The success of the Standard Model of particle physics in describing the wide range of precise experimental measurements is a remarkable achievement. However, the Standard Model is just a model and there are many unanswered questions. This short concluding chapter provides a broad overview of the current state of our understanding of particle physics and describes some of the more important open issues.
The Standard Model
The ultimate theory of particle physics might consist of a (simple) equation with relatively few free parameters, from which everything else followed. Whilst the Standard Model (SM) is undoubtedly one of the great triumphs of modern physics, it is not this ultimate theory. It is a model constructed from a number of beautiful and profound theoretical ideas put together in a somewhat ad hoc fashion in order to reproduce the experimental data. The essential ingredients of the Standard Model, indicated in Figure 18.1, are: the Dirac equation of relativistic quantum mechanics that describes the dynamics of the fermions; Quantum Field Theory that provides a fundamental description of the particles and their interactions; the local gauge principle that determines the exact nature of these interactions; the Higgs mechanism of electroweak symmetry breaking that generates particle masses; and the wide-reaching body of experimental results that guide the way in which the Standard Model is constructed. The recent precision tests of the Standard Model and the discovery of the Higgs boson have firmly established the validity of the Standard Model at energies up to the electroweak scale. Despite this success, there are many unanswered questions.
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