The discussion of any new topic necessarily makes use of knowledge that is to some extent assumed to have been already acquired. One cannot start from the very beginning and teach the whole of physics every time something new is to be introduced – even though the Landau and Lifshitz series of books comes close to pulling it off. More pragmatically, I would like to make sure that we are all on the proverbial same page with some of the basic notions. And where, you may ask, are these basic notions learned? I have in mind what can be called the canon of physics, that is, the books where we, as students, first studied the basic concepts and equations, the books that everyone has read to study, say, classical mechanics, electromagnetism, quantum mechanics, thermodynamics and statistical mechanics – as well as the mathematical tools necessary to comprehend the equations and the statistics to make sense of the data analysis. I have them (most of them, at least) in my office, and so do most of the physicists I know. The covers of some of them are shown in Figure 2.1.

It is time to face the elephant in the room. Beautiful though they are, the gauge bosons of the weak interactions are massless and cannot be the mediators of the weak interactions. They do not reproduce Fermi’s interaction at low energy. They make the weak interactions long range while they are most definitely not.

Strong interactions are, well, strong. You have hadrons interacting and breaking up into a huge number of other hadrons. It is hard to understand what is going on. Physicists stumbled along during the 1960s trying out many ideas (the key words here are current algebra, analytic S-matrix, Regge trajectories and string theory) without much real understanding.

The Lagrangian for the charge and neutral currents of the Standard Model contains fermion fields, what we call matter – leptons and quarks – and their interaction with spin-1 fields. These spin-1 fields are the W-, Z- and γ-bosons. As will become clear as I proceed, these fields are at the very center of the definition of the Standard Model as a gauge theory.

“Marvelous is the working of our world!” N. Gogol, Nevsky Prospect, 1835 Should this have been the first chapter? Where to start from in teaching the Standard Model? The data painfully collected by experiments are the basis of everything – but they would be mute were it not for our models and theories. On the other hand, it seems that our brain is not very good at working all by itself. It needs the gentle prodding of experimental data, of finding out how (real) things are. Without external inputs, our mind is easily led astray, going round in circles (often of narrower and narrower radius).

What else? The Standard Model answers all our questions about high-energy physics. At the time of writing of this book, there was no evidence of physics beyond the Standard Model. The predicted values of all observables are in reasonable agreement with the experimental data; only few of them show some tension (that is, a discrepancy of more than three standard deviations) in the global fit.

The theory called the Standard Model represents the state of the art of our understanding of the physics of elementary particles and their interactions. It is, as explained in the following chapters, a gauge theory, that is, a theory in which the interactions among the matter constituents are determined by gauge invariance and carried by gauge bosons. Everything else in physics (except perhaps gravitational phenomena at very high energies), as well as everything in the Universe, from chemistry to biology, is but an application (admittedly, very convoluted in most cases) of this model.