Feature - DZero: Doing Double Duty

Summer 2006: A new layer of silicon detector is ready to fit in the center of the DZero detector. Image courtesy of Dmitri Densiov, Fermilab

On the hunt for physics beyond the Standard Model, some DZero physicists search for traces of super-symmetry, lepto-quarks, quark substructure and other curiosities. Others continue to probe the Model, the theoretical backbone of modern high energy physics, by making ever more precise measurements.

DZero, located at Fermi National Accelerator Laboratory, Illinois, U.S., is a “general-purpose high energy physics experiment.” As such, the DZero detector slices and dices a broad range of particles and phenomena that only Fermilab’s Tevatron, currently the world’s highest energy particle accelerator, can produce.

Last summer the DZero detector was treated to a substantial upgrade, including the addition of new components. After any significant change to a detector, a precise calibration is required to properly quantify subsequent data.  DZero’s recalibration, completed in January 2007, involved measuring, reconstructing and comparing millions of data events. 

The events used in calibration do double duty: in order to be used in physics analyses, they must first be “reprocessed” using the new calibration values.  In other words, during the calibration, these data were used to refine the reconstruction “recipe”; now these same (raw) data are being reprocessed using the new-and-improved recipe.

Total events reprocessed up to mid-May 2007 on OSG (blue), DZero-SAM (red) and WLCG (green). Image courtesy of Mike Diesburg

The experiment itself is also doing double duty—DZero’s local computing farm is busy with incoming data and unable to accommodate all the needed calculations. Part of the needed computing power comes from DZero’s distributed computing resources, DZero-SAM, and the Worldwide Large Hadron Collider Computing Grid. But the bulk of reprocessing takes place on twelve shared Open Science Grid sites via opportunistic access. On OSG, DZero has sustained execution of over 1,000 simultaneous jobs, and has consumed and produced hundreds of gigabytes of data.

“This is the first major analysis of real high energy physics data, as opposed to simulations, ever run on OSG resources," said Brad Abbott of the University of Oklahoma, head of the DZero computing group. “There were some start-up problems on both the DZero and OSG sides, but we worked together and solved them.”

Reprocessing is currently about 80% complete, and at five million events per day, is progressing two to three times faster than originally planned.

“We try to get results out as soon as we can,” says Fermilab's Dmitri Denisov, DZero co-spokesperson. “There’s a natural analysis cycle that’s influenced by the Tevatron schedule, conference dates, graduate student and postdoc terms, and of course, competition.  With OSG we’re making good progress.”

In addition to the reprocessing effort, Denisov noted that OSG provided 300,000 CPU-hours to DZero for one of the most precise measurements to date of the top quark mass (170.5±2.7GeV/c2). DZero was able to achieve this result in time for the yearly Moriond conference last March, an important event for the high energy physics community.

Learn more about DZero at their Web site.

This article originally appeared as a OSG Research Highlight. 

- Anne Heavey, OSG Contributing Editor