LATER this spring, electrons and positrons will collide in the heart of the new BaBar Detector at the Department of Energy's Stanford Linear Accelerator Center (SLAC), creating an alphabet soup of subatomic particles. These particles will shoot out, interacting with BaBar's layered subsystems and leaving clues about their identities. Once these data are gathered and processed, physicists from both ends of the physics continuum will examine the results, looking for evidence that will illuminate worlds both infinite and infinitesimal.
For cosmologists, the experiments may point the way to a clearer picture of the earliest moments in our universe. For particle physicists, the experiments will yield insights into unexplored regions of the fundamental interactions of matter. The BaBar Detector,* the work of 500 physicists from over 70 organizations worldwide, is part of DOE's B Factory shown in Figure 1. (For more information about the B Factory project, see S&TR, January/February 1997, pp. 4-13.) When it's up and running, SLAC's B Factory will produce B and anti-B mesons, particle-antiparticle pairs that scientists believe will open a new window on our understanding of nature and matter.





B Mesons and the Big Bang
Prevailing theory holds that at the time of the creation of the universe-by the so-called Big Bang-matter and antimatter existed in equal quantities. Fifteen billion years later, we look around and see a universe primarily of matter. The question is: "What happened to all the antimatter?"
Current theoretical models of elementary particles predict that an effect called charge parity violation favors the decay of antimatter over matter on the subatomic scale. Although a small charge-parity-violation effect was first observed in the 1960s, the theoretical explanation remains unresolved.
To detect charge parity violation in the laboratory, physicists measure the difference in the decay rates of particles and their antiparticles. Prime candidates for studying this effect are the B meson and its antiparticle, the anti-B meson. The electrically neutral B and anti-B mesons are short-lived, existing about 1.5 trillionths of a second before decaying. To determine the rates of decay for each B and anti-B meson, physicists measure how far the particles have traveled from their creation point. By knowing the velocity and the distance traveled for each, physicists can determine how long each particle existed before it decayed. The distances are exceedingly small-a few hundred micrometers. Subtle variations in the distribution of the distance traveled for the B and anti-B mesons will be the evidence for charge parity violation.
A thorough investigation of charge parity violation requires a "factory" that can produce 30 million pairs of B and anti-B mesons each year. The B Factory-a virtual time machine back to the earliest moments of the Big Bang-will do just that by colliding electrons with their antiparticles, positrons. The electrons are accelerated to a higher energy than the positrons-9 billion electron volts for the electron versus 3 billion electron volts for the positron. The particles created from the collision will then move together in the same direction. Only a few of the electron-positron collisions, about one in a billion, will result in B meson-anti-B meson pairs.
The B and anti-B mesons have a "rich" decay chain; that is, they decay into a variety of subatomic particles-leptons, neutrinos, and lighter hadrons-some of which decay in turn. This decay process repeats, creating hundreds of different decay pathways. About one in a thousand B-anti-B meson pairs is expected to take a certain decay pathway that can be used to search for the violation of charge parity. The B Factory's BaBar Detector will gather information about the decay products and pathways. Physicists will then use sophisticated computer programs to reconstruct the millions of recorded events, looking for the few that will shed light on the matter-antimatter paradox.

Touching the Elephant
The BaBar Detector has seven subsystems, each of which has a different purpose in the effort to identify all the decay products. It's a bit like the fable of the blindfolded wise men trying to identify an elephant by touching different parts-the trunk, the tail, the leg.
Livermore physicist Doug Wright explained, "We identify a particle's velocity from one subsystem, its position and charge from another, and so on. We then pull those bits of information together and say: `Aha! These are the characteristics of such-and-such a particle.' Some of those particles will lead us back to the B-anti-B meson pairs we're looking for."
The subsystems, shown in Figure 2, are the silicon vertex detector, the drift chamber, the DIRC (for detection of internally reflected Cerenkov light), the calorimeter, the cylindrical resistive plate chamber (RPC), the superconducting solenoid magnet, and the instrumented flux return (IFR). Working with research groups and manufacturers in the U.S., Italy, Britain, Japan, China, and Russia, Lawrence Livermore played a major role in the design, development, and delivery of the last four systems. Those leading these efforts included physicists Doug Wright, Richard Bionta, Marshall Mugge, and Craig Wuest and engineer Thomas O'Connor.






The first three subsystems look for clues about particles that carry a negative or positive charge. The silicon vertex detector subsystem detects the direction a charged particle travels, providing the position of a particle's decay to within 80 micrometers. The drift chamber and the DIRC measure, respectively, the momentum and velocity of charged particles. With this information, the B Factory investigators determine a charged particle's mass and sign (negative or positive).
The calorimeter, codesigned by Wuest and engineer Alan Brooks, primarily detects electrons, positrons, and photons. When these particles enter one of the calorimeter's 6,800 cesium iodide crystals, the crystal emits a flash of light. From this flash, physicists can then estimate a particle's position and energy.
The cylindrical RPC subsystem, developed by Wright, Bionta, and Mugge, detects charged particles that escape the calorimeter. The RPC is a gas-filled chamber between two conductive plates. When a charged particle goes through the detector and hits a gas atom, it knocks electrons off and causes a spark. From this spark, investigators ascertain the position of the particle.
The BaBar Detector's superconducting solenoid and 800-ton steel flux return, designed with O'Connor's assistance, are key to providing additional clues in this identification game. The solenoid's strong magnetic fields bend the path of charged particles-negatively charged particles in one direction, positively charged particles in another. The steel flux return is the main support structure for the detector and has been designed to withstand up to a 7.9 earthquake with minimal damage. Located just 2 miles from the San Andreas fault, the entire BaBar Detector is sitting on seismic isolators that protect the delicate physics equipment inside the detector.
The final BaBar subsystem is the IFR, which detects charged particles and provides a target for long-lived neutral particles. The IFR, codeveloped by Bionta and Wright, has 2,000 square meters of resistive plate chambers, each one layered between steel plates. These chambers detect muons (a heavier cousin of the electrons) and other charged, high-energy particles. As Bionta noted, "These particles are all very penetrating; they go right through the other subsystems." The IFR has another important function: its 800 tons of steel plates trap the enormous magnetic field produced by the superconducting magnet, confining the field effects to BaBar. "Otherwise," said Bionta, "the magnetic field would simply extend outward in all directions, affecting the electron and positron beams in the accelerator beam tubes as well as other B Factory equipment."
Once all the data are gathered from the BaBar subsystems, it's time to put the puzzle pieces together. This is where computer simulation and reconstruction programs come in, taking the data and completing a coherent picture of all the particles and their decay pathways.

Simulating the Physics of Particles
About 50 BaBar physicists, with contributions from Laboratory physicists including Xiaorong Shi, Torre Wenaus, and Doug Wright, developed computer programs that translate the predictions of particle theory into quantities that can be directly compared with the signals coming out of the BaBar Detector. The programs also simulate in detail how the subatomic particles interact with all the materials in each subsystem and provide the electronic responses of those interactions. In the end, B Factory physicists will have computer-generated results-simulated from theory-that can be compared with actual experimental results once BaBar is up and running. Checking previous known physics results with simulations validates the simulation programs and builds confidence in their predictive power.
Last year, using the Livermore Computing Center's computers, Shi simulated 7 million of the 10 million events needed for a mock data challenge that tested the BaBar simulation and reconstruction programs. The results from the data challenge set the guidelines for all the physics analysis.

The Answer to the $64,000 Question Is...
So, is charge parity violation "the" reason that we live in a universe of matter, instead of antimatter? When the data from BaBar begin to arrive, B Factory collaborators may find the long-sought clues. The information provided promises to open a new window on the subatomic world. Ultimately, the B Factory and BaBar will provide scientists with a more complete and accurate picture of the fundamental nature of matter and energy.

-Ann Parker

Key Words: BaBar Detector, B Factory, Big Bang, B mesons and anti-B mesons, charge parity violation, particle physics, Stanford Linear Accelerator Center (SLAC).

For further information contact Doug Wright (925) 423-2347 (wright20@llnl.gov).
More information about the BaBar Detector is available on the Internet at http://www.slac.stanford.edu/BF/doc/www/bfHome.html.


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