Discovery of the Top Quark

Particle Detectors at Fermilab

This particle detector dwarfs even Einstein, whose picture is included to provide a sense of the detector's size. The detector's chambers are crammed with intricate layers of materials and electronics to detect and record the results of billions of high-speed collisions between protons and antiprotons at Fermilab's Tevatron particle accelerator.

Beams of protons and antiprotons collide at nearly the speed of light in Fermilab's Tevatron particle accelerator. Four hundred thousand times a second, protons and antiprotons burst into showers of secondary particles: quarks and electrons, muons, neutrinos, B mesons and W bosons... there are more than a hundred possibilities. The collisions take place inside each of two huge collider detectors on the accelerator ring. The detectors' job is to observe as many collisions as possible, to recognize and record the particles that come flying out, and to preserve the information for later study. By analyzing the stored data from the detectors, physicists make discoveries about the fundamental nature of matter and energy. Physicists at Fermilab studied data from collisions at CDF (the Collider Detector at Fermilab) and DZero, the Laboratory's two collider detectors, to discover the top quark, an elementary particle of matter.



A photograph of Dzero

Portrait of a particle collision

A detector's first task is to decide which of the almost half million collisions that occur each second are worth recording. It would be impossible to save them all. Computers are instructed to weed out ho-hum events, registering only the most interesting collisions in detail for later analysis. Next, the detector must record as much information as possible about every particle in the events selected for recording--the particle's path, its energy, and its charge. Because each kind of particle that flies off from a collision has its own distinctive behavior, the detector can use this information to identify the particle's characteristic electronic signature and determine that it was present at the scene of the collision. To create a complete picture of an event, a detector uses layers of different kinds of materials intricately arranged around the central collision point.

Each layer collects and transmits a different kind of information about the moving particles that interact with the material in that layer. Electronic signals transmit information about the interactions to computers, tracing the paths of the particles, along with their energy and charge. Putting all these signals together, the detector builds up a kind of electronic photograph, as though the detector functioned as a 5,000-ton camera, creating a portrait of each collision it records.



The CDF detector

Open 24 hours

To accomplish its task, a detector must contain enough layered materials for all the collision-born particles to interact with, and it must have enough elec tronic channels to read the information about the interactions to computers. Fermilab's detectors are three stories high, crammed with the intricate circuitry for 100,000 electronic channels of information. Millions of lines of computer code control and monitor the detectors' operations. During collider operations at the Tevatron accelerator, control-room crews operate the detectors, taking eight-hour shifts, 24 hours a day.

At the end of the collider run, when the data from billions of collisions have been recorded and stored, experimenters move the detector--all 5,000 tons-- out of the accelerator ring and into the experiment's assembly hall, to make improvements that will upgrade its performance for the next collider run. For the moment, the detector's job is done. The job of the experiment collaboration, however, is only beginning. They search the data their detector has delivered, analyzing the electronic portraits of particle collisions to move another step forward in the understanding of matter and energy. From months of analyzing data from particle collisions at Fermilab came the discovery of the top quark.