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of unwanted interactions. A further merit of sequestering is that it can even protect particles from the effects of symmetry breaking. So long as symmetry breaking happens sufficiently far away from those particles, it will have very little effect on them. When symmetry breaking is sequestered, it is quarantined, much as a contagious disease is contained when everyone with the disease is kept within a restricted region. Or, to use our other analogy, dramatic events that occur outside Xenophobia would have no effect in Xenophobia without an intervening communicator. Without porous borders, Xenophobia could function independently of the rest of the world.

Sequestering and Supersymmetry

The particular problem that Raman and I investigated in the summer of 1998 dealt with how sequestering might operate in nature to yield a universe with broken supersymmetry that has the properties of the universe we see. We have seen that supersymmetry can elegantly protect the hierarchy and guarantee that all the large quantum mechanical contributions to the Higgs particle’s mass add up to zero. But, as we saw in Chapter 13, even if supersymmetry exists in nature, it must be broken in order to explain why we’ve observed particles but not their superpartners.

Unfortunately, most models with broken symmetry predict interactions that don’t occur in nature, and such models cannot possibly be right. Raman and I wanted to find a physical principle that nature might use to protect itself from these unwanted interactions so that we could incorporate it into a more successful theory.

We focused on supersymmetry breaking in a braneworld context. Braneworlds can preserve supersymmetry. But just as in four dimensions, supersymmetry can be spontaneously broken when some part of the theory contains particles that don’t preserve supersymmetry. Raman and I realized that if all the particles responsible for supersymmetry breaking were separated from the Standard Model particles, the model with broken supersymmetry would be less problematic.

We therefore assumed that Standard Model particles were confined to one brane, and that the particles responsible for supersymmetry breaking were sequestered on another. We observed that with such a setup, the dangerous interactions that quantum mechanics could induce don’t necessarily occur. Apart from the supersymmetry-breaking effects that might be communicated via intermediary particles in the bulk, the interactions of Standard Model particles would be the same as in a theory with unbroken supersymmetry. So just as in a theory with exact supersymmetry, unwanted flavor-changing interactions that are inconsistent with experiments should not happen. Bulk particles that interact with particles on both the supersymmetry-breaking brane and the Standard Model brane would determine precisely which interactions are possible—and they wouldn’t necessarily include the forbidden ones.

Of course, some supersymmetry breaking has to be communicated to the Standard Model particles. Unless supersymmetry breaking is communicated to them, nothing will raise the superpartners’ masses. Although we don’t know the exact values for the superpartners’ masses, experimental constraints, combined with supersymmetry’s role in protecting the hierarchy, tell us approximately what the superpartners’ masses should be.

The constraints tell us the qualitative relationships among the masses of the superpartners. Roughly speaking, all the superpartners have about the same mass, and those masses are all approximately the weak scale mass, 250 GeV. We needed to ensure that the masses of the superpartners fell in this range, while still preventing unwanted interactions from occurring. All the pieces had to fit for the theory of sequestered supersymmetry breaking to have a chance of being right.

The key to our model’s success was finding the intermediary particle that could carry the news of supersymmetry breaking to the Standard Model particles and give the superpartners the masses they needed to have. But we also wanted to be sure that our intermediary