Sep 19 2008
From a recent theoretical study, Japanese high-energy particle physicists have provided an important new method to apply to a fundamental concept known as supersymmetry.
In ‘lattice gauge theory’, space and time are represented by discrete points on a grid. This allows theorists to perform calculations numerically, using state-of—the-art computational methods.
Particle physicists have long struggled with what is known as the ‘hierarchy problem’: the relative weakness of the gravitational force compared with the other known forces in nature. The world we live in—and can measure—exists at energy scales up to about 100 GeV. Yet, there is a fundamental length scale—the Planck scale—at which gravitational interactions become so strong, they cannot be handled by existing quantum theory. At the Planck scale, energies reach 1019 GeV.
To deal with this huge discrepancy in energy scales, theorists developed the supersymmetry concept, which predicts that every fermion—particles like electrons and protons—has a ‘supersymmetric’ partner that is a boson—particles like photons or helium atoms—of equal mass. The existence of these partner states provides a framework in which theorists can naturally connect our low-energy world with the Planck scale.
“What this means is that there must be, in nature, a boson that has precisely equal mass to the electron. That boson is termed the ‘selectron’,” explains Hiroshi Suzuki of RIKEN’s Nishina Center for Accelerator-Based Science, Wako. But, no such particle has been observed in nature. According to Suzuki, this shows that although the underlying theory might be supersymmetric; supersymmetry must somehow break spontaneously in nature.
But how does supersymmetry break? Most efforts to understand this from first principles are restricted to models that can be studied without taking quantum fluctuations—energy fluctuations consistent with a principle known as the Heisenberg uncertainty principle—into account. However, as reported in the journal Physical Review D, Suzuki and co-authors have developed the computational tools that allow them to treat symmetry breaking that occurs only through quantum fluctuations*.
The calculations were performed using so-called ‘lattice gauge theory’. However, even with advancements in their computational tools, the group must use a simplified, or ‘toy’, model. Although the results of their calculations using this model are not immediately applicable to a real experimental situation in particle physics, Suzuki and colleagues believe that their study provides an important clue for subsequent computational approaches to understanding supersymmetry breaking.
Advancements in understanding supersymmetry are particularly timely as one objective for the new Large Hadron Collider at CERN in Switzerland will be the detection of superpartner particles.
- Kanamori, I., Suzuki, H. & Sugino, F. Euclidean lattice simulation for dynamical supersymmetry breaking. Physical Review D 77, 091502(R) (2008).