Warped Passages - Lisa Randall [204]
Black Holes, Strings, and Other Surprises
In addition to KK particles, other remarkable signals of extra dimensions could turn up when the LHC turns on. Although the effects of five-dimensional gravity are minuscule at ordinary energies, five-dimensional gravity will be a major player when colliders create high-energy particles. In fact, when energies reach about a TeV, the effects of five-dimensional gravity would be enormous—they would overwhelm the interactions of the feebly interacting four-dimensional graviton, which has a small probability function on the Weakbrane where we live (and experiments take place).
The enormous strength of five-dimensional gravity means that five-dimensional black holes might be produced, as well as five-dimensional strings. Furthermore, once energies reach about a TeV, everything located on or near the Weakbrane would interact strongly with everything else. That is because the effects of gravity and the extra KK particles would be enormous at TeV energies, and they would conspire to make everything interact with everything else. Such strong interactions among all known particles and gravity wouldn’t occur in a four-dimensional scenario; they would be a definite signal of something new. As was the case with large extra dimensions, we don’t yet know if the energy will be high enough to see these new objects. But if interactions are strong at energies not too much greater than a TeV, experiments won’t miss it.
Coda
The connection between a solution to the hierarchy problem and experimental consequences at TeV energies is robust, but the details of what we will see depend on the model. Different models have distinctive experimental consequences, and that is very reassuring. These distinctive signatures mean that once the LHC is up and running, we have a good chance of identifying which—if any—of these models applies to our world.
What’s New
Spacetime can be dramatically curved in the presence of bulk and brane energy, even when the brane itself is completely flat.
The model we considered in this chapter has two branes, the Gravitybrane and the Weakbrane, at each of the ends of a finite-sized fifth dimension. Energies in the bulk and on the branes warp spacetime.
A single extra dimension introduces an entirely new way to solve the hierarchy problem. The fifth dimension in this model is not big, but it is very warped. The strength of gravity depends strongly on where you are in the fifth dimension. Gravity is strong on the Gravitybrane and is feeble on the Weakbrane, where we are located.
From the perspective of an observer who thinks he is in four dimensions, objects should have different sizes and masses if they originate from different places in the fifth dimension. Objects that are confined to the Gravitybrane should be very heavy (with mass of about the Planck scale mass), whereas objects confined to the Weakbrane should have much lower masses, about a TeV.
All forces can unify and the hierarchy problem can be solved if the Higgs particle (but not the gauge bosons) is confined to the Weakbrane.
The Kaluza-Klein partners of the graviton should give rise to very distinctive particle collider events in which they decay into Standard Model particles within the detector.
In models in which Standard Model particles are in the bulk, other KK particles can be produced and observed.
21
The Warped Annotated “Alice”*
Go ask Alice.
When she’s ten feet tall.
Jefferson Airplane
Athena stepped out of the dreamworld’s elevator into the warped five-dimensional world and was astonished to see only three spatial dimensions. Was the Rabbit playing games, pretending to take her to a world with four spatial dimensions when in fact there were only three? What a funny way to travel to what looked like an ordinary world!†
With great gallantry, a local received the puzzled new arrival. “Welcome to Branesville,‡ our glorious capital. Permit me to show you around.” Athena, who was tired and confused, blurted out, “Branesville doesn’t look all that special.