Elementary
Particles
Elementary Particles

Atoms
◦ From the Greek for “indivisible”
◦ Were once thought to be the elementary
particles

Atom constituents
◦ Proton, neutron, and electron
◦ Were viewed as elementary because they
are very stable
Quarks

Physicists recognize that most particles are
made up of quarks
◦ Exceptions include photons, electrons and a few
others
The quark model has reduced the array of
particles to a manageable few
 The quark model has successfully predicted
new quark combinations that were
subsequently found in many experiments

Fundamental Forces

All particles in nature are subject to
four fundamental forces
◦
◦
◦
◦
Strong force
Electromagnetic force
Weak force
Gravitational force
Strong Force
Is responsible for the tight binding of the
quarks to form neutrons and protons
 Also responsible for the nuclear force
binding the neutrons and the protons
together in the nucleus
 Strongest of all the fundamental forces
 Very short-ranged

◦ Less than 10-15 m
Electromagnetic Force
Is responsible for the binding of atoms
and molecules
 About 10-2 times the strength of the
strong force
 A long-range force that decreases in
strength as the inverse square of the
separation between interacting
particles

Weak Force

Is responsible for instability in certain nuclei
◦ Is responsible for beta decay
A short-ranged force
 Its strength is about 10-6 times that of the
strong force
 Scientists now believe the weak and
electromagnetic forces are two
manifestations of a single force, the
electroweak force

Gravitational Force
A familiar force that holds the planets, stars
and galaxies together
 Its effect on elementary particles is
negligible
 A long-range force
 It is about 10-43 times the strength of the
strong force

◦ Weakest of the four fundamental forces
Explanation of Forces

Forces between particles are often
described in terms of the actions of
field particles or quanta
◦ For electromagnetic force, the photon is
the field particle
◦ The electromagnetic force is mediated, or
carried, by photons
Forces and Mediating Particles (also
see table 30.1)
Interaction (force)
Mediating Field
Particle
Strong
Gluon
Electromagnetic
Photon
Weak
W± and Z0
Gravitational
Gravitons
Antiparticles

For every particle, there is an antiparticle
◦ From Dirac’s version of quantum mechanics that
incorporated special relativity


An antiparticle has the same mass as the particle,
but the opposite charge
The positron (electron’s antiparticle) was
discovered by Anderson in 1932
◦ Since then, it has been observed in numerous experiments

Practically every known elementary particle has a
distinct antiparticle
◦ Exceptions – the photon and the neutral pi particles are
their own antiparticles
Classification of Particles
Two broad categories
 Classified by interactions

◦ Hadrons
 Interact through strong force
 Composed of quarks
◦ Leptons
 Interact through weak force
 Thought to be truly elementary
 Some suggestions they may have some internal
structure
Hadrons
Interact through the strong force
 Two subclasses

◦ Mesons
 Decay finally into electrons, positrons, neutrinos and
photons
 Integer spins
◦ Baryons
 Masses equal to or greater than a proton
 Noninteger spin values
 Decay into end products that include a proton (except for
the proton)

Composed of quarks
Leptons
Interact through weak force
 All have spin of ½
 Leptons appear truly elementary

◦ No substructure
◦ Point-like particles

Scientists currently believe only six leptons
exist, along with their antiparticles
◦ Electron and electron neutrino
◦ Muon and its neutrino
◦ Tau and its neutrino
Quarks
Hadrons are complex particles with size
and structure
 Hadrons decay into other hadrons
 There are many different hadrons
 Quarks are proposed as the elementary
particles that constitute the hadrons

◦ Originally proposed independently by Gell-Mann
and Zweig
Quark Model

Three types
◦
◦
◦
◦
◦
◦

u – up
d – down
s – strange
c – charmed
t – top
b – bottom
Associated with each quark is an antiquark
◦ The antiquark has opposite charge, baryon
number and strangeness
Quark Model, cont

Quarks have fractional electrical
charges
◦ +1/3 e and –2/3 e

All ordinary matter consists of just u
and d quarks
Quark Model – Rules

All the hadrons at the time of the
original proposal were explained by
three rules
◦ Mesons consist of one quark and one
antiquark
 This gives them a baryon number of 0
◦ Baryons consist of three quarks
◦ Antibaryons consist of three antiquarks
Numbers of Particles

At the present, physicists believe the
“building blocks” of matter are
complete
◦ Six quarks with their antiparticles
◦ Six leptons with their antiparticles
◦ See table 30.3 for quark summary
Color

Isolated quarks
◦ Physicist now believe that quarks are permanently
confined inside ordinary particles
 No isolated quarks have been observed experimentally
◦ The explanation is a force called the color force
 Color force increases with increasing distance
 This prevents the quarks from becoming isolated particles
Colored Quarks

Color “charge” occurs in red, blue, or
green
◦ Antiquarks have colors of antired, antiblue,
or antigreen
Color obeys the Exclusion Principle
 A combination of quarks of each color
produces white (or colorless)
 Baryons and mesons are always
colorless

Quark Structure of a Meson
A green quark is
attracted to an
antigreen quark
 The quark –
antiquark pair forms
a meson
 The resulting meson
is colorless

Quark Structure of a Baryon
Quarks of different
colors attract each
other
 The quark triplet
forms a baryon
 The baryon is
colorless

Quantum Chromodynamics
(QCD)
QCD gave a new theory of how quarks
interact with each other by means of color
charge
 The strong force between quarks is often
called the color force
 The strong force between quarks is carried
by gluons

◦ Gluons are massless particles
◦ There are 8 gluons, all with color charge

When a quark emits or absorbs a gluon, its
color changes
More About Color Charge

Like colors repel and opposite colors attract
◦ Different colors also attract, but not as strongly as a
color and its anticolor

The color force between color-neutral hadrons is
negligible at large separations
◦ The strong color force between the constituent quarks
does not exactly cancel at small separations
◦ This residual strong force is the nuclear force that
binds the protons and neutrons to form nuclei
Electroweak Theory
The electroweak theory unifies
electromagnetic and weak interactions
 The theory postulates that the weak
and electromagnetic interactions have
the strength at very high particle
energies

◦ Viewed as two different manifestations of
a single interaction
The Standard Model
A combination of the electroweak theory
and QCD form the standard model
 Essential ingredients of the standard model

◦ The strong force, mediated by gluons, holds the
quarks together to form composite particles
◦ Leptons participate only in electromagnetic and weak
interactions
◦ The electromagnetic force is mediated by photons
◦ The weak force is mediated by W and Z bosons
The Standard Model – Chart
The Big Bang

This theory of cosmology states that during
the first few minutes after the creation of the
universe all four interactions were unified
◦ All matter was contained in a quark soup
As time increased and temperature
decreased, the forces broke apart
 Starting as a radiation dominated universe,
as the universe cooled it changed to a
matter dominated universe

A Brief History of the Universe
Some Questions


Why so little antimatter in the Universe?
Do neutrinos have mass?
◦ How do they contribute to the dark mass in the
universe?
Explanation of why the expansion of the universe
is accelerating?
 Is there a kind of antigravity force acting between
widely separated galaxies?
 Is it possible to unify electroweak and strong
forces?
 Why do quark and leptons form similar but
distinct families?

More Questions





Are muons the same as electrons, except for
their mass?
Why are some particles charged and others
neutral?
Why do quarks carry fractional charge?
What determines the masses of fundamental
particles?
Do leptons and quarks have a substructure?