In Physics, What Is Sequestering?

Phil Riddel

In the context of physics, sequestering is a proposed means by which certain particles and forces can be confined to extra dimensions, preventing or minimizing their interaction with the particles and forces that comprise the Standard Model. The idea, which has particular relevance to string theory, M-theory and supersymmetry (SUSY), was developed by the theoretical physicists Lisa Randall and Raman Sundrum. Sequestering may solve some major problems in particle physics. In particular, it offers a solution to what is known as the “hierarchy problem” through the breaking of supersymmetry, while avoiding another problem known as “flavor violation.”

Scientist with beakers
Scientist with beakers

Physicists have long sought a Grand Unified Theory (GUT) that unites the four forces of nature — the electromagnetic force, the strong and weak nuclear forces, and gravity — as well as explaining the properties of all the elementary particles. The big problem that any such theory must address is the apparent incompatibility of general relativity with quantum theory and the Standard Model. String theory, in which the most fundamental units of matter, such as electrons and quarks, are regarded as extremely tiny, one-dimensional, string-like entities, is one attempt at such a theory. This has been developed into M-theory, in which strings can be extended into two and three-dimensional “branes” floating in a higher dimensional space, known as the “bulk.”

In addition to the problems involved in bringing gravity into the picture, there is an issue with the Standard Model itself, known as the hierarchy problem. To put it simply, the hierarchy problem centers on why the gravitational force is enormously weaker than the other forces of nature, but it also involves predicted values for the masses of some hypothetical force-carrying particles that differ enormously from one another. One hypothetical particle in particular, the Higgs particle, is predicted to be relatively light, while it seems that quantum contributions from virtual particles must make it enormously more massive, at least without an extraordinary degree of fine-tuning. This is considered extremely unlikely by most physicists, so some underlying principle is sought to explain the disparities.

The theory of supersymmetry (SUSY) provides one possible explanation. This states that for every fermion — or matter-forming particle — there is a boson — or force-carrying particle — and vice-versa, so that every particle in the Standard Model has a supersymmetrical partner or “superpartner.” Since these superpartners have not been observed, it means that the symmetry is broken, and that supersymmetry only exists at very high energies. According to this theory, the hierarchy problem is solved by the fact that the mass contributions of the virtual particles and their superpartners cancel out, removing the apparent discrepancies in the Standard Model. There is, however, a problem with supersymmetry.

Fundamental matter forming particles such as quarks comes in three generations or “flavors,” with differing masses. When supersymmetry is broken, it seems that a whole host of interactions can occur, some of which would change the flavors of these particles. Since these interactions are not observed experimentally, any theory of supersymmetry breaking must somehow include a mechanism that prevents what are known as flavor violations.

This is where sequestering comes in. Returning to the concept of three-dimensional branes floating in a higher dimensional bulk, it is possible to sequester supersymmetry breaking to a separate brane from that on which the particles and forces of the Standard Model reside. The supersymmetry breaking effects could be communicated to the Standard Model brane by force-carrying particles that are able to move within the bulk, but otherwise, the Standard Model particles would behave in the same way as in unbroken supersymmetry. Particles in the bulk that could interact with both the symmetry-breaking brane and the Standard Model brane would determine what interactions can occur, and might exclude the flavor-changing interactions we do not observe. The theory works well if the graviton — the hypothetical gravity force-carrying particle — plays this role.

Unlike many other ideas relating to string theory and M-theory, it seems possible to test sequestered supersymmetry. It makes predictions for the masses of the superpartners of the bosons — force-carrying particles — that are within the range of energies achievable by the Large Hadron Collider (LHC). If these particles are observed by the LHC, their masses can be matched to what is predicted. As of 2011, however, experiments at the LHC have failed to detect these superpartners at the energies at which they were expected to appear, a result that seems to rule out the simplest version of SUSY, although not some more complex versions. Even if SUSY is proven wrong, the idea of sequestering may still have useful applications with regard to other problems and mysteries in physics.

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