The standard model – augmented (say) with dimension-5 operators to account for neutrino oscillations – explains all particle physics experiments performed to date (2006). Yet there are a number of reasons to believe that it is incomplete, and should be regarded at best as being the effective theory describing particle physics at the energy scales which have been probed experimentally (roughly several hundred GeV).

This chapter aims to summarize these reasons, with an eye to identifying the main themes which govern the searches for the standard model's replacement. These themes typically revolve about “puzzles,” which either center around attempts to explain the values of some of the standard model's couplings, or around speculations about what kinds of new particles might exist at very large masses, and what their implications might be for experiments at accessible energies. Our goal in this summary is not to be exhaustive, but rather to provide a conceptual framework for further reading of the many research directions within the literature on the broad topic of physics “beyond the standard model.”

The organizing theme for our discussion is the assumption that any particles which have not yet been discovered must be heavy compared with the energies to which we presently have experimental access. This assumption has three motivations, not least of which is the outstanding success of the standard model itself.