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   from the issue of February 28, 2008

Physicists use pixels to track particles


Physicist Aaron Dominguez puts on a lab coat, head cover and booties. He opens the door to a clean room at CERN and enters a sterile zone with silvery walls and glossy floors.


The room contains multiple cables, computing equipment and four long, white wooden boxes that house the Forward Pixel Detector, a key component of the Compact Muon Solenoid experiment.

The CMS experiment is being conducted at the Large Hadron Collider, the world's largest particle accelerator, under construction at the European Organization for Nuclear Research (CERN) near Geneva, Switzerland. Scheduled to go online this summer, the experiment will explore the frontiers of energy, matter, space and time.

Other UNL physicists involved in the experiment are Ken Bloom, Dan Claes and Greg Snow, plus postdoctoral researchers Mike Eads and Sudhir Malik and graduate students Tony Kelly, Jason Keller and Emily Peterman.

Dominguez and his UNL colleagues are readying the detector, nicknamed FPIX, for installation in the core of the five-story experiment at the Large Hadron Collider.

FPIX comprises both ends of the cylindrical CMS detector and contains millions of tiny electronic sensors, or pixels, to track charged particles. FPIX can pinpoint particle locations to rectangular-shaped pixels comparable to the width of a strand of hair.

"The pixel detector is useful because its pixels are so small they can effectively be used to give you a precise three-dimensional point in space that the particle passed through," Dominguez said.

If many of the pixels fail, FPIX will lose its accuracy. So, the Nebraska team created a system of electronics and computer boards to test the detector one piece at a time.

"Since we could not manually test each of the 18 million channels (of data from FPIX), we had to come up with a more sophisticated way to do it," Dominguez said.

Roughly 1,000 multiple-layered slabs, or plaquettes, make up the FPIX detector. Each plaquette contains 8,320 to 41,600 pixels.

In order to earn a passing grade from the Nebraska team, and a place in FPIX, the plaquettes had to pass several computer-administered tests done at Fermilab in close collaboration with Northwestern University and University of Milan. Fermilab (the Fermi National Accelerator Laboratory), in Batavia, Ill., near Chicago, is the world's highest-energy physics laboratory. It is operated by the U.S. Department of Energy.



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