Theoretical Particle Physics

The work of this part of the group is concerned with the standard model and possible deviations from, and extensions to, it. This model consists of the Glashow-Weinberg-Salam model of weak and electromagnetic interactions and quantum chromodynamics (QCD) that describes the strong interactions. Successful as it is in describing a vast amount of experimental data, the model is incomplete. It offers no explanation of the pattern of quark and lepton masses and mixings and no explanation of the scale of weak interactions. If the standard model is correct there is one particle yet to be found; the Higgs boson which is the remainder of the mechanism that generates the mass for the W and Z bosons. This particle might be discovered at LEP or at the LHC .

Some extension of the standard model is needed if the value of the W mass ( i.e. the scale of weak interactions) is to be understood. One option is a supersymmetric version of the standard model. This type of theory holds the possibility that the theory could ultimately be unified to include gravitational interactions. Supersymmetric theories have a rich spectrum of new particles that are expected to have masses of the order of a few hundred GeV. In addition, they contain a stable neutral particle that is a natural candidate to account for the dark matter that is believed to pervade the whole of space.

Another option is that the weak scale could be generated dynamically in a manner similar to that of the proton mass in QCD. In this case, high energy scattering of W and Z bosons will reveal the presence of a new force.

A unification beyond that of the standard model is needed if the pattern of quark and lepton masses is to be understood. Different patterns are predicted by different models. An understanding of the general features that are required by unified models is essential if progress is to be made. These models can be more tightly constrained once the quark mixing angles, particularly those involving the bottom quark are known. If the unified theory is supersymmetric, the most likely possibility involves a unification including gravity in a string theory. At present, no testable predictions have emerged from string theory and, in particular, the pattern of supersymmetry breaking and hence the masses of the new supersymmetric particles is unknown.

All of these problems require a collaborative effort among experimenters and theorists. The Physics Division at LBNL is involved in many of these experiments.

Members of the LBNL senior staff and UC Berkeley faculty involved in work directly related to the problems discussed above are Michael Barnett, Robert Cahn, Mike Chanowitz, Mary K Gaillard, Larry Hall, Ian Hinchliffe, Hitoshi Murayama and Mahiko Suzuki.

Ian Hinchliffe
October 1997