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
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.