4
1. The Particles and Forces of the Standard Model
if neutral, only weakly. By contrast, the quarks – which are the constituents
of hadrons, and thence of nuclei – interact via all three interactions, strong,
electromagnetic and weak. The weak and electromagnetic interactions of both
quarks and leptons are described in a (partially) unified way by the electroweak
theory of Glashow, Salam and Weinberg (GSW), which is a generalization
of quantum electrodynamics or QED; the strong interactions of quarks are
described by quantum chromodynamics or QCD, which is also analogous to
QED. The similarity with QED lies in the fact that all three interactions are
types of gauge theories, though realized in different ways. In the first volume
of this book, we will get as far as QED; QCD and the electroweak theory are
treated in volume 2.
The reader will have noticed that the most venerable force of all – gravity
– is absent from our story. In practical terms this is quite reasonable, since its
effect is very many orders of magnitude smaller than even the weak force, at
least until the interparticle separation reaches distances far smaller than those
we shall be discussing. Conceptually also, gravity still seems to be somewhat
distinct from the other forces which, as we have already indicated, are encouragingly similar. There are no particular fermionic sources carrying ‘gravity
charges’: it seems that all matter gravitates. This of course was a motivation
for Einstein’s geometrical approach to gravity. Despite the lingering promise
of string theory (Green et al. 1987, Polchinski 1998, Zwiebach 2004), it is
fair to say that the vision of the unification of all the forces, which possessed
Einstein, is still some way from realization. Gravitational interactions are not
part of the SM.
This book is not intended as a completely self-contained textbook on particle physics, which would survey the broad range of observed phenomena and
outline the main steps by which the picture described here has come to be
accepted. For this we must refer the reader to other sources (e.g. Perkins
2000, Bettini 2008). We proceed with a brief review of the matter (fermionic)
content of the SM.
1.2 The fermions of the Standard Model
1.2.1 Leptons
Forty years after Thomson’s discovery of the electron, the first member of
another generation of leptons (as it turned out) – the muon – was found independently by Street and Stevenson (1937), and by Anderson and Neddermeyer
(1937). Following the convention for the electron, the μ
− is the particle and
the μ
+ the antiparticle. At first, the muon was identified with the particle
postulated by Yukawa only two years earlier (1935) as the field quantum of
the ‘strong nuclear force field’, the exchange of which between two nucleons
1. The Particles and Forces of the Standard Model
if neutral, only weakly. By contrast, the quarks – which are the constituents
of hadrons, and thence of nuclei – interact via all three interactions, strong,
electromagnetic and weak. The weak and electromagnetic interactions of both
quarks and leptons are described in a (partially) unified way by the electroweak
theory of Glashow, Salam and Weinberg (GSW), which is a generalization
of quantum electrodynamics or QED; the strong interactions of quarks are
described by quantum chromodynamics or QCD, which is also analogous to
QED. The similarity with QED lies in the fact that all three interactions are
types of gauge theories, though realized in different ways. In the first volume
of this book, we will get as far as QED; QCD and the electroweak theory are
treated in volume 2.
The reader will have noticed that the most venerable force of all – gravity
– is absent from our story. In practical terms this is quite reasonable, since its
effect is very many orders of magnitude smaller than even the weak force, at
least until the interparticle separation reaches distances far smaller than those
we shall be discussing. Conceptually also, gravity still seems to be somewhat
distinct from the other forces which, as we have already indicated, are encouragingly similar. There are no particular fermionic sources carrying ‘gravity
charges’: it seems that all matter gravitates. This of course was a motivation
for Einstein’s geometrical approach to gravity. Despite the lingering promise
of string theory (Green et al. 1987, Polchinski 1998, Zwiebach 2004), it is
fair to say that the vision of the unification of all the forces, which possessed
Einstein, is still some way from realization. Gravitational interactions are not
part of the SM.
This book is not intended as a completely self-contained textbook on particle physics, which would survey the broad range of observed phenomena and
outline the main steps by which the picture described here has come to be
accepted. For this we must refer the reader to other sources (e.g. Perkins
2000, Bettini 2008). We proceed with a brief review of the matter (fermionic)
content of the SM.
1.2 The fermions of the Standard Model
1.2.1 Leptons
Forty years after Thomson’s discovery of the electron, the first member of
another generation of leptons (as it turned out) – the muon – was found independently by Street and Stevenson (1937), and by Anderson and Neddermeyer
(1937). Following the convention for the electron, the μ
− is the particle and
the μ
+ the antiparticle. At first, the muon was identified with the particle
postulated by Yukawa only two years earlier (1935) as the field quantum of
the ‘strong nuclear force field’, the exchange of which between two nucleons