9 The Standard Model of Fundamental Particles
169
to be a fermion, not a boson, as anticipated from Quantum Field Theory. In
fact, the particle appeared not to participate in the strong interaction at all.
It therefore lost its status as a meson, and led to the disgruntled comment by
Rabi at the head of this Chapter. It is now called the muon. The pi meson,
or pion, which is now recognised as the real Yukawa particle, was discovered
in 1947 by Cecil Powell. As a consequence, Yukawa was awarded the Nobel
Prize in physics in 1949, and Powell joined him a year later.
We shall leave Yukawa and Powell celebrating their triumphs, and digress
for a few paragraphs to discuss briefly the fourth of the forces occurring in
nature, and its consequences for the field of particle physics. The weak interaction was originally proposed in 1933 by Enrico Fermi to explain beta decay,
which is the emission of high-speed electrons from an atomic nucleus. It is
also the force that is responsible for the decay of other fundamental particles,
e.g. the decay of a free neutron into a proton, an electron and a neutrino:
n → p + e + ν .
(We shall see later that the particle emitted in this decay, for reasons of
symmetry, is actually an anti-neutrino, the anti-particle of the neutrino.)
The neutrino 3 was originally proposed by Wolfgang Pauli when he realised
that without some new particle to carry away excess angular momentum
in beta decay, conservation of angular momentum was not possible. The
neutron is a fermion, with a spin of ½. The proton and electron are also
fermions with spins of ½. Without the presence of a neutrino (a fermion
with a spin of ½) it would not be possible to balance the angular momenta
before and after the decay process. Pauli chose what he considered the lesser of
two evils. Rather than violate the law of conservation of angular momentum,
he invented a new particle, a fermion with zero, or near zero, mass. Not
that his action left him with a clear conscience: “I have done a terrible thing,”
he admitted. “I have postulated a particle that cannot be detected.” He was,
however, wrong: the neutrino has since been detected, and about 100 trillion
of them pass through our bodies every second.
As is often the case in physics, the discovery of a new entity can have
implications that no one has foreseen. The weak interaction was found to
have a unique characteristic that differentiates it from the other three forces in
a way that was not detected, and largely not even suspected, for two decades.
In the last Chapter, we stated that the laws of physics have a symmetry
with regard to the reversal of time. If we watch a video of two balls colliding
3 The name “neutrino” is Italian for “little neutral one”. Perhaps Pauli thought the Italian had a better
ring to it than “kleines Neutralteilchen”, the German equivalent.
169
to be a fermion, not a boson, as anticipated from Quantum Field Theory. In
fact, the particle appeared not to participate in the strong interaction at all.
It therefore lost its status as a meson, and led to the disgruntled comment by
Rabi at the head of this Chapter. It is now called the muon. The pi meson,
or pion, which is now recognised as the real Yukawa particle, was discovered
in 1947 by Cecil Powell. As a consequence, Yukawa was awarded the Nobel
Prize in physics in 1949, and Powell joined him a year later.
We shall leave Yukawa and Powell celebrating their triumphs, and digress
for a few paragraphs to discuss briefly the fourth of the forces occurring in
nature, and its consequences for the field of particle physics. The weak interaction was originally proposed in 1933 by Enrico Fermi to explain beta decay,
which is the emission of high-speed electrons from an atomic nucleus. It is
also the force that is responsible for the decay of other fundamental particles,
e.g. the decay of a free neutron into a proton, an electron and a neutrino:
n → p + e + ν .
(We shall see later that the particle emitted in this decay, for reasons of
symmetry, is actually an anti-neutrino, the anti-particle of the neutrino.)
The neutrino 3 was originally proposed by Wolfgang Pauli when he realised
that without some new particle to carry away excess angular momentum
in beta decay, conservation of angular momentum was not possible. The
neutron is a fermion, with a spin of ½. The proton and electron are also
fermions with spins of ½. Without the presence of a neutrino (a fermion
with a spin of ½) it would not be possible to balance the angular momenta
before and after the decay process. Pauli chose what he considered the lesser of
two evils. Rather than violate the law of conservation of angular momentum,
he invented a new particle, a fermion with zero, or near zero, mass. Not
that his action left him with a clear conscience: “I have done a terrible thing,”
he admitted. “I have postulated a particle that cannot be detected.” He was,
however, wrong: the neutrino has since been detected, and about 100 trillion
of them pass through our bodies every second.
As is often the case in physics, the discovery of a new entity can have
implications that no one has foreseen. The weak interaction was found to
have a unique characteristic that differentiates it from the other three forces in
a way that was not detected, and largely not even suspected, for two decades.
In the last Chapter, we stated that the laws of physics have a symmetry
with regard to the reversal of time. If we watch a video of two balls colliding
3 The name “neutrino” is Italian for “little neutral one”. Perhaps Pauli thought the Italian had a better
ring to it than “kleines Neutralteilchen”, the German equivalent.