In the early 1980s I was asked to teach our University of Michigan undergraduate course in particle physics. Soon after agreeing, I found that there was no book available at the undergraduate level which presented particle physics as the successful theory of quarks, leptons, and their interactions that it has become. In 1970 there was no theory of weak or strong interactions; no confidence that a fundamental set of constituents had been identified; no way to calculate or explain a variety of results. Today there is a theory of strong, weak, and electromagnetic interactions, with the latter two unified. There have been several extraordinary experimental discoveries, and there are no experimental results that appear to fall outside of the framework of that theory. The so-called Standard Model of particle physics that accomplishes all of that is now widely tested in a variety of ways, and it is here to stay. It is expected that deviations from the Standard Model will someday be found, as clues to improved understanding and to new physics, but the Standard Model will describe physics on the scale where strong, weak, and electromagnetic interactions are important.

Given that development, it seemed essential to have a presentation of the Standard Model that could be used for advanced undergraduate teaching or broadly accessible first year grad courses. In addition, it seemed even more important to have a book that any scientist who understood the necessary background material could read in order to learn about the developments in particle physics. Those developments should become a part of the education of anyone interested in what mankind has learned about the basic constituents of matter and the forces of nature, but there was no place that many people who had the required background could go to learn them. After teaching the course once, I was convinced that an introductory course in quantum theory was the essential background.

With the discovery of the Higgs boson in 2012, a major voyage was successfully over, and a variety of new opportunities began to emerge. The mass (and properties) of the Higgs boson points toward how to extend the Standard Model, strengthen its foundations, and work toward an “ultraviolet completion,” a theory valid to near or at the Planck scale. Surprisingly, the data seems to allow more than one qualitatively different interpretation. In this chapter we describe the data and some of the interpretations.

We have already described the properties the Higgs field needed to have to make the Standard Model a complete effective theory of the world we see at the electroweak scale and below in Chapter 8, and the Higgs mechanism that led to breaking the electroweak symmetry to allow masses simultaneously for gauge bosons, quarks, and charged leptons. In this chapter we describe the successful search for the Higgs boson experimentally, its production at LHC, how it was detected, and the tests so far that it is indeed the Higgs boson. Its properties, such as its mass and decay branching ratios, are somewhat surprising and ironic, and have implications for physics beyond the Standard Model. Some, but not all, people think the results imply that the correct interpretation is the supersymmetric extension of the Standard Model.

The Higgs boson h 0 was difficult to observe because its couplings are proportional to mass, as we have seen, so they are small for the light particles that are most copiously available in beams. Another reason is that the mass of h 0 is unknown in the Standard Model theory. As we have seen, mh depends on the coefficient λ of the Higgs self-interaction in the Higgs potential. Since there is no understanding in the Standard Model of the physical origin of λ, its numerical value is not known. Nor does any other observable depend on λ in a way that allows λ to be extracted.

Since mh was unknown, searches had to be planned for all mh , which is much more difficult than designing an experiment to look for h at a specific mass. Different techniques work best for different mass ranges.