Warped Passages - Lisa Randall [89]
The Tevatron, which is still in operation, is located at Fermilab, an accelerator center that was commissioned in 1972 and named after the physicist Enrico Fermi. I was very amused when I first visited Fermilab and found there were wild corn, geese, and for some strange reason, buffalo on the site. Buffalo aside, the region is fairly flat and boring. The movie Wayne’s World was set in Aurora, about five miles south of Fermilab, and if you are familiar with this movie, you might have some idea of the Fermilab surroundings. Fortunately, the physics there is exciting enough to keep people happy anyway.
The Tevatron gets its name because it accelerates both protons and antiprotons to an energy of a TeV (pronounced T-e-V, although the “Tev” in “Tevatron” rhymes with “Bev”), which is the same as 1,000 GeV, the highest energy that has been achieved so far at any accelerator. The energetic beams of protons and antiprotons that the Tevatron produces circulate in a ring and smash together every 3.5 microseconds at two collision points.
Two separate experimental collaborations set up detectors at each of the two collision points, where the beams of particles and antiparticles cross paths and the interesting physical processes can happen. One of these experiments was named CDF (Collider Detector at Fermilab) and the other was called D0, the designation of the collision point between protons and antiprotons at which the detector was located. The two experiments searched extensively for new physical particles and processes, but in the early 1990s the top quark was their Holy Grail. Each experimental collaboration wanted to be the first to find it.
Many heavy particles are unstable and decay almost immediately. When that is the case, experiments search for visible evidence of a particle’s decay products, rather than the particle itself. The top quark, for example, decays into a bottom quark and a W (the charged gauge boson that communicates the weak force). And the W also decays, either into leptons or quarks. So experiments seeking the top quark look for the bottom quark in conjunction with other quarks or leptons.
Particles do not come with nametags, however, so detectors have to identify them by their distinguishing properties, such as their electric charge or the interactions in which they participate, and separate components of the detectors are needed to record these properties. The two detectors at CDF and D0 are each segmented into several pieces, each of which records different characteristics. One piece is a tracker, which detects charged particles by the electrons from ionized atoms that they leave in their wake. Another piece, called a calorimeter, measures the energy that particles deliver as they pass through. The detectors have other components which can identify particles with other specific distinguishing properties, such as a bottom quark, which lasts longer than most other particles before it decays.
Once a detector registers a signal, it transmits the signal through an extensive array of wires and amplifiers, and records resulting data. However, not everything that is detected is worth recording. When a proton and an antiproton collide, the interesting particles such as the top and antitop quarks are only rarely produced. Much more often, collisions produce only lighter quarks and gluons, and more often still, nothing of real interest. In fact, for every top quark that was produced at Fermilab, there were ten trillion collision events that didn’t contain a top quark.
No computer system is sufficiently powerful to find the one interesting event in such a crowd of useless data. For this reason, experiments always include triggers—devices in which hardware and software elements act like nightclub bouncers and permit only potentially interesting events to be recorded. Triggers in CDF and D0 reduced the number of events that experimenters had to sift through to about one in one hundred thousand—still an enormously challenging task, but far more tractable than one in ten trillion.
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