Modern Accelerators

Accelerating Particles

While cosmic rays are incredibly energetic, they are not a suffciently reliable source of particles for most experimental purposes. Thus physicists have created their own "cosmic ray generators" here on Earth. These are called accelerators ("atom smashers" in the popular press). The basic idea is get a particle moving very fast and smash it into another one. Sometimes new particles are created (see below); sometimes not; but the physicists always learn something about how particles interact with each other.

It is fairly easy to obtain the particles which get accelerated. Physicists get electrons by heating metals, they get protons by robbing hydrogen of its electron, etc.

Accelerators speed up charged particles by creating large electric fields which attract or repel the particles. This field is then moved down the accelerator, "pushing" the particles along.

In a linear accelerator the field is due to traveling electromagnetic (E-M) waves. When an E-M wave hits a bunch of particles, those in the back get the biggest boost, while those in the front get less of a boost. In this fashion, the particles "ride" the front of the E-M wave like a bunch of surfers.

Particle Accelerators

One of the biggest accelerators in the world is at CERN, in Geneva. The dashed line in the photo shows the site of the Large Electron-Positron (LEP) collider (so called because electrons and positrons are both accelerated and made to collide head on). It is 27 km long and accelerates the electrons (and positrons) to 50 GeV in energy.

The Cornell Electron Storage Ring (CESR) is located at Cornell University in Ithaca, New York. The ring on the left accelerates electrons. It can also accelerate positrons which are stored in the ring on the right, so that they may collided with electrons at a later time. This setup is ideal for studying bottom particles.

Detecting Particles

It is not enough to smash particles together -- we must be able to see what happened! We have already seen how cloud chambers, bubble chambers, and photographic emulsion can assist in this. Modern computers have allowed this old technologies to be carried to new heights of sensitivity. Typical detectors are as large as houses and cost tens of millions of dollars to build. Here is a picture of UA1 (Underground Area 1) at CERN. Note the person on the scaffolding. This detector is famous for the discovery of the W and Z particles -- the carriers of the weak force.

Scattering events produced at UA1 are analyzed by computer. Here is an example of a proton-antiproton collision. Two high energy jets (in blue) may be seen emerging from the collider.

This is an example of a W particle decaying into an electron and a neutrino. The neutrino is the thick blue line moving off to the top (this was reconstructed by the computer as neutrinos are extremely difficult to detect). The neutrino's momentum is balanced by the electron, which travels to the lower left side (dashed blue line). The other lines represent low energy particles which were produced after the decay.

Analyzing Data

It is often very difficult to make sense of the wealth of data that accelerators produce. One way in which this is done is with Dalitz plots (named after Dick Dalitz who introduced them). In this example from the Crystal Barrel experiment at CERN, the patterns are formed by the interference of different produced particles. In this case evidence for a new type of particle, the glueball, is visible.