11.1 Introduction to particle physics (Page 4/22)

University physics volume 3 Page 4 / 22

Soon it was discovered that for every particle in nature, there is a corresponding antiparticle . An antiparticle has the same mass and lifetime as its associated particle, and the opposite sign of electric charge. These particles are produced in high-energy reactions. Examples of high-energy particles include the antimuon ( $μ^{+}$ ), anti-up quark ( $\overset{−}{u}$ ), and anti-down quark $(\overset{−}{d}) .$ (Note that antiparticles for quarks are designated with an over-bar.) Many mesons and baryons contain antiparticles. For example, the antiproton ( $\overset{−}{p}$ ) is $\overset{−}{u} \overset{−}{u} \overset{−}{d}$ and the positively charged pion $(π^{+})$ is $u \overset{−}{d}$ . Some neutral particles, such as the photon and the $π^{0}$ meson, are their own antiparticles. Sample particles, antiparticles, and their properties are listed in [link] .

Particles and their properties
	Particle name	Symbol	Antiparticle	Mass $(MeV/ c^{2})$	Average lifetime (s)
Leptons
	Electron	$e^{−}$	$e^{+}$	0.511	Stable
	Electron neutrino	$υ_{e}$	$\overset{—}{υ_{e}}$	$\approx 0$	Stable
	Muon	$μ^{−}$	$μ^{+}$	105.7	$2.20 \times 10^{−6}$
	Muon neutrino	$υ_{μ}$	$\overset{—}{υ_{μ}}$	$\approx 0$	Stable
	Tau	$τ^{−}$	$τ^{+}$	1784	$< 4 \times 10^{−13}$
	Tau neutrino	$υ_{τ}$	$\overset{—}{υ_{τ}}$	$\approx 0$	Stable
Hadrons
Baryons	Proton	p	$\overset{−}{p}$	938.3	Stable
	Neutron	n	$\overset{−}{n}$	939.6	920
	Lambda	$Λ^{0}$	$\overset{—}{Λ^{0}}$	1115.6	$2.6 \times 10^{−10}$
	Sigma	$Σ^{+}$	$Σ^{−}$	1189.4	$0.80 \times 10^{−10}$
	Xi	$Ξ^{+}$	$Ξ^{−}$	1315	$2.9 \times 10^{−10}$
	Omega	$Ω^{+}$	$Ω^{−}$	1672	$0.82 \times 10^{−10}$
Mesons	Pion	$π^{+}$	$π^{−}$	139.6	$2.60 \times 10^{−8}$
	$π$ -Zero	$π^{0}$	$π^{0}$	135.0	$0.83 \times 10^{−16}$
	Kaon	$K^{+}$	$K^{−}$	493.7	$1.24 \times 10^{−8}$
	k -Short	$K_{S}^{0}$	$\overset{—}{K_{S}^{0}}$	497.7	$0.89 \times 10^{−10}$
	k -Long	$K_{L}^{0}$	$\overset{—}{K_{L}^{0}}$	497.0	$5.2 \times 10^{−8}$
	J / $ψ$	J / $ψ$	J / $ψ$	3100	$7.1 \times 10^{−21}$
	Upsilon	$Υ$	$Υ$	9460	$1.2 \times 10^{−20}$

The same forces that hold ordinary matter together also hold antimatter together. Under the right conditions, it is possible to create antiatoms such as antihydrogen, antioxygen, and even antiwater. In antiatoms, positrons orbit a negatively charged nucleus of antiprotons and antineutrons. [link] compares atoms and antiatoms.

Figure a shows a hydrogen atom and an antihydrogen atom. The former has a circle labeled p at the center and another, smaller circle labeled e minus in an orbit around it. The latter has a circle labeled p bar at the center and another, smaller circle labeled e plus in an orbit around it. Figure b shows a helium atom and an antihelium atom. The former has a circle labeled 2p 2n at the center and two smaller circles labeled e minus in an orbit around it. The latter has a circle labeled 2p bar 2 n bar at the center and two smaller circles labeled e plus in an orbit around it. — A comparison 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 atom consists of two positrons that orbit a nucleus of two antiprotons and two antineutrons.

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

e^{−} + e^{+} \to 2 γ .

The electron and positron vanish completely and two photons are produced in their place. (It turns out that the production of a single photon would violate conservation of energy and momentum.) This reaction 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 production. Reactions of this kind are measured routinely in modern particle detectors. The existence of antiparticles in nature is not science fiction.

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

Summary

The four fundamental forces of nature are, in order of strength: strong nuclear, electromagnetic, weak nuclear, and gravitational. Quarks interact via the strong force, but leptons do not. Both quark and leptons interact via the electromagnetic, weak, and gravitational forces.
Elementary particles are classified into fermions and boson. Fermions have half-integral spin and obey the exclusion principle. Bosons have integral spin and do not obey this principle. Bosons are the force 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 fundamental forces? Briefly describe them.

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

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

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List the quark and lepton families

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

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Distinguish between elementary particles and antiparticles. Describe their interactions.

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Problems

How much energy is released when an electron and a positron at rest annihilate each other? (For particle masses, see [link] .)

1.022 MeV

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If $1.0 \times 10^{30} MeV$ 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?

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

$e^{−} + e^{+} \to γ + γ .$

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

0.511 MeV, $2.73 \times 10^{−22} kg \cdot m / s$ , $1.23 \times 10^{20} Hz$

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What is the total kinetic energy carried away by the particles of the following decays?
$\begin{matrix} (a) π^{0} \to γ + γ \\ (b) K^{0} \to π^{+} + π^{−} \\ (c) Σ^{+} \to n + π^{+} \\ (d) Σ^{0} \to Λ^{0} + γ . \end{matrix}$

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Source: OpenStax, University physics volume 3. OpenStax CNX. Nov 04, 2016 Download for free at http://cnx.org/content/col12067/1.4

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