Knocking on Heaven's Door (51 page)

In fact, the KK modes of the warped geometry have an important and distinctive feature. Whereas the graviton itself has extraordinarily feeble interaction strength—after all, it communicates the extremely weak gravitational force—the KK modes of the graviton interact far more strongly, almost as strongly as the force called the weak force, which is in actuality trillions of times stronger than gravity.

The reason for the KK gravitons’ surprisingly strong interaction strength is the warped geometry they travel in. Owing to spacetime’s dramatic curvature, the interactions of KK gravitons have far greater strength than those of the graviton that communicates the gravitational force we experience. In the warped geometry, not only do masses get rescaled, but gravitational interactions do as well. Calculations demonstrate that in the warped geometry, KK gravitons have interactions comparable to that of weak scale particles.

This means that unlike supersymmetric models, and unlike large extra-dimensional ones, the experimental evidence for this scenario will not be missing energy where the interesting particle escapes unseen. Instead, it will be a much cleaner, and easier to identify, signature, consisting of the particle decaying inside the detector into Standard Model particles that leave visible tracks. (See Figure 69, in which a KK particle is produced and decays into an electron and positron for example.)

[
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 difficult is that KK particles could be heavier than we hope. We know the range of masses we might anticipate for KK particles, but we don’t yet know the precise values. If KK particles are nice and light, the LHC will readily produce them in abundance and discovery will be easy. But if the particles are heavier, the LHC might create only a few of them. And if they turn out to be heavier still, the LHC might not produce any at all. In other words, the new particles and new interactions might only be produced or occur at higher energies than the LHC will achieve. This was always a concern for the LHC with its fixed tunnel size and constrained energy reach.

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 view of the underlying nature of reality, or at least the matter of which we are composed. When the results are in, whole new worlds could emerge. Within our lifetimes, we just might see the universe very differently.

BOTTOM-UP VERSUS TOP-DOWN

Nothing substitutes for solid experimental results. But we physicists haven’t just been sitting on our thumbs for the last quarter century waiting for the LHC to turn on and produce meaningful data. We’ve thought long and hard about what it is that experiments should look for and what the implications of the data are likely to be. We have also studied results from experiment s that have been operational during this time frame, and these have taught us details about known particles and interactions and helped orient our thinking.

This interim period has also been a tremendous opportunity to think more deeply about ideas that at least for the time being are more removed from data. Some of the more interesting and speculative models and theoretical insights of the last twenty-five years resulted from these more mathematical pursuits. I doubt that I, for one, would have thought about extra dimensions or more mathematical aspects of supersymmetry had data been more plentiful. Even if measurements that would ultimately support these ideas had been made, the implications would have taken a while to unravel without the luxury of previous mathematical pursuits.

Experiments and mathematics both lead to scientific advances. But the road to progress is rarely clear, and physicists have been divided as to the best strategy. Model builders use the “bottom-up” approach introduced in Chapter 15 to start with what is known from experiments and then address residual puzzling unexplained features—often employing more theoretical mathematical developments. The last chapter presented some specific examples of models and how they influence the searches experimenters at the LHC will perform.

Others, most notably string theorists, apply a “top-down” way of thinking, in which they start with the theory that they believe is true—namely string theory—and try to use its underlying concepts to formulate a consistent quantum theory of gravity. Top-down theories are defined at high energies and small distances. The label refers to the theoretical notion that everything can be derived from fundamental premises defined at high-energy scales. Although the name can be confusing since high energies correspond to short distances, recall that the ingredients at small distances are the fundamental building blocks of matter. In this way of thinking, everything can be derived from basic principles and fundamental ingredients, which are defined at small distances and high energies—hence the label “top-down.”

This chapter is about the top-down and bottom-up approaches and the ways in which they contrast with each other. We’ll explore the differences, but also reflect on how they occasionally converge to yield remarkable insights.

STRING THEORY

Unlike model builders, more mathematically inclined physicists try to work from pure theory. The hope is to start with a single elegant theory and derive the consequences, and only then apply the ideas to data. Most any attempt at a unified theory embodies such a top-down approach. String theory is perhaps the most prominent such example. It is a conjecture for the ultimate underlying framework from which all other known physics phenomena would in principle follow.

String theorists take a major leap in the physics scales they try to conquer—jumping from the weak scale to the Planck scale at which gravity becomes strong. Experiments probably won’t directly test these ideas anytime soon (although the extra-dimensional models of the last chapter might be an exception). But even though string theory itself is difficult to test, elements of string theory do provide ideas and concepts that potentially observable models have incorporated.

The question physicists ask when deciding on model building versus string theory is whether to follow the Platonic approach, which tries to gain insights from some more fundamental truth, or the Aristotelian one, rooted in empirical observations. Do you take the “top-down” or the “bottom-up” approach? The choice could also be phrased as “Old Einstein versus Young Einstein.” Einstein originally did thought experiments that were grounded in physical situations. Nonetheless, he also valued beauty and elegance. Even when an experimental result contradicted his ideas about special relativity, he confidently (and ultimately correctly) decided that the experiment had to be wrong since its implications would have been too ugly to believe.