Knocking on Heaven's Door - Lisa Randall [159]
As a theorist, I can only do so much about that. The LHC energy is what it is. But we can try to find subtle clues about the existence of extra dimensions, even if the KK modes turn out to be too heavy. When Patrick Meade and I did our calculations about the production rate of possible higher-dimensional black holes, we focused not only on the negative result—the much lower black hole production rate than had originally been claimed—but also thought about what would happen if higher-dimensional gravity was strong, even if no black holes were produced. We asked whether the LHC might produce any interesting signals of higher-dimensional gravity at all. We found that even without discovering new particles or exotic objects like black holes, experimenters should be able to observe deviations from Standard Model predictions. Discovery is not guaranteed, but experimenters will do everything they can with the existing machine and detectors. In other more advanced research, colleagues have thought about improved methods for finding KK modes, even if Standard Model particles reside in the bulk.
There is also a chance that we could be lucky and that the scales for new particle masses and interactions might turn out to be lower than we anticipate. If that turns out to be the case, we would not only find KK modes sooner than expected, but we would also see other new phenomena. If string theory is the underlying theory of nature and the scale of new physics is low, the LHC could even produce—in addition to KK particles and new interactions—additional particles associated with oscillating underlying strings. These particles would be much too heavy to create under more conventional assumptions. But with warping, there is hope that some string modes will be much lighter than anticipated and could thus appear at the weak energy scale.
Clearly there are several interesting possibilities for warped geometry and we eagerly await experimental results. If the consequences of this geometry are discovered, they will change our view of the nature of the universe. But we will only know which—if any—of these possibilities is realized in nature after the LHC has done its search.
REDUX
Experiments at the LHC are currently testing all the ideas in this chapter. We hope that if any of these models are right, hints will soon appear. There might be solid evidence like KK modes, or there might be subtle changes to Standard Model processes. Either way, both theorists and experimenters are alert and waiting. Every time the LHC does or does not see something, it constrains the possibilities further. And if we’re lucky, one of the ideas that have been discussed might prove right. As we learn more about what the LHC will produce and how detectors work, we will hopefully also learn more about how to extend the LHC’s reach to test as large a range of possibilities as possible. And as data become available, theorists will incorporate that data into their proposals.
We don’t know how long it will be before we start getting answers since we don’t know what is there or what the masses and interactions might be. Some discoveries may happen within a year or two. Others could take more than a decade. Some might even require higher energies than the LHC will ever achieve. The wait is a little anxiety provoking, but the results will be mind-blowing. That should make the nail-biting worth it. They could change our