Knocking on Heaven's Door (48 page)

[
FIGURE 59
]
A squark can decay into a quark and the lightest super-symmetric particle.

In fact, because the squark is never created on its own, but only in conjunction with another strongly interacting object (such as another squark or an antisquark), the experimenters will measure at least two jets (see Figure 60 for an example). If two squarks are created by a proton collision, they would give rise to two quarks that detectors would record. The net missing energy and momentum would escape undetected, but their absence would be noted and provide evidence for new particles.

[
FIGURE 60
]
The LHC might produce two squarks together, both of which decay into quarks and LSPs, leaving a missing energy signature.

One major advantage of all the delays in the LHC schedule was that experimenters had time to fully understand their detectors. They calibrated them so that measurements were very precise from the day the machine went on line, so missing energy measurements should be robust. Theorists, on the other hand, had time to think about alternative search strategies for supersymmetric and other models. For example, together with a theorist from Williams College, Dave Tucker-Smith, I found a different—but related—way to search for the squark decay just described. Our method relies on measuring only the momentum and energy of the quarks emerging from the event, with no need to explicitly measure missing momentum, which can be tricky. The great thing about the recent LHC excitement was that a number of CMS experimenters immediately ran with the idea and not only showed that it worked, but generalized and improved it within a few months. It’s now part of the standard supersymmetry search strategy and the first supersymmetry search from CMS used the technique we had so recently suggested.
⁶²

Down the road, even if supersymmetry is discovered, experimenters won’t stop there. They will try their best to determine the entire supersymmetric spectrum, and theorists will work to interpret what the results could mean. A lot of interesting theory underlies supersymmetry and the particles that could spontaneously break it. We know which supersymmetric particles should exist if supersymmetry is relevant to the hierarchy problem, but we don’t yet know the precise masses they should have or how those masses arise.

Different mass spectra will make an enormous difference to what the LHC should see. Particles can only decay into other particles that are lighter. The decay chain, the sequence of possible decays of supersymmetric particles, depends on the masses—what is heavier and what is lighter. The rates of various processes also depend on particle masses. Heavier particles in general decay more quickly. And they are usually more difficult to produce since only collisions with a good deal of energy can create them. Combining all the results together could give us important insights into what underlies the Standard Model and what awaits at the next energy scales. This will be true of any analysis of new physical theories that we might find.

Nonetheless, one should keep in mind that despite supersymmetry’s popularity among physicists, there are several reasons for concern about whether it truly applies to the hierarchy problem and the real world.

The first, and perhaps the most worrisome, is that we have not yet seen any experimental evidence. If supersymmetry exists, the only explanation for why we haven’t yet seen evidence is that the superpartners are heavy. But a natural solution to the hierarchy problem would require that superpartners be reasonably light. The heavier the superpartners are, the more inadequate supersymmetry appears as a solution to the hierarchy problem. The fudge required is determined by the ratio of the mass of the Higgs boson to the supersymmetry breaking scale. The bigger this is, the more “fine-tuned” the theory.

Not yet having seen the Higgs boson either compounds the problem. It turns out that in a supersymmetric model, the only way to make the Higgs heavy enough to have eluded detection is to have big quantum mechanical contributions that can come only from heavy superpartners. But again, those masses need to be so heavy that the hierarchy becomes a little unnatural, even with supersymmetry.

The other problem with supersymmetry is the challenge of finding a fully consistent model that includes supersymmetry breaking and agrees with all experimental data to date. Supersymmetry is a very specific symmetry that relates many interactions and prohibits interactions that quantum mechanics would otherwise permit. Once supersymmetry is broken, the “anarchic principle” takes over. Anything that can happen will. Most models would predict decays that have either never been seen in nature or are seen only much too infrequently to agree with predictions. Because of quantum mechanics, a whole can of worms is opened once supersymmetry is broken.

Physicists might simply be missing the right answers. We certainly cannot say definitively that no good models exist or that a little fine tuning doesn’t happen. Certainly, if supersymmetry is the correct resolution of the hierarchy problem, we should find evidence for it soon at the LHC. So it is certainly worth pursuing. A discovery of supersymmetry would mean that this exotic new spacetime symmetry applies not just in a theoretical formulation on a piece of paper, but also in the real world. However, in the absence of discovery, it is also worth considering alternatives. The first we’ll consider is known as
technicolor
.

TECHNICOLOR

Back in the 1970s, physicists also first considered an alternative potential solution to the hierarchy problem known as
technicolor
. Models under this rubric involve particles that interact strongly via a new force, playfully named the
technicolor force
. The proposal was that technicolor acts similarly to the strong nuclear force (which is also known as the color force among physicists), but binds particles together at the weak energy scale—not the proton mass scale.

If technicolor is indeed the answer to the hierarchy problem, the LHC wouldn’t produce a single fundamental Higgs boson. Instead it would produce a bound state, something like a hadron, that would play the role of the Higgs particle. The experimental evidence in support of technicolor would be lots of bound state particles and many strong interactions—very much like the hadrons we are familiar with, but that appear only at much higher energy—at or above the weak scale.

Not yet having seen any evidence poses a significant constraint on technicolor models. If technicolor is truly the solution to the hierarchy problem, we would expect to have already found evidence—though of course we could be missing something subtle.

On top of that, model building with technicolor is even more challenging than with supersymmetry. Finding models that agree with everything we have observed in nature has posed significant challenges, and no entirely suitable model has been found.

Experimenters will nonetheless keep an open mind and search for technicolor and any other evidence of new strong forces. But hopes are not overly high. If, however, technicolor turns out to be the underlying theory of the world, maybe Microsoft Word will stop automatically spellcorrecting and inserting a capital “T” whenever I write about it.

EXTRA DIMENSIONS

Neither supersymmetry nor technicolor are obviously perfect solutions to the hierarchy problem. Supersymmetric theories don’t readily accommodate experimentally consistent supersymmetry breaking and deriving technicolor theories that predict the correct quark and lepton masses is even more difficult. So physicists decided to look further afield and considered ideas that are superficially even more speculative alternatives. Remember, even if an idea seems ugly or not obvious at first, only after we fully understand all the implications can we decide which idea is most beautiful—and, more importantly, correct.

The better understanding of string theory and its components that physicists gained in the 1990s led to new suggestions for addressing the hierarchy problem. These ideas were motivated by elements of string theory—though not necessarily directly derived from its very constrained structure—and involved extra dimensions of space. If extra dimensions exist—and we have reason to think they might—they could hold the key to solving the hierarchy problem. If that is indeed the case, they would give rise to experimental evidence of their existence at the LHC.

Additional spatial dimensions is an exotic concept. If the universe has such dimensions, space would be very different from what we observe in our everyday lives. In addition to the three directions—left-right, up-down, forward-backward, or alternatively longitude, latitude, and altitude—space would extend in directions no one has ever observed.

Clearly, since we don’t see them, these new dimensions of space must be hidden. That could be because they are too small to directly influence anything we could possibly see, as physicist Oskar Klein suggested back in 1926. The idea is that as much that owing to our limited resolution, the dimensions might be too small to discern. We might not notice a curled-up dimension that we cannot travel through—much as a tightrope walker would view his path as one-dimensional, whereas a tiny ant on the wire might experience two, as illustrated in Figure 61.
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Man on tightrope

Ant on tightrope

[
FIGURE 61
]
A person and a tiny ant experience a tightrope very differently. For the person, it appears to have one dimension, whereas the ant experiences two.

Another possibility is that dimensions can be hidden because space-time is curved or warped, as Einstein taught us will happen in the presence of energy. If the curving is sufficiently dramatic, the effects of the additional dimensions are obscured, as Raman Sundrum and I determined in 1999. This meant that warped geometry might also provide a way in which a dimension might hide.
⁶⁴

But why would we even think extra dimensions could be out there if we have never seen them? The history of physics holds many examples of finding things no one could see. No one could “see” atoms and no one could “see” quarks. Yet we now have strong experimental evidence of the existence of both.