Warped Passages - Lisa Randall [133]
The Large Hadron Collider will explore this energy range. If supersymmetry is not discovered at the LHC, which will search for particles up to a few thousand GeV in mass, it will mean that superpartners are too heavy to solve the hierarchy problem, and the supersymmetry solution will be ruled out.
But if supersymmetry solves the hierarchy problem, it will be an experimental windfall. A particle accelerator that explores energies of about a TeV (1,000 GeV) will find, in addition to the Higgs particle, a host of supersymmetric partners of Standard Model particles. We should see gluinos and squarks, as well as sleptons, winos (pronounced “weenos,” not like Bowery bums), a zino, and a photino. The new particles would have all the same charges as Standard Model particles, but would be heavier. With sufficient energy and collisions, these particles would be hard to miss. If supersymmetry is right, we will soon see it confirmed.
Supersymmetry: Weighing the Evidence
This leaves us with the outstanding question: does supersymmetry exist in nature? Well, the jury is still out. Without more facts, any answer is mere conjecture. At the moment both the defense and the prosecution have compelling arguments in their favor.
We have already mentioned two of the strongest reasons to believe in supersymmetry: the hierarchy problem and the superstring. A third compelling piece of evidence in favor of supersymmetry is the potential unification of forces in supersymmetric extensions of the Standard Model. As discussed in Chapter 11, the interaction strengths of the electromagnetic, weak, and strong forces depend on energy. Although Georgi and Glashow originally found that the Standard Model forces unify, better measurements of these three forces showed that unification in the Standard Model doesn’t quite work. A plot of the three interaction strengths as a function of energy is presented in the upper graph in Figure 66.
However, supersymmetry introduces many new particles that interact via these three forces. This changes the distance (or energy) dependence of the forces because supersymmetric partners also appear as virtual particles. These additional quantum contributions enter the renormalization group calculation and influence how the interaction strengths of the electromagnetic, weak, and strong forces depend on energy.
The lower graph in Figure 66 shows how the strengths of the forces depend on energy when the effect of virtual superpartners is included. Remarkably, with supersymmetry the three forces appear to unify more precisely than ever. This is more significant than the earlier unification attempts because we now have much better measurements of the interaction strengths. The intersection of three lines could be coincidence. But it might also be taken as evidence in support of supersymmetry.
Another nice feature of supersymmetric theories is that they contain a natural candidate for dark matter. Dark matter is the nonluminous matter that pervades the universe and has been discovered through its gravitational influence. Even though about one-quarter of the energy in the universe is stored in dark matter, we still don’t know what it is.* A supersymmetric particle that does not decay and has the right mass and interaction strength would be a suitable dark matter candidate. And indeed, the lightest supersymmetric particle doesn’t decay, and could have the right mass and the right interactions to be the particle of which dark matter is composed. This lightest superpartner could be the photino, the partner of the photon. Or, in the extra-dimensional scenario that we’ll consider later on, it could be the wino, the partner of the W gauge boson.
However, the case for supersymmetry is not airtight. The strongest argument against supersymmetry is that neither the Higgs particle nor its supersymmetric partners