Download Fundamental Particles

Fundamental Particles
Jean Brainard, Ph.D.
Say Thanks to the Authors
Click http://www.ck12.org/saythanks
(No sign in required)
To access a customizable version of this book, as well as other
interactive content, visit www.ck12.org
CK-12 Foundation is a non-profit organization with a mission to
reduce the cost of textbook materials for the K-12 market both
in the U.S. and worldwide. Using an open-content, web-based
collaborative model termed the FlexBook®, CK-12 intends to
pioneer the generation and distribution of high-quality educational
content that will serve both as core text as well as provide an
adaptive environment for learning, powered through the FlexBook
Platform®.
Copyright © 2013 CK-12 Foundation, www.ck12.org
The names “CK-12” and “CK12” and associated logos and the
terms “FlexBook®” and “FlexBook Platform®” (collectively
“CK-12 Marks”) are trademarks and service marks of CK-12
Foundation and are protected by federal, state, and international
laws.
Any form of reproduction of this book in any format or medium,
in whole or in sections must include the referral attribution link
http://www.ck12.org/saythanks (placed in a visible location) in
addition to the following terms.
Except as otherwise noted, all CK-12 Content (including
CK-12 Curriculum Material) is made available to Users
in accordance with the Creative Commons Attribution/NonCommercial/Share Alike 3.0 Unported (CC BY-NC-SA) License
(http://creativecommons.org/licenses/by-nc-sa/3.0/), as amended
and updated by Creative Commons from time to time (the “CC
License”), which is incorporated herein by this reference.
Complete terms can be found at http://www.ck12.org/terms.
Printed: July 14, 2013
AUTHOR
Jean Brainard, Ph.D.
www.ck12.org
C ONCEPT
•
•
•
•
Concept 1. Fundamental Particles
1
Fundamental Particles
Outline the search for fundamental particles.
Compare and contrast quarks, leptons, and bosons.
Relate bosons to the four fundamental forces.
Describe the standard model.
Who says scientists don’t have a sense of humor? Look at the odd names and images in this chart. Do you have any
idea what they represent? Would it surprise you to learn that they represent the most fundamental particles of matter?
In fact, particles with funny names like charm quarks and strange quarks make up all the matter in the universe. For
an excellent video introduction to fundamental particles inside the atom, go to this URL:
http://www.youtube.com/watch?v=Vi91qyjuknM (7:44)
MEDIA
Click image to the left for more content.
The Search for Fundamental Particles
Scientists have long wanted to find the most basic building blocks of the universe. They asked, “what are the
fundamental particles of matter that cannot be subdivided into smaller, simpler particles,” and “what holds these
1
www.ck12.org
particles together?” The quest for fundamental particles began thousands of years ago. Scientists thought they had
finally found them when John Dalton discovered the atom in 1803 (see the timeline in Table 1.1). The word atom
means “indivisible,” and Dalton thought that the atom could not be divided into smaller, simpler particles.
TABLE 1.1: Timeline of Discovery of Fundamental Particles
2
Year
1803
Discovery
John Dalton discovers the atom.
1897
J.J. Thomson discovers the electron, the first lepton to
be discovered.
1905
Albert Einstein discovers the photon, the first boson to
be discovered.
www.ck12.org
Concept 1. Fundamental Particles
TABLE 1.1: (continued)
Year
1911
Discovery
Ernest Rutherford discovers the proton, the first particle
to be discovered in the nucleus of the atom.
1932
James Chadwick discovers the neutron, another particle
in the nucleus.
1964
Murray Gell-Mann proposes the existence of quarks,
the fundamental particles that make up protons and
neutrons.
1964-present
Through the research of many scientists, many other
fundamental particles (except gravitons) are shown to
exist.
For almost 100 years after Dalton discovered atoms, they were accepted as the fundamental particles of matter. But
starting in the late 1890s with the discovery of electrons, particles smaller and simpler than atoms were identified.
Within a few decades, protons and neutrons were also discovered. Ultimately, hundreds of subatomic particles were
found. You can learn how such tiny particles are discovered by watching the video at this URL: http://www.ehow.
com/video_5112658_particle-accelerator_.html.
3
www.ck12.org
Leptons and Quarks and Bosons, Oh My!
Today, scientists think that electrons truly are fundamental particles that cannot be broken down into smaller, simpler
particles. They are a type of fundamental particles called leptons. Protons and neutrons, on the other hand, are no
longer thought to be fundamental particles. Instead, they are now thought to consist of smaller, simpler particles of
matter called quarks. Scientists theorize that leptons and quarks are held together by yet another type of fundamental
particles called bosons. All three types of fundamental particles—leptons, quarks, and bosons—are described below.
The following Table 1.1 shows the variety of particles of each type.
FIGURE 1.1
• There are six types of quarks. In ordinary matter, virtually all quarks are of the types called up and down
quarks. All quarks have mass, and they have an electric charge of either +2/3 or -1/3. For example, up quarks
have a charge of +2/3, and down quarks have a charge of -1/3. Quarks also have a different type of charge,
called color charge, although it has nothing to do with the colors that we see. Quarks are never found alone
but instead always occur in groups of two or three quarks.
• There are also six types of leptons, including electrons. Leptons have an electric charge of either -1 or 0.
Electrons, for example, have a charge of -1. Leptons have mass, although the mass of electrons is extremely
small.
• There are four known types of bosons, which are force-carrying particles. Each of these bosons carries a
different fundamental force between interacting particles. In addition, there is a particle which may exist,
called the "Higgs Boson", which gives objects the masses they have. Some types of bosons have mass; others
are massless. Bosons have an electric charge of +1, -1, or 0.
Q: Protons consist of three quarks: two up quarks and one down quark. Neutrons also consist of three quarks: two
down quarks and one up quark. Based on this information, what is the total electric charge of a proton? Of a neutron?
4
www.ck12.org
Concept 1. Fundamental Particles
A: These combinations of quarks give protons a total electric charge of +1 (2/3 + 2/3 – 1/3 = 1) and neutrons a total
electric charge of 0 (2/3 – 1/3 – 1/3 = 0).
Force-Carrying Particles
The interactions of matter particles are subject to four fundamental forces: gravity, electromagnetic force, weak
nuclear force, and strong nuclear force. All of these forces are thought to be transmitted by bosons, the forcecarrying fundamental particles. The different types of bosons and the forces they carry are shown in Table 1.2.
Consider the examples of gluons, the bosons that carry the strong nuclear force. A continuous exchange of gluons
between quarks binds them together in both protons and neutrons. Note that force-carrying particles for gravity
(gravitons) have not yet been found.
TABLE 1.2: Bosons and Fundamental Forces
Type of Bosons
Gluons
W bosons
Z bosons
Photons
Gravitons (hypothetical)
Fundamental Force They
Carry
strong nuclear force
weak nuclear force
Particles They Affect
quarks
leptons and quarks
Distance over Which
They Carry Force
only within the nucleus
only within the nucleus
electromagnetic force
force of gravity
leptons and quarks
leptons and quarks
all distances
all distances
Q: Which type of boson carries force between the negative electrons and positive protons of an atom?
A: Photons carry electromagnetic force. They are responsible for the force of attraction or repulsion between all
electrically charged matter, including the force of attraction between negative electrons and positive protons in an
atom.
Q: Gravitons have not yet been discovered so they have only been hypothesized to exist. What evidence do you think
leads scientists to think that these hypothetical particles affect both leptons and quarks and that they carry force over
all distances?
A: Gravity is known to affect all matter that has mass, and both quarks and leptons have mass. Gravity is also known
to work over long as well as short distances. For example, Earth’s gravity keeps you firmly planted on the ground
and also keeps the moon orbiting around the planet.
The Standard Model
Based on their knowledge of subatomic particles, scientists have developed a theory called the standard model to
explain all the matter in the universe and how it is held together. The model includes only the fundamental particles
in the table above. No other particles are needed to explain all kinds of matter. According to the model, all known
matter consists of quarks and leptons that interact by exchanging bosons, which transmit fundamental forces. The
standard model is a good theory because all of its predictions have been verified by experimental data. However,
the model doesn’t explain everything, including the force of gravity and why matter has mass. Scientists continue
to search for evidence that will allow them to explain these aspects of force and matter as well. You can learn much
more about the standard model at the URL below. Click on “the standard model” and follow the sequence titled
“What is fundamental?”
http://particleadventure.org/
5
www.ck12.org
Summary
• For centuries, scientists searched for the fundamental particles of matter and the “glue” that holds them
together. At first, scientists thought that atoms were the fundamental particles. Now they know that there
are smaller, simpler particles than atoms that make up matter and carry the forces that hold matter together.
• Protons and neutrons are made up of fundamental particles of matter called quarks. Electrons are another type
of fundamental particles of matter called leptons. Bosons are fundamental particles that carry forces between
fundamental particles of matter.
• Scientists think that a different type of boson carries each of the four fundamental forces in the universe (strong
and weak nuclear forces, electromagnetic force, and gravity).
• The standard model is a simple theory that explains all the matter in the universe and its interactions (except
for the mass of matter and the force of gravity). According to the standard model, all known matter consists
of quarks and leptons, which interact by exchanging force-carrying particles called bosons.
Vocabulary
• fundamental particle: Particle of matter, such as an electron, that cannot be subdivided into smaller, simpler
particles.
• quark: Type of particle that makes up protons and neutrons.
• standard model: Theory that all known matter consists of fundamental particles called quarks and leptons.
Practice
Explore fundamental particles with the sequence, “What is the world made of?” at the following URL (access it by
clicking on “the standard model”). Then check your knowledge of fundamental particles by taking the quiz at the
end of the sequence.
http://particleadventure.org/
Review
1. Outline the order in which fundamental particles were discovered.
2. Make a table comparing and contrasting the three types of fundamental particles. Include an example of each
type in your table.
3. Make a two- or three-dimensional model of a hydrogen atom (1 proton and 1 electron) that represents all of
its fundamental particles, including force-carrying particles.
4. What is the standard model? In what ways is it incomplete?
References
1. MissMJ. . CC-BY 3.0
6