R.S. Chivukula, A. Cohen, K. Lane, E. Simmons
The problems of the breakdown of electroweak and flavor symmetries are among the most pressing facing particle physics today. Electroweak symmetry breaking is manifested by the nonzero masses of the weak W and Z bosons. To solve the problem of flavor and its symmetry breaking, one must understand the number of quarks and leptons and why they have such a peculiar pattern of masses. Today, there is no generally accepted theory of these phenomena nor any experimental evidence for their dynamical basis. Several theoretical approaches to electroweak and flavor symmetry breaking are being actively investigated at Boston University. All of them require new particles and interactions near the electroweak energy scale of one trillion electron volts. Our research is aimed at formulating consistent theoretical models of these scenarios and at working out their experimental consequences.
The Big Bang, Dark Matter, and Cosmology
R.S. Chivukula, A. Cohen, S.-Y. Pi
Many of the questions of current theoretical interest are beyond the energy reach of existing accelerators. However, extremely high energies were once realized in the largest natural particle accelerator, the Big Bang. Shortly after the Big Bang, the universe was composed of a hot plasma of elementary particles. The nature of this plasma, which depends on the properties of the constituent elementary particles, determined the subsequent evolution of the universe. By observing the universe today, we can obtain information about its state shortly after the Big Bang and obtain information about the interactions of these particles at high energy. There are several aspects of cosmology currently under study at Boston University, including inflation of the early universe, the mechanism for producing the baryon excess, nucleosynthesis, "dark" matter, and the formation of large scale structure in the universe.
C. Rebbi
A crucial component of the scientific process consists in deriving quantitative predictions from assumed theoretical models. The power of modern computers has added a new dimension to this aspect of research. Today, physicists can use advanced numerical techniques to simulate the behavior of very complex systems and thus solve problems that defy the more traditional methods of mathematical analysis. Scientists in the particle theory group have been applying forefront computational methods to the study of quantum chromodynamics (QCD, the theory of interacting quarks and gluons) and to other particle models. Space-time is approximated by a lattice of points and the fundamental fields, defined over this lattice, are represented by an extremely large collection of numbers stored in the memory of a supercomputer. Calculating at the rate of billions of operations per second, the computer simulates the effects of quantum fluctuations of the fields. From such techniques, one calculates fundamental observables such as particle masses, the temperature at which quarks and gluons become unbound or the surface tension of a nucleating hadron. Students, research staff, and faculty working on these problems avail themselves of the supercomputer resources and support structure of the Center for Computational Science (see below). While doing research in the fascinating and challenging field of subatomic particles, students also acquire invaluable expertise in the use of the most modern and powerful supercomputer technologies.
R. Brower, R. Giles, C. Rebbi
Boston University has a strong tradition of support for advanced scientific computing. It was one of the first universities to install a massively-parallel supercomputer when it acquired the CM-2 Connection Machine in 1988, which was made available as a university-wide resource for research and education. The Center for Computational Science was established as a focal point for interdisciplinary research and education using massively-parallel computation. Continuing its commitment to advanced scientific computing, the University installed the next generation Connection Machine, the CM-5, in June of 1992. The CM-5 has 64 processing nodes, 3 control processors, a 24 gigabyte scalable disk array, 2 gigabytes of memory, and a theoretical peak speed of 8 gigaflops.
The Center for Computational Science, which houses the CM-5, brings together researchers and students from a wide variety of fields, providing advanced resources and support as well as seminars, tutorials, and educational programs. The Connection Machine user community has grown to over 300 scientists collaborating on over 70 research projects. In 1993, the Center, in coordination with the departments of Chemistry, Computer Science, Electrical, Computer, and Systems Engineering, and Physics was awarded a grant from the National Science Foundation to introduce a new, interdisciplinary sequence of courses in massively parallel computing for undergraduates and a second grant to develop a high-performance computing laboratory for these courses. The new laboratory of networked, multimedia workstations linked to the CM-5 supercomputer provides a unique opportunity for undergraduate and graduate students to develop expertise in leading-edge technologies.
Students, research staff, and faculty from the Physics Department are engaged in a wide variety of challenging computational research projects in theoretical and experimental particle physics, condensed-matter physics, statistical mechanics, biophysics, and polymer physics. Students that would like to specialize in computational physics will find excellent opportunities for interesting and stimulating research within the Center for Computational Science.
Supersymmetry and String Theories
R. Rohm
Several more mathematical approaches to problems in particle physics are also pursued by the theory group. One concerns the many questions surrounding the possibility of unifying the known forces at an energy scale approaching the Planck mass. For the unification schemes arising from supersymmetric string theories, many of the fundamental difficulties of doing reliable calculations have yet to be resolved and may require a different conception of the short-distance structure of space-time; we currently have only hints of what this might be. Within the scope of approximations that make these theories tractable, there are also interesting questions about their low-energy manifestation, in particular as regards supersymmetry breaking and its effect on physics at the weak scale.