Professor John Gunion

E-Mail gunion@physics.ucdavis.edu

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My Research

What is the origin of mass? Are quarks, leptons and gauge bosons truly elementary? Do the strong, electromagnetic, weak, and gravitational forces ultimately unify into a single force at very short distances (i.e. very high energies)? Are there compelling reasons to postulate new types of forces and particles that have not yet been observed? How can theoretical hypotheses regarding such questions be probed at current and planned experimental facilities? These are the types of issues that research in high energy/elementary particle theory focuses upon.

The last two decades have seen tremendous progress in our understanding of the strong, electromagnetic and weak interactions. We have experimentally observed the gluon, photon, and $W/Z$ bosons that are responsible for mediating these respective forces, and the quarks and leptons on which they act. We have also discovered that neutrinos have mass. Aside from neutrino mass (which is easily accomodated by a small extension), all observations fit into the `Standard Model' in which the forces are part of an elegant `gauge' theory based on a certain symmetry group. However, this symmetry group must be broken in order to explain the different masses of the gauge bosons (the gluon and photon are massless, whereas the $W/Z$ bosons are quite massive), and in order to give masses to the quarks, leptons and neutrinos. The process of generating mass for some gauge bosons (and not others) is generally referred to as electroweak symmetry breaking, and the Standard Model mechanism for accomplishing electroweak symmetry breaking is called the `Higgs' mechanism in honor of the theorist who proposed it. The single physical remnant of this mechanism is the `Higgs' boson, an as-yet unobserved spin-zero particle.

A crucial goal of current experiments and of the next generation of high energy accelerators will be to detect the Higgs boson (if it exists) or, more generally, to unravel the mechanism responsible for electroweak symmetry breaking. This task is much more subtle and complex than it appears at first sight. In particular, there is no guarantee that the simple Higgs mechanism of the Standard Model is correct. Indeed, there are many unsatisfactory features of the Standard Model. It contains too many unexplained parameters (most notatably, those associated with quark, lepton and neutrino masses and mixings), the forces are not truly unified, and there is no natural control on the mass of the Higgs boson (which must not be too heavy if the theory is to make sense). In addition, the Standard Model does not explain many important known features of the universe. In particular, it does not: predict the observed magnitude of the cosmological constant; provide a good candidate for the dark matter of the universe; or contain a natural mechanism for generation of the baryon/antibaryon asymmetry that arose in the early universe following the big bang.

For these, and many other reasons, we believe that the Standard Model may be simply a highly successful approximation (valid at the limited energies of current experiments) to the so-called Theory of Everything (TOE). Many candidate theories for the TOE have been proposed. In general, all theoretical proposals predict the existence of new phenomena (new dimensions, new particles and/or new forces) at or below an energy scale of roughly 1000 times the proton mass (a TeV). The next generation of high energy accelerators (in particular the Large Hadron Collider and a future TeV scale linear collider) are designed precisely to probe extra dimensions and particle interactions at the TeV energy scale. Our research program focuses on exploring models for the TOE and on the development of techniques for experimentally revealing any given TOE at existing and future accelerators. Both analytic mathematical analysis and highly sophisticated computer programming are employed in most research projects.

The research program is coordinated with those of Professors Steve Carlip (Quantum Gravity), Hsin-Chia Cheng (model building and phenomenology), Markus Luty (model building and phenomenology), John Terning (model building and phenomenology) and Joe Kiskis (lattice QCD). The result is a group consisting of six professors, three to four post-doctoral researchers, a number of graduate students and perhaps several visiting scientists. There is considerable collaboration and overlap in research interests and projects. In addition, we interact strongly with the Cosmology program, which includes Professors Nemanja Kaloper and Andy Albrecht who work in areas closely allied to high energy theory. The result is a hospitable and productive environment in which students perform research of direct relevance to the future of high energy physics, while developing a deep understanding of the mathematical and physical subtleties that will characterize a complete theory of particles and interactions.

Courses I teach