1
The Particles and Forces of the Standard
Model
1.1 Introduction: the Standard Model
The traditional goal of particle physics has been to identify what appear to be
structureless units of matter and to understand the nature of the forces acting between them; all other entities are then to be successively constructed as
composites of these elementary building blocks. The enterprise has a two-fold
aspect: matter on the one hand, forces on the other. The expectation is that
the smallest units of matter should interact in the simplest way; or that there
is a deep connection between the basic units of matter and the basic forces.
The joint matter/force nature of the enquiry is perfectly illustrated by Thomson’s discovery of the electron and Maxwell’s theory of the electromagnetic
field, which together mark the birth of modern particle physics. The electron
was recognized both as the ‘particle of electricity’ – or as we might now say,
as an elementary source of the electromagnetic field, with its motion constituting an electromagnetic current – and also as an important constituent of
matter. In retrospect, the story of particle physics over the subsequent one
hundred years or so has consisted in the discovery and study of two new (nonelectromagnetic) forces – the weak and the strong forces – and in the search
for ‘electron-figures’ to serve both as constituents of the new layers of matter
which were uncovered (first nuclei, and then hadrons) and also as sources of
the new force fields. In the last quarter of the twentieth century, this effort
culminated in decisive progress: the identification of a collection of matter
units which are indeed analogous to the electron; and the highly convincing
experimental verification of theories of the associated strong and weak force
fields, which incorporate and generalize in a beautiful way the original electron/electromagnetic field relationship. These theories are collectively called
‘the Standard Model’ (or SM for short), to which this book is intended as an
elementary introduction.
In brief, the picture is as follows. The matter units are fermions, with
spin1 (in units of ħ). They are of two types, leptons and quarks. Both are
2
structureless at the smallest distances currently probed by the highest-energy
accelerators. The leptons are generalizations of the electron, the term denoting
particles which, if charged, interact both electromagnetically and weakly; and
3
The Particles and Forces of the Standard
Model
1.1 Introduction: the Standard Model
The traditional goal of particle physics has been to identify what appear to be
structureless units of matter and to understand the nature of the forces acting between them; all other entities are then to be successively constructed as
composites of these elementary building blocks. The enterprise has a two-fold
aspect: matter on the one hand, forces on the other. The expectation is that
the smallest units of matter should interact in the simplest way; or that there
is a deep connection between the basic units of matter and the basic forces.
The joint matter/force nature of the enquiry is perfectly illustrated by Thomson’s discovery of the electron and Maxwell’s theory of the electromagnetic
field, which together mark the birth of modern particle physics. The electron
was recognized both as the ‘particle of electricity’ – or as we might now say,
as an elementary source of the electromagnetic field, with its motion constituting an electromagnetic current – and also as an important constituent of
matter. In retrospect, the story of particle physics over the subsequent one
hundred years or so has consisted in the discovery and study of two new (nonelectromagnetic) forces – the weak and the strong forces – and in the search
for ‘electron-figures’ to serve both as constituents of the new layers of matter
which were uncovered (first nuclei, and then hadrons) and also as sources of
the new force fields. In the last quarter of the twentieth century, this effort
culminated in decisive progress: the identification of a collection of matter
units which are indeed analogous to the electron; and the highly convincing
experimental verification of theories of the associated strong and weak force
fields, which incorporate and generalize in a beautiful way the original electron/electromagnetic field relationship. These theories are collectively called
‘the Standard Model’ (or SM for short), to which this book is intended as an
elementary introduction.
In brief, the picture is as follows. The matter units are fermions, with
spin1 (in units of ħ). They are of two types, leptons and quarks. Both are
2
structureless at the smallest distances currently probed by the highest-energy
accelerators. The leptons are generalizations of the electron, the term denoting
particles which, if charged, interact both electromagnetically and weakly; and
3