LHC Lunch

Meet Ricardo Vasquez Sierra

Precious cargo

In his job as a physicist on one of the experiments at the Large Hadron Collider, Ricardo Vasquez Sierra has helped build devices that are unique in the world. Vasquez Sierra, a postdoc for the University of California, Davis, took part in the team that made the CMS experiment’s end-cap pixel detectors, some of the most sensitive detectors on Earth.

CMS commissioned a U.S. subgroup, based at Fermilab, for the job in 2000.

The pixel detectors are C-shaped electronic grids that light up where charged particles hit them. The grids overlap each other surrounding the collision site in the CMS detector.

Vasquez Sierra began working on the pixel detectors in 2006. “I got to Fermilab when they were going to start putting together the final product,” he said.

Scientists across the U.S. contributed to the effort. Fermilab received sensors made at Purdue University, parts tested at Kansas State and chips built at Rutgers, among others. Vasquez Sierra and a few others put it all together and made sure it worked as expected. With limited resources, they had to make everything right the first time.

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“They were all very small jobs, but they were very specialized,” Vasquez Sierra said. “Some of the steps required wire-bonding that only one person at Fermilab had the expertise to do.”

Since almost every piece of the cathedral-sized experiment is one-of-a-kind, scientists learned as they went. “There is no kind of production line for [the detectors],” Vasquez Sierra said. “We’re not making phones.”

After the team at Fermilab connected, debugged and mounted hundreds of sensors, the detectors were ready to go. But sending them to CERN was not as simple as calling the post office. A few structural pieces could be shipped, but all the sensitive C-shaped pieces required personal escorts. Vasquez Sierra and his supervisor took them two at a time in suitcases on commercial flights from Chicago, Illinois, to Geneva, Switzerland.

The experiment transported the precious pixels across the border from CERN’s campus in Switzerland to the CMS detector in France at night to reduce the risk of traffic accidents. Around 1 a.m., a convoy flanked by two cars, with lights flashing, began the slow, 20-kph drive.

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A few days later, Vasquez Sierra and his team got the call; it was time for installation. From a two-story scaffold about 90 meters underground, Vasquez Sierra helped connect the pixel detectors to CMS. The pixels are the detectors closest to the collision point, so they are installed last into cylindrical cavities left open especially for the job. Then came the moment of truth: moving them into the heart of the 14,000-ton experiment.

The parts fit to the millimeter. “The incredible thing is that we built these in the U.S. and they slid in so perfectly – no last-minute modifications required,” Vasquez Sierra said.

The LHC began running in 2008. “It was really exciting to see them working,” Vasquez Sierra said. “I was just happy.”

These days, physicists on shift in the CMS control room remotely monitor the pixel detectors. Vasquez Sierra misses working in what he calls the “cozy environment” of the pixel group. “I made incredibly good friends in that collaboration and met very high-quality colleagues,” he said.

Vasquez Sierra, originally from Mexico, is now finishing his postdoc and applying for jobs around the world.  Like many physicists, he’s stayed in touch with his former colleagues over the years. “Maybe wherever I end up, there will be some good friend living there already -- that’s probably very likely,” he said.