9 The Standard Model of Fundamental Particles
173
collision indicated the presence of internal structure in the proton [2]. There
could be no doubt now: the proton definitely was not a fundamental particle.
Trying to get some sort of order into the assemblage of particles that had
turned up in their collision experiments became a priority for particle physicists in the last half of the 20th century. We will not follow the history too
closely, but rather attempt an overview of the currently accepted situation,
and highlight any outstanding problems.
We have already indicated that particles can be divided into two groups,
fermions and bosons, depending on their intrinsic spins. This division
is important, because fermions satisfy the Pauli Exclusion Principle (see
Chap. 8), while bosons do not.
However, there are other classifications that are important in attempting
to sort out our zoo. Just as a visitor to a normal zoo might expect the big
cats (lions, tigers, etc.) to be in a different area from the bears and insects, so
physicists have arranged the particles into leptons and hadrons. The hadrons
are themselves subdivided into baryons and mesons. We will treat these various
groups separately, and see what their differences are.
Leptons
Leptons are particles that interact only via the electroweak interaction,
and not the strong interaction. (We will not consider gravity further in our
discussion of particles.) The name comes from the Greek root leptos, meaning
“slight”. This name is now somewhat inappropriate, as since the name was
first adopted, leptons more massive than protons have been discovered, but
no one has yet succumbed to the urge to change the name.
Leptons are truly elementary particles, having no internal structure.
According to the Standard Model, all leptons are point particles; their volume
is zero. What this means exactly is complicated by the Heisenberg Uncertainty Principle, which requires the particle wavepacket to occupy a non-zero
volume. Physicists discuss the intrinsic “size” of a particle, i.e. the size of its
internal structure, rather than the size of its wavepacket. Leptons have no
internal structure, so their “size” is zero. Experimental evidence shows the
electron to be smaller than 10 −18 m, or at least 1000 times smaller than a
proton.
Leptons come in three generations, each containing two flavours, so that
we have six flavours in total. (We must now make our acquaintance with the
somewhat flowery terminology of particle physics, where words do not have
the same meaning as they do in everyday life. Particle physicists have adopted
the attitude of Humpty Dumpty: “When I use a word,” Humpty Dumpty said
173
collision indicated the presence of internal structure in the proton [2]. There
could be no doubt now: the proton definitely was not a fundamental particle.
Trying to get some sort of order into the assemblage of particles that had
turned up in their collision experiments became a priority for particle physicists in the last half of the 20th century. We will not follow the history too
closely, but rather attempt an overview of the currently accepted situation,
and highlight any outstanding problems.
We have already indicated that particles can be divided into two groups,
fermions and bosons, depending on their intrinsic spins. This division
is important, because fermions satisfy the Pauli Exclusion Principle (see
Chap. 8), while bosons do not.
However, there are other classifications that are important in attempting
to sort out our zoo. Just as a visitor to a normal zoo might expect the big
cats (lions, tigers, etc.) to be in a different area from the bears and insects, so
physicists have arranged the particles into leptons and hadrons. The hadrons
are themselves subdivided into baryons and mesons. We will treat these various
groups separately, and see what their differences are.
Leptons
Leptons are particles that interact only via the electroweak interaction,
and not the strong interaction. (We will not consider gravity further in our
discussion of particles.) The name comes from the Greek root leptos, meaning
“slight”. This name is now somewhat inappropriate, as since the name was
first adopted, leptons more massive than protons have been discovered, but
no one has yet succumbed to the urge to change the name.
Leptons are truly elementary particles, having no internal structure.
According to the Standard Model, all leptons are point particles; their volume
is zero. What this means exactly is complicated by the Heisenberg Uncertainty Principle, which requires the particle wavepacket to occupy a non-zero
volume. Physicists discuss the intrinsic “size” of a particle, i.e. the size of its
internal structure, rather than the size of its wavepacket. Leptons have no
internal structure, so their “size” is zero. Experimental evidence shows the
electron to be smaller than 10 −18 m, or at least 1000 times smaller than a
proton.
Leptons come in three generations, each containing two flavours, so that
we have six flavours in total. (We must now make our acquaintance with the
somewhat flowery terminology of particle physics, where words do not have
the same meaning as they do in everyday life. Particle physicists have adopted
the attitude of Humpty Dumpty: “When I use a word,” Humpty Dumpty said