Modern Elementary Particle Physics Explaining and Extending the Standard Model

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 h0 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 h0 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.