Knocking on Heaven's Door - Lisa Randall [55]
The LHC is an example of a collider. It bangs together two beams of particles that magnets deflect so that they will be aimed toward each other. The principal parameters that determine the capabilities of a collider such as the LHC are the type of particles that collide, their energy after acceleration, and the machine’s luminosity (the intensity of the combined beams and hence the number of events that occur).
TYPES OF COLLIDERS
Once we have decided that two beams colliding can provide higher energy (and hence explore shorter distances) than fixed-target experiments, the next question is what to collide. This leads to some interesting choices. In particular, we have to decide which particles to accelerate so that they participate in the collision.
It’s a good idea to use matter that’s readily available here on Earth. In principle, we could try to collide together unstable particles, such as particles called muons that rapidly decay into electrons, or heavy quarks such as top quarks that decay into other lighter matter.
In that case, we would first have to make these particles in a laboratory since they are not readily available. But even if we could make them and accelerate them before they decayed, we’d have to ensure that the radiation from the decay could be safely diverted. None of these problems are necessarily insurmountable, particularly in the case of muons, whose feasibility as particle beams is currently under investigation. But they certainly pose additional challenges that we don’t face with stable particles.
So let’s go with the more straightforward option: stable particles available here on Earth that don’t decay. This means light particles or at least bound stable configurations of light particles such as protons. We also would want the particles to be charged, so that we can readily accelerate them with an electric field. This leaves protons and electrons as options—particles that are conveniently situated in abundance.
Which should we choose? Both have their advantages and their downsides. Electrons have the advantage that they yield nice clean collisions. After all, electrons are fundamental particles. When you collide an electron into something, the electron doesn’t partition its energy into lots of substructure. So far as we know, the electron is all there is. Because the electron doesn’t divide, we can follow very precisely what happens when it collides with anything else.
That’s not true for protons. Recall that protons are composed of three quarks bound together by the strong nuclear force with gluons exchanged among the quarks that “glue” the whole thing together, as was discussed in Chapter 5. When a proton collides at high energy, the interaction you are interested in—that could produce some heavy particle—generally involves only one individual particle inside the proton, such as a single quark.
That quark certainly won’t carry all the energy of the proton. So even though the proton might be very energetic, the quark will generally have much less energy. It can still have quite a bit of energy, just not as much as if the proton could impart all its energy into that single quark.
On top of that, collisions involving protons are very messy. That’s because the other stuff in the proton still hangs around, even if it’s not involved in the super-high-energy collision we care about. All the remaining particles still interact through strong interactions (aptly named), which means there is a flurry of activity surrounding (and obscuring) the interaction you are interested in.
So why would anyone ever want to collide a proton in that case? The reason is that the proton is heavier than an electron. In fact, the proton mass is about 2,000 times greater than that of an electron. It turns out that’s a very good thing when we try to accelerate a proton to high energy. To get to these enormous energies, electric fields accelerate particles around a ring so that they can be accelerated more and more in each