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Reading the Data at Fermilab

Searching for the answers to the questions we ask. And what will be the tools, to help us with our task?
. . . Detectors, computers and human intuition, combining together making sense of these collisions.

Seeking out these mysteries is what we try to do. This is just the start, the rest is up to you!

Lyrics from the video Accelerating Science


This is the last of an article in the series that has traveled through the stages of physics research at Fermilab. In this article we try to understand how the data from the detectors is used.

Data that speeds through the detectors at a rate of 5,000,000 photons per second need a great deal of filtering and continuous monitoring. Physicists need to capture approximately 50,000 events per second for study. Of course, each experiment is uniqely designed and has different philosophies regarding the amount of data to keep or filter. Detectors are also monitored by computer.

Detectors which may consist of various components such as the silicon vertex tracker (see the last issue of sciencelines) must be handled differently. For example, the signal traveling through the scintillating fibers can be transferred by wire to electronic chips and then read by computers. Tom Carter of the Computing Division outlines the process with the following diagram:


It is crucial that the detectors operate correctly. Monitoring this process is done online so wires and individual detectors can be corrected if necessary. Graduate students often are responsible for checking graphs that indicate activity of the detectors. Art Kreymer, Applications Physicist with Experiment #831 (Focus, Photo Production of Charm Upgraded Spectrometer) also notes that during the monitoring procedure it is now possible to do physics. Changes in voltage of wire chambers, magnetic fields, alignments and calibrations can be performed.

In the experimental areas with the detectors are elaborate electronic programmable crates that can be programmed using NIM logic standards. This hardware provides the experimenters with a method of decision filtering. The experiment can categorize events into three levels: was the event okay, interesting or really interesting? In other words, how do the events reflect the information that the experiment is seeking to obtain? The NIM logic board allows technicians to plug wires from the detectors into a pattern that arranges and filters the data through a boolean process. The electronics temporarily hold the data for fractions of seconds before translating it to the computer. The three event categories reflect both a logic process and a different electronics path or method.

Many of the experiments use Fortran programs to sort data offline. Fortran is preferred for experiments where fast calculation is necessary while C programming language is useful for moving large portions of data.

Tom Carter emphasizes the importance of human intuition in the process and the importance of individuals who have the creativity and the courage to try a new approach in the experiment if the new approach will result in better physics.