Shortly after moving to Berkeley, Weinberg slips a disc and is bedbound. He reads Chandrasekhar’s stellar physics book, which helped spark his interest in astrophysics. They decide to stay on the San Francisco side of the bay. At that time, Berkeley was the world’s leading center of experimental research on elementary particles and the newly commissioned Bevatron was the latest particle accelerator. Weinberg resolves to do some work that will be useful to Berkeley experimenters and sets about studying muon physics. In Spring 1960, he is offered and accepts a tenure-track position as an assistant professor. He is invited to join JASON, the group of defense consultants. He begins teaching and learns that he loves it. He decides to take a year abroad via an Alfred Sloan Fellowship and he and Louise buy a round-the-world ticket.

In this chapter we will present the simplest calculations, the so-called tree-level calculations that do not yet require a deep knowledge of quantum field theory like renormalization and renormalization group. This step will give us leading formulas for the determination of various parameters of the SM like the Fermi constantand the parameters of the CKM matrix, which will be introduced and discussed in detail. These formulas will also give us a transparent picture of the leading decays ofmesons and leptons and will allow approximate determination of the parameters in question. Corrections to these leading expressions will be calculated or simply listed in later chapters. In this chapter we will also find out that in the case of meson decays some entries in the decay amplitudes cannot be calculated in perturbation theory. These are hadronicmatrix elements of quark currents and of four-quark operators. Their calculation will be done in other chapters.