212 14. THE STRUCTURE OF ENERGY AND MATTER
When we say that physical laws have symmetries such as invariance under translations in
space and time, we are making claims about Nature. But how do we test whether or not these
claims are true? Noether’s First eorem shows us the way. e conserved quantities associated
with these symmetries are measurable, and so we can measure them in experiments that test
whether or not they are conserved. As we build more and more experimental evidence for a
particular conservation law, Noether’s First eorem demonstrates that we also provide evidence
for that particular symmetry of Nature.
14.3 THE STANDARD MODEL OF PARTICLE PHYSICS
Over the past several decades, a picture has gradually emerged that attempts to bring together all
of the known forces and types of matter into one unified scheme. Much of the motivation for the
particulars comes from a search for various notions of symmetry acting at the most fundamental
level of subatomic particles and the interactions between them.
At this level it is more common to speak of an interaction than a force. An interaction can
be seen as a particular set of rules by which certain particles affect each other. ese interactions
are typically studied with large particle accelerators, which smack subatomic particles into each
other at extremely high energy densities. Two of the largest of these accelerators are at CERN,
in Geneva and Fermilab, near Chicago.
But it has been increasingly recognized that these human-made accelerators will never
reach energies that are high enough to probe the deepest levels, in order to reach an understand-
ing of all of these interactions. We do, however, live in the aftermath of such a high-powered
accelerator experiment—the Big Bang itself. And so increasingly, there is a synergy between
particle physics and cosmology; each informs the other.
14.3.1 THE PARTICLES AND THEIR INTERACTIONS
Apart from the familiar protons, neutrons and electrons that make up ordinary matter, and the
photons that make up light, there are a myriad of other particles that can be identified—each
with its own particular properties—from nuclear reactions and particle accelerator experiments.
But it has become increasingly clear that most of these particles are not fundamental themselves;
they are combinations of more fundamental particles. is is even true of our familiar friends,
the proton and the neutron.
e Standard Model of Elementary Particles makes sense of this seeming chaos of par-
ticles, showing how they can all be formed from only three families of matter, along with two
groups of interaction particles. e interaction particles produce the forces between particles—
and so both matter and the fundamental forces are explained in one framework
A summary of the Standard Model can be seen in Figure 14.4. First, it is separated into
bosons (red and yellow in the diagram) and fermions (purple and green in the figure). Fermions
have, at some level, solidity; there is a limit to how tightly packed they can be. Electrons, for
example, can only be packed into a density of about 1.4 M
@
per Earth volume—the maximum
14.3. THE STANDARD MODEL OF PARTICLE PHYSICS 213
Standard Model of Elementary Particles
three generations of matter
(fermions)
I II III
interactions / force carriers
(bosons)
QUARKS
u
≃ 2.2 MeV/c²
⅔
½
up
d
≃ 4.7 MeV/c²
−⅓
½
down
c
≃ 1.28 GeV/c²
⅔
½
charm
s
≃ 96 MeV/c²
−⅓
½
strange
t
≃ 173.1 GeV/c²
⅔
½
top
b
≃ 4.18 GeV/c²
−⅓
½
bottom
LEPTONS
e
≃ 0.511 MeV/c²
−1
½
electron
ν
e
<2.2 eV/c²
0
½
electron
neutrino
μ
≃ 105.66 MeV/c²
−1
½
muon
ν
μ
<0.17 MeV/c²
0
½
muon
neutrino
τ
≃ 1.7768 GeV/c²
−1
½
tau
ν
τ
<18.2 MeV/c²
0
½
tau
neutrino
GAUGE BOSONS
VECTOR BOSONS
g
0
0
1
gluon
γ
0
0
1
photon
Z
≃ 91.19 GeV/c²
0
1
Z boson
W
≃ 80.39 GeV/c²
±1
1
W boson
SCALAR BOSONS
H
≃ 124.97 GeV/c²
0
0
higgs
mass
charge
spin
Figure 14.4: A diagram showing the families and types of fundamental particles and their in-
teractions. (Graphic by MissMJ, Public Domain.)
density of a white dwarf. If compressed this tight, they push back, and if forced any tighter,
they must change to some other form of particle. is is why a white dwarf can withstand the
enormous force of its own gravity without collapsing—it is held up by the enormous pressure
of electrons confined to their minimum possible space. A neutron star is similar in that it is also
held up by fermions compressed to their smallest possible volume. But for a neutron star it is
the much smaller volume of the neutron.
Each of the three families (also called generations) of fermions is broken into two groups:
quarks and leptons. Leptons are fundamental in and of themselves; the most familiar example
is the electron. e quarks combine in twos or threes to make other, usually relatively mas-
sive particles. e most familiar of these are the proton and neutron, each of which is made of
combinations of up and down quarks.
Notice that the most familiar particles—protons, neutrons, and electrons—are all part of
the first family. Particles made from combinations of quarks in the second and third families are
unstable, and simply do not appear at the low energy densities that occupy our daily lives. But
they can be seen in collisions made by particle accelerators. e particles in these three families
..................Content has been hidden....................
You can't read the all page of ebook, please click here login for view all page.