Ever since scientists first looked to the skies, they have tried to piece together the cosmic puzzle that would reveal the origins of the universe. If a group of Syracuse University high-energy particle physicists and their colleagues succeed in a $150 million experiment, their work may move scientists a step closer to solving the puzzle—yielding answers to some key questions about how Earth and the universe around it were formed.
SU physics professors Sheldon Stone, Marina Artuso, and Tomasz Skwarnicki are among the lead researchers in a consortium of 28 universities participating in the initiative with the Fermi National Accelerator Laboratory in Batavia, Illinois. The experiment—called the BTeV project (pronounced bee-tev)—is designed to detect and study subatomic particles called heavy quarks. Stone, a co-spokesperson for BTeV, says these quarks were present in great numbers when the universe was created, but largely disappeared as the universe cooled. There are several different kinds of heavy quarks, including the “charm” and the “bottom.” Scientists working on the BTeV project are primarily interested in the bottom quarks, or b-quarks, and anti-b quarks. “By furthering our understanding of the behavior of these subatomic particles,” Stone says, “we will learn more about the fundamental symmetries that determine the structure of our universe.”
Fermilab, which is funded by the U.S. Department of Energy, operates the world’s highest energy particle accelerator—the Tevatron. More than 2,200 scientists from 36 states and 20 countries use Fermilab’s facilities to carry out research at the frontier of particle physics. The accelerator consists of a four-mile-long circular ring that holds two beams of atomic particles (protons and antiprotons) that move in opposite directions at nearly the speed of light. When the particles collide, b-quarks and anti-b quarks (heavy quarks) are produced.
"BTeV represents the most important experimental program that can be conducted in the United States this decade,” says Fermilab Director Michael Witherell, who approved the project in June, following a unanimous recommendation by Fermilab’s Physics Advisory Committee (PAC). Witherell also notes that BTeV could establish the Tevatron as the premier B factory in the world.
The project will involve construction of an array of particle detectors that will fill an entire building located on the lab’s circular ring, Stone says. Construction is expected to be completed in 2006. Scientists will then collect and analyze the results of the experiments and disseminate the information.
Stone and Joel Butler, a Fermilab physicist and BTeV co-spokesperson, spent the past five years building the BTeV consortium and developing the proposal. The process included creating a computer model of the BTeV detector array. “We built a virtual detector and made a computer model of all the possible interactions of the particles of matter,” Stone says. “The simulator software doesn’t give us answers to the physics, but it enables us to design the entire detector, test it, and try to address contingency issues before actually building the detector. It was an enormous effort. A lot of people put a lot of creative thought into developing the software.”
The BTeV detector array will be composed of seven parts—three of which will be partially designed and tested at SU. These include two Ring Imaging Cherenkov Counters (RICH), which will be similar to the RICH detector that Stone, Artuso, and Skwarnicki designed for the CLEO III particle detector housed at Cornell University. The $5 million RICH detector, funded by the National Science Foundation, was built in collaboration with physicists from Southern Methodist University and the University of Minnesota and was completed in June 1999. Skwarnicki is the principal investigator for the BTeV RICH detector, which will be much larger than the CLEO III detector and incorporate different technology, Stone says.
In addition to being co-spokesperson for the BTeV project, Stone is co-principal investigator with University of Minnesota physicist Yuichi Kubota in the design and construction of the BTeV Lead Tungstate Electromagnetic Calorimeter. The calorimeter will be used to detect high-energy photons that result from the collisions.
The heart of the BTeV project is the pixel detector and its trigger mechanism. “Without the pixel detector, the BTeV experiment doesn’t exist,” says Artuso, principal investigator for the pixel project at Syracuse University. Artuso and her collaborators from the University of Iowa and Fermilab have worked on the design for more than two years and have already tested prototype models.
The pixel detector will be composed of 62 large planes; each will contain hundreds of thousands of pixel cells, which are rectangular, microscopic patterns deposited on silicon chips. These millions of active elements record the trail left by subatomic particles in their extremely short and interesting lives. These traces are the clues that identify intriguing events that warrant further studies. The position of the particles crossing the pixel sensors can be determined within a resolution of a few microns—less than a human hair, Artuso says.
Each pixel cell will be attached to a microscopic electronic system using a technique called bump bonding. The tiny electronic circuits will transmit the data collected by the cells hit by the subatomic particles to a triggering device, which will decide whether the event warrants further study. “The triggering mechanism is a crucial and unique component of the pixel detector,” Artuso says. “It will act as a high-speed gatekeeper that will instantaneously decide whether the particles detected are the ones the scientists are interested in. The trigger will distinguish the good data from the bad—the flowers from the weeds.”
Fermilab theorist and PAC member Andreas Kronfeld says the BTeV detector, especially the trigger, breaks new ground in its design. “We held the bar [for approving the experiment] very high,” Kronfeld says. “The consortium just kept jumping over it, even if we moved the bar while they were still in the air.”