Warped Passages - Lisa Randall [172]
The graviton, a bulk particle that interacts with energetic particles no matter where they are, looked like the perfect candidate. The graviton interacts with particles on the supersymmetry-breaking and on the Standard Model branes. Furthermore, the interactions of the graviton are known—they follow from the theory of gravity. We could show that the graviton’s interactions, while generating the necessary superpartner masses, do not generate the interactions that would cause quarks or leptons to confuse their identities—the interactions that are known not to occur in nature. The graviton therefore looked like a promising choice.
When Raman and I worked out the superpartner masses that would follow from a mediating messenger graviton, we found that, despite the simple elements, the calculation was surprisingly subtle. Classical contributions to supersymmetry-breaking masses turn out to be zero, and only quantum mechanical effects communicated supersymmetry breaking. When we realized this, we called the graviton-induced communication of supersymmetry-breaking anomaly mediation. We chose the name because, like the anomalies I discussed in Chapter 14, the specific quantum mechanical effects broke a symmetry that would otherwise be present. The great thing was that since the masses of the superpartners depended on known quantum effects in the Standard Model, rather than unknown higher-dimensional interactions, we could predict the relative sizes of the superpartners’ masses.
It took a few days to get it all straight, which meant that I could go from disappointment to relief in the same day. I remember startling my dinner companion one evening when I became completely distracted because I recognized an error and solved a problem that had worried me earlier in the day. In the end, Raman and I discovered that if gravity communicates supersymmetry breaking, sequestered supersymmetry breaking works surprisingly well. All the superpartners had the right masses, and the relationship between the gaugino and squark masses was in the range where we wanted it to be. Although not everything worked quite as simply as we had initially hoped, important relations among the superpartners’ masses fell into place without inducing the impossible interactions that are problematic for other supersymmetry-breaking theories. And with only slight modifications, everything worked.
And, best of all, thanks to the distinctive predictions for the superpartners’ masses, our idea can be tested. A very significant feature of sequestered supersymmetry breaking is that, even though the extra dimension could be extraordinarily tiny, something like 10-31 cm in size, only about a factor of a hundred bigger than the minuscule Planck scale length, there would still be visible consequences. This goes against standard wisdom, which says that only much larger dimensions could have visible consequences, through either a modified gravitational force law or new heavy particles.
Although it is indeed true that we won’t see either of the above experimental consequences when the extra dimension is small, the graviton communicates supersymmetry breaking to the gauginos in a very particular way that we could calculate from the known gravitational interactions and the known interactions that occur in a theory with supersymmetry. The sequestered supersymmetry-breaking model predicts distinctive mass ratios for the gauginos, the partners of the gauge bosons, and those masses can be measured.30
This is very exciting. If physicists discover superpartners, they can then determine whether the relationships among their masses agree with what we predict. An experiment to search for these gauge superpartners is under way right now at the Tevatron—the proton-antiproton collider at Fermilab in Illinois. If we are very lucky, we will see results in the next few years.
In the end, Raman and I were both reasonably confident that we had discovered something interesting. But we both had some residual concerns. I was a little afraid that such an