Warped Passages - Lisa Randall [88]
Finding heavy particles is not easy. Yet that’s what we must do if we are to discover the structure underlying the Standard Model and, ultimately, the physical makeup of the universe. Most of what we know about particle physics comes from high-energy particle accelerator experiments, which first accelerate a rapidly moving beam of particles and then smash them into other matter.
In a high-energy particle collider, the accelerated beam of particles actually collides with an accelerated beam of antiparticles so that they meet in a small collision region containing a huge amount of energy. This energy is then sometimes converted into heavy particles not readily found in nature. High-energy particle colliders are the only place where the heaviest known particles have appeared since the time of the Big Bang, when the much hotter universe contained all particles in abundance. Colliders can create pairs of any kind of particle and antiparticle, in principle, as long as they have enough energy for that particular pair, the energy given by Einstein’s E = mc2.
But the goal of high-energy physics is not merely to find new particles. Experiments at high-energy colliders will tell us about fundamental laws of nature that cannot be observed in any other way—laws that operate at too close a range to be visible more directly. High-energy experiments are the only way to probe any short-distance interactions that operate at extremely tiny distance scales.
This chapter is about two of the collider experiments that were important in confirming the predictions of the Standard Model and constraining what physical theories might lie beyond. These experiments are both impressive in their own right. But they should also give you a sense of what physicists will be up against when they will search for new phenomena, such as extra dimensions, in the future.
The Top Quark Discovery
The search for the top quark beautifully illustrates the difficulties of finding a particle at a collider when the collider’s energy is barely adequate to produce it, and the ingenuity with which experimenters can rise to this challenge. Although the top quark is not part of any atom or known matter, the Standard Model would be inconsistent without it, so most physicists had been confident of its existence since the 1970s. Yet as recently as 1995, no one had ever detected one.
At that time, experiments had been looking for the top quark in vain for many years. The bottom quark, the next-heaviest Standard Model particle, which weighs in at five times the mass of a proton, was discovered in 1977. But although physicists back then thought the top quark would soon show up, and experimenters raced to find it and claim the glory, to everyone’s surprise experiment after experiment failed. It wasn’t found at colliders that operated at 40 times, or 60 times, or even 100 times the energy required to produce a proton. The top quark was evidently heavy—remarkably heavy compared with the other quarks, all of which had been detected. When it finally made its appearance after twenty years of searching, it turned out to have a mass almost 200 times that of the proton.
Because the top quark is so heavy, the relations of special relativity tell us that only a collider that operated at extremely high energy could produce it. High energy always requires a very large accelerator, which is technically difficult to design and expensive to construct.
The collider that eventually produced the top quark was the Tevatron in Batavia, Illinois, thirty miles west of Chicago. The collider at Fermilab was initially designed with far too low an energy to produce a top quark, but engineers and physicists had made many changes that improved its potential enormously. By 1995 the Tevatron, the culmination of these improvements, operated at far higher energy and produced many more collisions than the original machine could