, but leptons practice non. Both quark and leptons interact via the electromagnetic, weak, and gravitational forces.
Elementary particles are classified into fermions and boson. Fermions have one-half-integral spin and obey the exclusion principle. Bosons take integral spin and do non obey this principle. Bosons are the strength carriers of particle interactions.
Quarks and leptons belong to particle families composed of three members each. Members of a family share many properties (charge, spin, participation in forces) but not mass.
All particles have antiparticles. Particles share the same properties as their antimatter particles, but carry opposite charge.

Conceptual Questions

What are the four cardinal forces? Briefly describe them.

Strong nuclear force: interaction between quarks, mediated by gluons. Electromagnetic force: interaction between accuse particles, mediated photons. Weak nuclear forcefulness: interactions between fermions, mediated by heavy bosons. Gravitational force: interactions between material (massive) particle, mediate by hypothetical gravitons.

Distinguish fermions and bosons using the concepts of indistiguishability and exchange symmetry.

List the quark and lepton families

electron, muon, tau; electron neutrino, muon neutrino, tau neutrino; down quark, strange quark, bottom quark; upwards quark, charm quark, top quark

Distinguish between elementary particles and antiparticles. Describe their interactions.

Problems

How much free energy is released when an electron and a positron at residuum demolish each other? (For particle masses, come across (Effigy).)

ane.022 MeV

If $1.0\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{30}\phantom{\rule{0.2em}{0ex}}\text{MeV}$ of energy is released in the annihilation of a sphere of matter and antimatter, and the spheres are equal mass, what are the masses of the spheres?

When both an electron and a positron are at rest, they can annihilate each other co-ordinate to the reaction

${e}^{\text{−}}+{e}^{+}\to \gamma +\gamma .$

In this case, what are the energy, momentum, and frequency of each photon?

0.511 MeV, $2.73\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{-22}\phantom{\rule{0.2em}{0ex}}\text{kg}·\text{m}\text{/}\text{s}$ , $1.23\phantom{\rule{0.2em}{0ex}}×\phantom{\rule{0.2em}{0ex}}{10}^{20}\phantom{\rule{0.2em}{0ex}}\text{Hz}$

What is the total kinetic energy carried abroad by the particles of the following decays?
$\begin{array}{}\\ \\ \left(\text{a}\right)\phantom{\rule{0.2em}{0ex}}{\pi }^{0}\to \gamma +\gamma \hfill \\ \left(\text{b}\right)\phantom{\rule{0.2em}{0ex}}{\text{K}}^{0}\to {\pi }^{+}+{\pi }^{\text{−}}\hfill \\ \left(\text{c}\right)\phantom{\rule{0.2em}{0ex}}{\Sigma }^{+}\to n+{\pi }^{+}\hfill \\ \left(\text{d}\right)\phantom{\rule{0.2em}{0ex}}{\Sigma }^{0}\to {\Lambda }^{0}+\gamma .\hfill \end{array}$

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Source: https://opentextbc.ca/universityphysicsv3openstax/chapter/introduction-to-particle-physics/

The Two Families of Fundamental Particles Out of Which All Ordinary Matter Is Made Are

81 Introduction to Particle Physics

Learning Objectives

Four Primal Forces

Classifications of Unproblematic Particles

Nomenclature past spin

Classification past force interactions

Particles and Antiparticles

Summary

Conceptual Questions

Problems

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