Knocking on Heaven's Door - Lisa Randall [158]
[ FIGURE 69 ] In Randall-Sundrum models, a KK graviton can be produced and decay inside the detector into visible particles, such as an electron and a positron.
This is in fact how experimenters have discovered all new heavy particles so far. They don’t see the particles directly. But they observe the particles that they decay into. That’s a lot more information in principle than would be provided by missing energy. By studying the properties of these decay products, experimenters can figure out the properties of the particle that was initially present.
If the warped geometry scenario is correct, we will soon see particle pairs originating from the decay of KK graviton modes. By measuring the energies and charges and other properties of the final state particles, experimenters will be able to deduce the mass and other properties of the KK particles. These identifying features, along with the relative frequency with which the particle decays into various final states, should help experimenters determine whether they have discovered a KK graviton or some other new and exotic entity. The model tells us the nature of the particle that should be found so that physicists can make predictions to distinguish among the possibilities.
A friend of mine (a screenwriter who both extols and satirizes the excesses of human nature) doesn’t understand how, given the potential implications of the discoveries that might happen, I’m not sitting on the edge of my seat waiting for results. Whenever I see him, he insistently asks me, “Won’t the results be life-changing? Might they not confirm your theories?” He also wants to know, “Why aren’t you over there (in Geneva) talking to people all the time?”
Of course, in some sense his instincts are right. But experimenters already know what to look for, so much of the job of theorists is already done. When we have new ideas about what to look for, we communicate them. We don’t necessarily have to be at CERN or even in the same room to do that. Experimenters can be found all over the United States and almost anywhere on the globe for that matter. And remote communication works pretty well—in part due to the initial Internet insight that Tim Berners-Lee had many years ago at CERN.
I also know enough to know what a challenge these searches might be, even once the LHC is fully operational. So I know we might have a bit of a wait. Fortunately for us, the KK modes just described are one of the most straightforward things experimenters can look for. The KK gravitons decay into all particles—after all, every particle experiences gravity—so experimenters can focus on the final states that they find easiest to identify.
However, there are two cautionary notes—two reasons that the searches might be more challenging than initially anticipated and why we might have to wait awhile for discovery, even if the underlying idea is correct.
One is that other candidate models of warped geometry could lead to messier experimental signatures that will be more difficult to find. Models describe the underlying framework—which here involve an extra dimension and branes. They also suggest specific implementations of the general principles the framework embodies. Our original scenario suggested that only gravity was spread throughout the higher-dimensional space known as the bulk. But some of us later worked on alternative implementations. In these alternative scenarios, not all particles are on branes. This would mean more KK particles since each bulk particle would have its own KK modes. But it also turns out that these KK particles would be considerably harder to find. This challenge has prompted a great deal of research into how to discover these more elusive scenarios. The investigations that followed will prove useful not only in the search for KK particles, but also for any energetic massive particles that might be present in any new model.
The other reason that searches might prove to be