University physicists work toward rebirth of Collider Detector

Subatomic particles called bottom quarks are among the basic building blocks of matter, but they live fast and die young. In colliding laboratory beams of protons and antiprotons, bottom quarks move at the speed of light and decay into other types of particles only a trillionth of a second after their birth.

University physicists are developing an instrument to help reconstruct the bottom quark family tree, which may hold the secret to the very existence of matter in the universe and otherwise radically alter scientists’ understanding of the physical world.

Fermi National Accelerator Laboratory in Batavia, Ill., houses the world’s premier delivery room for bottom quarks. But until now it has been impossible for physicists to rapidly distinguish a bottom quark from the flood of other particles produced in Fermilab’s Tevatron, the most powerful particle accelerator in the world.

That inability will change late this summer, when tests begin on the refurbished Collider Detector at Fermilab. “Although we will still call it CDF, more than 90 percent of the detector will be brand new,” said Chicago’s Melvyn Shochet, the Elaine and Samuel Kersten Jr. Professor in Physical Sciences.

A massive undertaking, the CDF collaboration consists of more than 500 scientists from 52 institutions representing 11 countries. The Chicago team is developing several key components of the experiment’s trigger system, which determines which droplets in the data flood will be collected.

One of the Chicago components will provide an entirely new capability––triggering on bottom quarks. “If you can’t trigger on things, then you can’t look at them because they’re not recorded,” said Henry Frisch, Professor in Physics at the University. “To be able to trigger on bottom quarks is going to put us into a whole new realm.”

Frisch and Shochet have been members of the CDF team since its inception in 1977. Shochet served as co-spokesman of the team in 1995, when it announced the discovery of the top quark, the last undiscovered quark of the six predicted by current scientific theory. Since then, new technology and concepts have emerged that could lead to further discoveries of even greater significance when CDF begins gathering data again in March 2001.

“What we are doing in simple terms is designing and building what is essentially a very special-purpose, extremely high-speed computer,” Shochet said.

As the part of the detector that examines each particle collision, the trigger has to operate fast. The CDF will need to process 5 million or more particle collisions per second, a vast increase over the 1 million collisions per second that was achieved during the experiment’s second run in 1994 and 1995. Each collision in turn produces 50 to 100 other particles of various types and energies that would generate approximately a quarter-million bytes of data to record.

But the experimenters can record the data from only 30 to 50 collisions per second. The trigger’s task: determine which 30 to 50 collisions to record, all within microseconds after they occur.

“You don’t want to select a random 30 to 50 events,” Shochet said. “You want to select the 30 to 50 that have the greatest potential for finding new phenomena or for better understanding important phenomena that we’ve studied before.”

The three-level trigger system will decide whether a collision is sufficiently interesting 5 million times per second by determining what types of particles were produced and their measured energies, locations in the detector and directions of travel.

The trigger’s first level will reduce the data flow to 50,000 events per second, before sending it to the second level. Even then, Frisch said, “It’s like 2 million 56-kilobit modems on your desk running at full speed into your phone line.”

A Chicago component, developed in collaboration with the University of Pisa, at the trigger’s second level will screen this fire hose of data for bottom quarks. Unlike most other types of particles, a bottom quark’s tracks will not point back to the original collision.

“Because they travel at the speed of light, bottom quarks travel away from their creation point by a few millimeters before they decay,” Shochet said. “This new set of hardware will try very quickly, in a few microseconds, to identify that there was an object that traveled a few millimeters and then decayed into two, three, four, five or 10 other particles. On a fast commercial computer, such calculations would take almost a second to carry out.”

The expanded CDF experiment will open multiple avenues of new research. Physicists are especially excited about studying the decay of bottom quarks to better understand why matter exists at all.

“It’s a very small effect, but a very important one,” Frisch said. “The universe is matter. If it were equal amounts of matter and antimatter, it would annihilate, and we wouldn’t be here.”

The experiment also could find evidence supporting the existence of a 10-dimensional universe and explain why gravity is so much weaker than nature’s three other fundamental forces: electromagnetism; the weak interaction, which is associated with radioactivity and particle decay; and the strong interaction, which binds protons and neutrons in an atom’s nucleus.

This gravitational problem puzzled Albert Einstein for years and continues to confound physicists today in their quest for a so-called “Theory of Everything.”

“We’re about to embark in some really mind-boggling directions,” Frisch said.