Warped Passages (59 page)

It would be as if someone hinted something to you in such a subtle manner that you didn’t take it to heart the first time you heard it. But afterwards, fifty people repeated the same thing. Even though you wouldn’t take much notice the first time you heard the message, by
the fiftieth time the message would register. Similarly, although the light KK particles are light enough to be produced at current accelerators, they interact so weakly that we can’t detect any individual one. However, when an accelerator achieves sufficiently high energy to produce a lot of them, KK particles will leave observable signals.

The Large Hadron Collider, which will study TeV-scale energies, could produce KK particles at a measurable rate if the ADD idea is correct. That might sound like a fortunate coincidence—why should an energy of about a TeV be relevant to KK production rates when neither the KK masses nor the mass that determines the interaction strength of the KK particles (that is,
M
_Pl
) are a TeV? The answer is that an energy of about a TeV determines the strength of higher-dimensional gravity, and higher-dimensional gravity ultimately determines what a collider will produce. Because the interactions of the many KK partners of the graviton are equivalent to the interaction of a single higher-dimensional graviton, and the higher-dimensional graviton interacts strongly at energies of about a TeV, the sum of the contributions of all KK particles must also be significant at this scale.

Experimenters are already looking for KK particles at the Tevatron at Fermilab. Although the Tevatron doesn’t reach energies as high as the LHC will achieve, it does attain energies where it makes sense to start looking. But the LHC will do better, and has a much greater chance of finding ADD KK particles, should they exist.

What would these KK particles look like? The answer is that the collisions that produce graviton KK partners will look like ordinary collider events, except that it will appear that energy is missing. At the LHC, when two protons collide they could produce a Standard Model particle and a graviton KK partner. The Standard Model particle could be a gluon, for example—the protons would collide to produce a virtual gluon, and this virtual gluon could turn into a true physical gluon and a graviton KK partner.

However, any individual KK particle would interact too feebly for it to be detected—remember, graviton KK partners interact very weakly and might be detected only because there are so many of them. But because the detector would register the gluon—or, more accurately, the jet (see Chapter 7) that surrounds the gluon—the event that produced the graviton KK partner would be recorded, even if
the graviton KK partner is not. The key to identifying the event’s extra-dimensional origin would be that the unseen KK partner carrying away energy into the extra dimensions so that energy would seem to be missing. By studying single jet events where the energy of the emitted gluon is less than the energy that entered the collision, experimenters could deduce that they had produced a graviton KK partner (see Figure 77). This would be similar to the way in which Pauli surmised the existence of the neutrino (as we saw in Chapter 7).

Figure 77.
KK particle production in the ADD model. Protons collide, and a quark and an antiquark annihilate into a virtual gluon. The virtual gluon turns into an undetected KK particle and an observable jet. The gray lines are sprays of additional particles that protons always emit when they collide.

Because all we would know about the new particle is that it carries away energy, in actuality we wouldn’t know for sure that the accelerator had produced a KK particle and not some other particle that interacts too weakly to detect. However, by doing detailed studies of the missing-energy events—how the production rate depends on energy, for example—experimenters could hope to determine whether the KK particle interpretation is correct.

KK particles would be the most accessible extra-dimensional interlopers in our four-dimensional world because they are likely to be the lightest of the objects that could signal extra dimensions. But, if we’re lucky, other signatures of the ADD model could appear along with them, including even more exotic objects. If ADD are correct, higher-dimensional gravity would become strong at about a TeV, which is to say at far lower energy than would be true in a conventional
four-dimensional world. If that is the case, black holes might be produced at close to a TeV energy, and such higher-dimensional black holes would be a gateway to a better understanding of classical gravity, quantum gravity, and the shape of the universe. If the relevant energies of the ADD proposal are sufficiently low, black hole production could be imminent; they could be formed at the LHC.

The higher-dimensional black holes that would form at colliders would be far smaller than the ones in the universe around us. They would be comparable in size to the very tiny extra dimensions. In case you are worried, rest assured that these small, very short-lived black holes won’t pose a danger to us or to our planet: they’ll be gone well before they could do any damage. Black holes don’t last for ever: they evaporate by emitting radiation through the phenomenon known as
Hawking radiation
. But just as a small drop of coffee evaporates more quickly than a full cup, a small black hole evaporates more quickly than a big one, so the small black holes that could conceivably be produced at colliders will evaporate almost immediately. Nevertheless, if they are produced, these higher-dimensional black holes would last long enough to leave visible signs of their existence at a detector. They would have a very distinctive appearance since they would produce many more particles than you would find in ordinary particle decays, and these particles would go off in all directions.

Furthermore, if the ADD model is correct, black holes and KK partners of the graviton might not be the only exotic new discoveries. If ADD and string theory are both right, colliders could produce strings at very low energies, almost as low as a TeV. Once again, this is because the fundamental gravity scale is so low in the ADD models. Higher-dimensional gravity would become strong at about a TeV, and quantum gravity could contribute measurable effects.

The strings of the ADD theory would not be nearly as massive as the inaccessible Planck scale mass. If you think of strings as notes, the strings of the ADD proposal are far less high-pitched. The low-pitched strings of the ADD models would have mass not much bigger than a TeV. If we’re lucky, they’ll be light enough for the LHC to produce. Collisions with high enough energy would then produce the light strings of this model in abundance, along with new objects called
string balls
, containing many long strings.

However, despite the appeal of such potential discoveries, you should bear in mind that in all likelihood the energy at the LHC will be close to, but not as high as, the energy needed to make strings and black holes. Whether or not the ADD strings and black holes will be visible depends on the precise energy of higher-dimensional gravity (and, of course, on whether the proposals are correct).

The Fallout

The ADD proposal was fascinating. Who would have thought that extra dimensions could be so large, or that they could have so much bearing on problems of immediate interest (to particle physicists at least), such as the hierarchy problem? However, this proposal did not actually
solve
the hierarchy problem. It turned the hierarchy problem into another question: can additional dimensions be this large? This remains an outstanding question for the ADD scenario. Without some new and as yet undetermined physical principles, dimensions are not expected to be so extraordinarily large. At the very least, according to known theories, you would still need supersymmetry to maintain the large flat space that is needed for the ADD proposal. In essence, supersymmetry would stabilize and reinforce large dimensions that would otherwise collapse. Since one nice feature of ADD seemed to be that it could eliminate the need for supersymmetry, this is a bit disappointing.

The other weakness of the theory is its cosmological implications. For the theory to agree with known facts about the evolution of the universe, some of its numbers have to be very carefully chosen. And the bulk has to contain very little energy, or else cosmological evolution won’t agree with observations. Again, this might be possible, but the whole point of a solution to the hierarchy problem is to eliminate the necessity for large fudges.

Nonetheless, many physicists were open to the idea of taking extra-dimensional theories seriously and trying to devise ways to search for them. Experimenters, especially, were excited. As Joe Lykken, a particle physicist working at Fermilab, said to me when describing experimenters’ reaction to large extra dimensions, “To them, all this
‘beyond the Standard Model’ research is kooky. Supersymmetry or extra large dimensions? Who cares? Extra dimensions is no kookier.” Experimenters were hungry to search for something new, and extra dimensions provided a very interesting alternative to supersymmetry.

Theorists had a more mixed reaction. On the one hand, large extra dimensions seemed outlandish; no one had considered them before, since no one knew of any reason why extra dimensions should be so large. On the other hand, no one could identify a way to rule them out. In fact, before the first paper about large extra dimensions was written, Gia Dvali, one its the authors, spoke about them at Stanford. The authors, who were aware of the radical nature of their proposal, awaited the talk with trepidation, and were relieved when there were no serious objections. But they were also dismayed—how could people accept this pretty radical idea with such equanimity? Nima told me that when they first posted their paper on the Internet, they had a similar experience. Although they had expected a flood of responses, they received only two. Apparently the Italian physicist Riccardo Rattazzi and I were the only ones to comment on some potential problems. And even these two messages were not really independent: Riccardo and I had just discussed the paper at CERN, where we were both visiting.

Subsequently, as physicists absorbed the implications of the ADD model, they investigated the real-world consequences in more detail, considering tests of gravity, accelerator searches, astrophysical consequences, and cosmological implications. Reactions varied according to research interest or style.

Physicists whose research explored details of the Standard Model were happy to accept the possibility of a new idea, one which was in any case interesting. Surprisingly, there was more hostility from some model builders, who were unwilling to forfeit ideas about supersymmetry that had become entrenched over the years. Admittedly, altering the Standard Model so dramatically poses formidable challenges. Any new model would have to reproduce those features of the Standard Model that have already been experimentally verified, and theories that alter the Standard Model too dramatically will have a tough time meeting this challenge. Furthermore, the shining light of supersymmetry—the unification of couplings, the fact that at high energy all
forces would have equal strength—would have to be abandoned. However, younger theorists not so wedded to supersymmetry were more excited. The topic of extra dimensions was a new, not-yet-cornered idea, and posed new challenges and open questions.

The reaction from string theorists was mixed as well. When Savas Dimopoulos began his project, he foresaw that work on extra dimensions would bring string theory and particle physics closer together. And string theorists did pay attention, though most of them viewed large extra dimensions as an interesting idea that would never be relevant to string theory. For string theorists the major problem was theoretical: it is very difficult to understand how dimensions could be as large as assumed in the ADD proposal.

Personally, I don’t believe that extra dimensions, even if they exist, will turn out to be this large.
^*
Both for theoretical reasons (it’s hard to get dimensions that are this large) and for observational ones (it’s very tough to get cosmology to work out), the idea seems like a long shot. Even Nima, one of the protagonists, is skeptical at this point. But this was a very important theoretical idea. This new, previously unexplored suggestion highlighted the extent of our ignorance about gravity and the shape of the universe. The ADD paper stimulated a good deal of new thought, and whether or not the idea proves correct, it has had an important impact on physicists’ thinking. The large-dimension scenarios have led to many new proposals for extra dimensions and many ideas for experimental tests. After the LHC turns on, theoretical prejudices will be irrelevant in any case, since the implications of hard data will be irrefutable. Who knows? They might turn out to be right.

What’s New

• If Standard Model particles are confined to a brane, extra dimensions can be much larger than physicists previously thought: they can be as large as about a tenth of a millimeter.
  
• Extra dimensions can be so large that they can explain why gravity is so much weaker than the electromagnetic, weak, and strong forces.
  
• If large extra dimensions solve the hierarchy problem, higher-dimensional gravity would become strong at about a TeV.
  
• If higher-dimensional gravity becomes strong at about a TeV, the LHC will produce KK particles at a measurable rate. The KK particles would carry away energy from the collision, so their signature would be events with missing energy.