Warped Passages - Lisa Randall [188]
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