Why Does E=mc2_ - Brian Cox [82]
At LEP, magnetic fields were also exploited, this time to bend the particles in a circle so they followed the arc of the tunnel. The whole point of the venture was to bring the two beams of particles together so they would collide head-on. As we have already learned, the collision of an electron and a positron can lead to the annihilation of both, with their mass converting into energy. This energy is what physicists at LEP were most interested in, because it could be converted into heavier particles in accord with Feynman’s rules. During the first phase of the machine’s operation, the electron and positron had energies that were very precisely tuned to the value that greatly enhanced the chances of making a Z particle (you might want to check back to the list of Feynman’s rules in the Standard Model to check that electron-positron annihilation into a Z particle is allowed). The Z particle is actually pretty heavy by the standards of the other particles—it is nearly 100 times more massive than a proton and nearly 200,000 times more massive than the electron and positron. As a result, the electron and positron had to be pushed to within a whisker of the speed of light to have energy sufficient to bring the Z into being. Certainly the energy locked in their mass and liberated upon annihilation is nowhere near sufficient to make the Z.
FIGURE 21
The initial goal of LEP was simple: keep on producing Z particles by repeatedly colliding electrons and positrons. Every time the particle beams collide, there would be a reasonable chance of an electron in one beam annihilating against a single positron in the other beam, resulting in the production of a single Z particle. By quick-firing beams into each other, LEP managed to make over 20 million Z particles through electron-positron annihilation during its lifetime.
Just like the other heavy Standard Model particles, the Z is not stable and it lasts for a fleeting 10-25 seconds before it dies. Figure 21 illustrates the various possible Z particle processes that the 1,500 or so LEP physicists were so interested in, not to mention the many thousands more around the world who were eagerly awaiting their results. Using giant particle detectors that surround the point where the electron and positron annihilate each other, particle physicists could capture the stuff produced by the decay of the Z and identify it. Modern particle physics detectors, like those used at LEP, are a little like huge digital cameras, many meters across and many meters tall, that can track particles as they pass through them. They, like the accelerators themselves, are glorious feats of modern engineering. In caverns as big as cathedrals, they can measure a single subatomic particle’s energy and momentum with exquisite accuracy. They are truly at the edge of our engineering capabilities, which makes them wonderful monuments to our collective desire to explore the workings of the universe.
Armed with these detectors and vast banks of high-performance computers, one of the primary goals for the scientists involved a pretty simple strategy. They needed to sift through their data to identify those collisions in which a Z particle was produced and then for each collision, figure out how the Z particle decayed. Sometimes it would decay to produce an electron-positron pair; other times a quark and antiquark would be produced or maybe a muon and an antimuon (see Figure 21 again). Their job was to keep a tally of how many times