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November 11, 1996

Research Update: Physics

It's hard to imagine a more intriguing riddle in science than the one a machine at the Stanford Linear Accelerator Center (SLAC) will begin to investigate in 1999: Why do we exist?

More specifically, the new machine, called the "B Factory," will explore why the universe is dominated by matter--the stuff that composes galaxies, the atoms in our bodies, and everything in between. The fires of the Big Bang forged nearly equal amounts of matter and antimatter, scientists believe. However, the laws of physics led to some tiny preference for matter. That extra dollop survives to this day, spared from the mutual annihilation that consumed the rest of the matter and antimatter in an explosive fraction of a second.

The B Factory promises to lay bare those physical laws, which have remained tantalizingly out of reach of even the most powerful particle accelerators on earth.

"The B Factory will exploit to the maximum degree a collection of fortuitous scientific parameters that define the behaviors of a set of particles," says Bruce Schumm, assistant professor of physics at UCSC. "It's excellent science, opening a wonderfully elegant and remarkable window to explore the physics that played a fundamental role in the birth of our universe."

Schumm and several colleagues at the Santa Cruz Institute for Particle Physics, a highly regarded research unit at UCSC, play key roles in the B Factory's progress. The researchers are devising the sophisticated electronic components needed to make sense of the blizzards of particles that will rifle through the innermost sections of the machine's detector. Physicists must measure the flight paths and energies of these particles with incredible precision to unravel the collisions that create them--thus exposing the complex laws that govern the matter-antimatter pas de deux.

Jonathan Dorfan, professor of physics at Stanford University and leader of the SLAC team working on the B Factory, spoke recently at UCSC about the status of the $237 million project.

"The fate of all of the antimatter is one of the great mysteries of the evolution of the universe," Dorfan says. "The B Factory will provide us with the only conclusive experiments we can perform to distinguish between matter and antimatter."

The B Factory derives its name from the objects it will churn out by the hundreds of millions: B mesons, short-lived particles sparked when electrons collide with their antimatter counterparts, positrons. B mesons vanish quickly into sprays of other particles, but not before they disobey a physical principle known as charge-parity conservation. Under this principle, subatomic interactions should behave identically if one switches all particles into their antiparticles and reflects them into their mirror images (e.g., from "left-handed" to "right-handed" parity). Some B mesons, however, violate this symmetry. The way they do so is ideal for probing why there was a slight excess of matter in the infant universe.

More broadly, the B Factory will shine light into the deepest recesses of the "Standard Model," the theoretical edifice of particle physics that describes all known particles and forces of nature and how they interact. "If the Standard Model is incorrect, the B Factory will give us precision information about its flaws," Dorfan says. "We plan to surgically examine the Standard Model."

Like the layers of an onion, the particle detector planned for the B Factory will consist of many nested devices, all tuned to certain types of particles and measurements. UCSC physicists are working on the machine's hearts: the silicon vertex detector, a breadbox-sized apparatus that will record the initial frenzy of particles bursting from the collisions, and the drift chamber, a gas-and-wire-filled cavity the size of a phone booth.

Associate professor of physics Robert Johnson leads UCSC's work on prototypes of the integrated circuit chips that will read data from the silicon vertex detector. In the finished machine, 150,000 electronic readout channels will record the flight paths of particles to an accuracy of better than 50 microns--less than half the width of a human hair. Physicists at Santa Cruz have pioneered the use of this silicon-strip technology in detectors for accelerators around the world.

Other Santa Cruz contributors to this project are professor of physics Abraham Seiden, postdoctoral researchers Ariane Frey and Wilko Kroeger, and research physicist Alex Grillo, who is working on data transmission from the readout channels to the computer system.

Another group at UCSC focuses on electronics for the drift chamber. This cavity uses an unusual combination of helium and isobutane gas to create ionized trails of electrons as particles whiz past, much as jets leave contrails of water vapor. The electrons drift in a magnetic field until they touch one of about 5,000 wires laced through the chamber, where they convert into electronic signals. Reconstructing the timing and locations of those signals will let the researchers pinpoint crucial aspects of the collisions. The main collaborators are professor of physics David Dorfan (Jonathan Dorfan's brother), adjunct professor of physics Hartmut Sadrozinski, and Bruce Schumm.

"We're pushing the speeds and capabilities of these electronics to the limit," Schumm says. "The real challenge will be for this system to discriminate the one event of physical interest out of every 10,000 or 100,000 events."

--Robert Irion