What are the fundamental symmetries of nature, and how are they broken?
The standard model of particle physics, while very successful, leaves many fundamental questions unanswered. These questions frequently have to do with symmetries: could certain aspects of particles and their interactions be better understood in terms of a new symmetry? How are these new symmetries, and indeed many of the symmetries of the standard model, to be broken? The symmetry of the electroweak interaction would imply that there is no difference between the left-handed neutrino and the left-handed electron! These particles differ only because the symmetry is broken — and yet we do not know how this symmetry breaking occurs. The difference between the masses of the electron and the muon are a sign that flavor symmetries are broken; but again, the origin is unknown. It is remarkable that we can address such problems, and that in the coming decade we shall find out the answers to at least some of the fundamental questions of symmetry breaking.
In recent years I have constructed and studied theories with enhanced spacetime symmetries: weak scale supersymmetry, and compact extra spatial dimensions with size from sub-mm to inverse TeV to inverse Planck mass. I have worked on grand unified theories in four and higher dimensions, and have considered a variety of frameworks for neutrino masses.
I continue to study constrained theories for the quark and charged lepton masses.Whether the electroweak symmetry turns out to be broken by the effects of supersymmetric interactions, a new strong force, or by extra spatial dimensions, the elucidation of the TeV scale will be as exciting as any previous discoveries in particle physics.
The cosmos apparently contains both dark energy and dark matter, contributing in roughly equal amounts to a critical universe. These contents of the universe, as well as the asymmetric baryons and background photons, arise from the underlying theory of particle and gravitational interactions. For all its successes, the standard model of particle physics does not allow the computation of the density of any of these components of the universe. What theory will?
My recent work has concentrated on the physics of extra spatial dimensions. I have constructed a supersymmetric theory with a fifth dimension at the TeV scale which is responsible for electroweak symmetry breaking. This allows a prediction for the mass of the Higgs boson and for the pattern of the masses of the superpartners and KK resonances. I continue to pursue extra-dimensional theories of the weak scale.
I have introduced a new framework for grand unification, in which the unified symmetry is realized only in higher dimensions and the higher dimensional field theory allows a precise calculation of the weak mixing angle. Such theories have new possibilities for understanding gauge symmetry breaking, quark and lepton masses and proton decay. The Higgs boson does not seem to fit naturally in the standard model. I am pursuing the possibility that it is not a fundamental spin zero particle at all, but originates from components of a gauge field in higher dimensions.
Why are the lepton flavor mixing angles large while those in the quark sector are small? I am pursuing a variety of ideas to explain this quark-lepton asymmetry. Possible schemes for large mixing between the tau neutrino and the muon neutrino were explored in.
Cosmological data suggest that we live in an interesting period in the history of the universe when the matter, radiation and vacuum contributions to the energy density are broadly comparable. The occurrence of any epoch with such a “triple coincidence” is puzzling, and yet it appears that we happen to live during this special epoch. I have recently proposed why this might be so, and am following this up with a study of theories which lead to such interesting times.
In the News
UC Berkeley alumni, Mark Levinson returned to the Bay Area to premiere his new documentary, ‘Particle Fever,’ about the discovery of the Higgs boson. The film includes Berkeley physicists, Lawrence Hall and Yasunori Nomura, along with several campus alums.
A July 13 lecture and panel discussion drew overflow crowds to hear about the newly discovered Higgs boson. Physicists Beate Heinemann and Lawrence Hall explained the theory and experiment behind this “third” kind of stuff, while three others explored the implications of the discovery.