Warped Passages - Lisa Randall [173]
He wasn’t far from the truth. Anomaly mediation of supersymmetry breaking was independently discovered around the same time by Gian Giudice at CERN, Markus Luty at Maryland, Hitoshi Murayama at Berkeley, and Riccardo Rattazzi at Pisa, who had been working together that same summer. They released a paper the day after ours came out. Their research was amazing to me. I couldn’t see how two groups of physicists could have traced the same tortuous journey through ideas in a single summer, but Raman had correctly guessed that others might have had similar interests. In fact, we were both right in a way. Although the other group had similar ideas, they developed them independently of the extra-dimensional motivation—without which anomaly-mediated masses were just a curiosity. As Riccardo generously said to the physicist Massimo Porrati, a mutual friend, Raman and I had done it better, not because our version of anomaly mediation was more correct, but because we had a reason anyone would care in the first place! That reason was extra dimensions. Without extra dimensions, supersymmetry breaking wouldn’t be sequestered and anomaly-mediated masses would be swamped by larger effects.
Other physicists have since gone on to investigate sequestered models of supersymmetry breaking. They have found ways to join this with other, older ideas to make even more successful models, ones that might represent the real world. People have even found ways to extend the lesson of sequestering back to four dimensions.
There are too many models to enumerate, but let me just mention two ideas I found particularly interesting. The first idea arose from a collaboration between Raman and Markus Luty. They used the insights from the warped geometry (described in Chapter 20) to reinterpret the consequences of sequestering in four dimensions. With these ideas, they developed a new class of four-dimensional symmetry-breaking models.
Another interesting idea was called gaugino mediation. The idea was to communicate supersymmetry breaking not through the graviton, but instead through gauginos, the supersymmetric partners of the gauge bosons. For this to work, gauge bosons and their partners couldn’t be stuck on a brane; they would have to be free to travel in the bulk. Raman reminded me that gaugino mediation was actually one of the many ideas we had dismissed early on. But the excellent model builders David E. Kaplan, Graham Kribs, and Martin Schmaltz, and, separately, Zacharia Chacko, Markus Luty, Ann Nelson, and Eduardo Ponton, demonstrated that we had been too hasty, and that gaugino mediation might work beautifully in communicating supersymmetry-breaking masses while preserving all the advantages of sequestered supersymmetry breaking.*
Sequestering and Shining Masses
Sequestered symmetry breaking is a powerful tool for model building. The real world could contain separated branes, and by constructing models with this assumption, physicists can explore the range of possibilities.
The previous section explained how problems with flavor-changing interactions might be solved in theories with supersymmetry. But another question challenging the model builder is why there should be different flavors of quarks and leptons with different masses in the first place. The Higgs mechanism gives particles their masses, but the precise values are different for each flavor. This can be true only if each of the flavors interacts differently with whatever plays the role of the Higgs particle. Given that the three flavors of each particle type, such as the up, charm, and top quarks, have exactly the same gauge interactions, it’s mysterious that they should all have different masses. Something has to distinguish them, but the particle physics