ONE of the great mysteries of the universe is the overwhelming preponderance of matter over antimatter. Physicists believe that a few trillionths of a second after the universe was created--the so-called Big Bang--matter and antimatter existed in equal amounts. And yet, the known universe today is overwhelmingly made of matter, the result of some process long ago that must have favored matter over antimatter (see the box below).|
Physicists believe the key to unraveling this mystery--and discovering the ancient origin of matter--is to follow closely the decay of a pair of artificially produced, extremely short-lived particles of matter and antimatter, the B meson and its antiparticle. A serious investigation, however, requires a "factory" designed to produce 30 million pairs of B mesons and anti-B mesons each year. Such a facility, a virtual "time machine" back to the earliest moments of the Big Bang, is now under construction at the Stanford Linear Accelerator Center (SLAC) near Menlo Park, California (see Figure 1 above). Total project cost is $300 million, including the accelerator ($177 million), the detector ($73 million), and research and development costs. The B-Factory accelerator portion is a combined effort of SLAC, Lawrence Berkeley National Laboratory, and Lawrence Livermore National Laboratory.
The Curious Case of the B Meson
Physicists believe a key to the matter-antimatter disparity in the universe lies in understanding an effect called charge parity violation. First observed in the 1960s, charge parity violation refers to the apparently small differences in the way that certain short-lived particles and their antiparticles decay. Many scientists, starting with the renowned Russian physicist Andrei Sakharov, have suggested that charge parity violation is the reason why the universe seems to be composed almost exclusively of matter. Or, as Stanford Linear Accelerator Center Director Burton Richter puts it, "Charge parity violation is why we're here."
Physicists think that the best way to understand the phenomenon is by studying the decay patterns of the rare B meson, a type of unstable but electrically neutral particle, and its antiparticle, the anti-B meson. The B meson consists of "anti-b" quark and a "d" quark (quarks are the fundamental building blocks of matter), while the anti-B meson consists of a "b" quark and an "anti-d" quark.
To measure the decay patterns of these extremely short-lived (1.5 trillionths of a second or 10-12 second) particles, investigators need a machine to produce a myriad of B mesons and anti B mesons. "B-Factories" at the Stanford Linear Accelerator Center and in Japan, both currently under construction, will provide literally millions of particles to study. In effect, says Livermore physicist Marshall Mugge, "The B-Factory will allow us to go back in time to reproduce the early conditions of the Big Bang."
At both factories, the B mesons and anti-B mesons will be created by colliding beams of electrons and positrons circling at different energies. Most of the electrons and positrons will miss each another. However, a few collisions will result in B meson- anti-B meson pairs. Because the electrons and positrons are circling at different energies, the B and anti-B mesons will be created with a push away from the point at which the two beams collide, clearly separating their decay vertices--up to a millimeter apart--and therefore making them easier to resolve.
The particle pairs will follow one of several different pathways as they decay into a host of subatomic particles like muons, leptons, neutrinos, and quarks. About one in every 1,000 pairs is expected to follow a unique pathway that results in a very special combination of particles that will signal to physicists a possible violation of charge parity. That is, these special pairs will decay to certain sets of particles at different rates.
Physicists will determine the rates of decay by measuring how far the particles have traveled from the interaction point. By knowing how fast the particles are traveling, scientists can determine the time that they existed before they decayed. The distances are exceedingly small--about a few hundred micrometers (less than one-thirty-second of an inch) in space. Subtle variations in the distribution of the distance traveled between the pairs will be evidence for charge parity violation.
Livermore physicist and B-Factory leader Karl van Bibber says that by the end of the first six months, there should be some "very interesting data" for 75 to 80 institutions worldwide to analyze. The data should enable scientists to better understand why the universe appears to contain essentially no antimatter. In so doing, they will be able to paint a much more complete and accurate picture of the fundamental nature of matter and energy.
The B-Factory's two underground rings, each 2,200 meters (a mile and a half) in circumference, will generate B mesons by colliding electrons and positrons (the antimatter counterpart
of electrons) moving at near the speed of light. In helping to design and manufacture many of the major components and systems for the B-Factory rings and its giant, three-story-tall detector, Lawrence Livermore is strengthening its reputation as a world-class center of excellence for accelerator science and technology and high-energy physics. Nearly 200 Laboratory specialists representing a broad range of disciplines, from electroplating to particle physics, are contributing to the B-Factory effort.|
The B-Factory work is only the latest chapter in a long history of Lawrence Livermore accelerator projects. Many have been built at Livermore, including the Advanced Test Accelerator, Flash X-Ray Facility, Experimental Test Accelerator, and LINAC, a 100-million-electron-volt linear accelerator. A team of Lawrence Livermore engineers and physicists contributed to designing parts of the Department of Energy's Superconducting Super Collider before that enormous project was canceled. Accelerator work continues today in support of projects nationwide for the U.S. Department of Energy and overseas for Europe's nuclear research agency, CERN.
Accelerators also form a basic component of Lawrence Livermore programs. Examples include heavy-ion fusion research for inertial confinement fusion, the Center for Accelerator Mass Spectrometry for environmental research, and a host of accelerator-driven projects supporting the DOE's science-based Stockpile Stewardship and Management Program. Accelerator science is even impacting Lawrence Livermore health-care research efforts, where the Peregrine computer simulation code adapts nuclear particle transport software to optimize radiation therapy for cancer patients.
Physicist Karl van Bibber, Livermore project leader for the B-Factory, suggests two key reasons for Lawrence Livermore's important contributions to past accelerator projects and its high potential for future successes. The first is the Laboratory's longstanding experience in managing large-scale, multidisciplinary, and multilaboratory projects. The second reason is the concentration of experts in such fields as computer simulation, lasers, advanced manufacturing, precision engineering, pulsed power, and materials science, who combine to form multidisciplinary teams producing innovative accelerator component designs, engineering concepts, and manufacturing technologies.
Serving as a U.S. Flagship|
Livermore accelerator expertise is most visible in its contributions to the B-Factory. Scheduled for completion in early 1999, the facility will be one of the flagships of the U.S. high-energy physics program, along with Fermi National Accelerator Laboratory's main ring injector upgrade to the Tevatron accelerator. Thousands of components, many of which will define the state of the art in accelerator technology, are being designed and built by the three partnering laboratories, which are working closely with a host of small and large U.S. contractors.
The B-Factory accelerator will consist of two storage rings built one above the other in an existing tunnel at the east end of SLAC (see Figure 2 above). The upper ring is for positrons; the lower for electrons (Figure 3). The rings will be connected to the existing 3.2-kilometer- (2-mile-) long SLAC linear accelerator, which will act as a particle injector. The positrons will be generated part way along the linear accelerator by crashing high-energy electrons into a cooled rotating tungsten target. Both the electrons and positrons are stored in existing damping rings, which will shrink the size of the beams, before they are reinjected and accelerated down to the storage rings. The streams of electrons and positrons travel in opposite directions at nearly the speed of light within 10-centimeter- (4-inch-) diameter metal beam pipes. Magnets guide these streams and narrow them to beams that are 1 to 2 millimeters wide. By the time the beams collide in the middle of the detector, they are flat "ribbons," about 6 micrometers high and 150 micrometers wide.
The construction project is making use of much of SLAC's existing PEP (Positron-Electron Project) facility. Work involves renovating the existing high-energy PEP storage ring for the electrons, adding a new low-energy storage ring for the positrons, and installing a huge detector called BaBar that encompasses the central part of the interaction region where the electrons and positrons are made to collide. (The BaBar detector is named after the elephant in Jean de Brunhoff's children's stories and is a playful pun on the physics notation for B and anti-B mesons which is pronounced "B, B bar.")|
A key feature of this collider is that electrons and positrons will circulate and collide with unequal (or asymmetric) energies so scientists can better study the particles generated in the collisions. The electrons will be accelerated to 9 billion electron volts and the positrons to 3.1 billion electron volts. The asymmetric energy of the colliding electrons and positrons will create B mesons and anti-B mesons with a "kick" forward, away from the collision point, making it easier for the massive detector to pinpoint the origin of the B particles' decay products.
The project is expected to generate enormous amounts of raw data each year. The data will be distributed within a collaboration that includes nearly 500 physicists representing 75 institutions in the U.S., Canada, the United Kingdom, France, Italy, Germany, Russia, China, and Taiwan. Lawrence Livermore physicists will be part of the American team analyzing the long-awaited data.
The cavities, which have an interior diameter of 50 centimeters (20 inches), were designed as the most powerful of their kind ever built. They posed exceptionally challenging manufacturing problems for Livermore engineers and technicians. For example, because the cavity wall must dissipate over 100,000 watts of microwave power, an innovative manufacturing process was developed to embed water channels in the cavity's outer surface to remove heat. First, the cavities' bowl shapes are pressed out of copper plate and then welded together using an electron beam. The channels are cut into the outside of the cavities, filled with wax, and plated with copper. The wax is melted and removed and the cavities precisely fitted with ports and flanges. (See Figure 5 for a summary of the cavity manufacturing process.)
Most of the machining is contracted to U.S. industry, with some extremely specialized fabrication and assembly activities centered at Lawrence Livermore. For example, the Laboratory's plating shop is one of the few places in the world capable of precisely electrodepositing a 1-centimeter (three-eighths-inch) layer of oxygen-free copper on the 200-kilogram (450-pound) cavities, a process that takes four weeks to complete (see May 1996 S&TR, pp. 28-30). All told, each cavity requires 50 different manufacturing steps and some 1,700 worker-hours to manufacture.|
"Rarely in the history of the Laboratory have we faced so complex a manufacturing task as the cavities," notes Jeff Williams, head of Manufacturing and Materials Engineering Division. Williams says the task is made particularly challenging by the number of units (most of the time, the division makes one-of-a-kind items), the number of steps involved, and the number of different shops within the division that have gotten involved.
Given the original presumption that the cavities would be sole-sourced to a foreign vendor, American industry has greatly benefited from the relationship, both in new sales and new skills. "The Laboratory is acting as a master contractor. We're very proud that we're able to keep all of the work in the U.S.," says physicist Marshall Mugge, deputy project leader of the Laboratory's B-Factory activities.
Livermore experts are also cleaning nearly a kilometer of 2.4-meter- (8-foot-) long straight sections of beam pipes through a process called glow-discharge cleaning that rids the metal of residual carbon and contaminants. The process, conducted in Livermore clean rooms, is essential because an electron beam tends to attract dust particles left in the pipes much as static electricity does (see Figure 6). "Having extremely clean pipes will help ensure that the accelerator starts out with a very good vacuum," says Mugge.
Another Livermore responsibility is a critical 5-meter- (16.4-foot-) long device called the distributed ion pump. The pump will be installed within each of the 192 dipole magnets around the high-energy ring. As the particle beams circle around, they will generate a huge amount of x-ray energy, which will heat the metal pipes. The pipes in turn will discharge hot gases, which must be immediately removed to maintain the high vacuum conditions of 10-9 torr, or one-trillionth the atmospheric pressure of Earth at sea level.|
Electrons streaming off the distributed ion pump will ionize the discharged gas, which will become quickly trapped inside the pump's cathode. Without such an effective pump, the beam would quickly attenuate by colliding with the contaminating gases. Because of the importance of maintaining a very high vacuum, successfully demonstrating the distributed ion pump was an important factor in showing that SLAC's B-Factory design was workable.
The pump is one of several key designs produced by the 60 engineers, designers, and technicians comprising Livermore's Accelerator Technologies Engineering Group, a part of the Mechanical Engineering Department's Applied Research Engineering Division. For supervisor Lou Bertolini, the group's success is due in part to working closely with physicists in the Physics and Space Technology Directorate. Another contributing factor is the broad range of group members' backgrounds that encourages taking innovative approaches to accelerator component design.
Where Beams Collide
Without such extreme vacuum conditions, the beams would lose energy by colliding with residual air molecules. The vacuum is also necessary to reduce "noise" in the detector from interactions with gas molecules that would be confused with the large number of particles produced by B-particle decay.|
"There are a tremendous number of components that have to come together at the same time in the interaction region. It's like assembling a one-of-a-kind exotic automobile," says Robert Yamamoto, deputy division leader of Applied Research Engineering Division and the coordinator for LLNL engineering work for the B-Factory.
The design of detector subsystems within BaBar has required close working relationships with research groups and manufacturers in Italy, Britain, China, and Russia. For example, the cesium iodide calorimeter, co-designed by Livermore physicists and engineers, is made of 6,000 cesium iodide crystals being manufactured in China and Russia. The 30-centimeter- (12-inch-) long crystals will measure various products from B-particle decay.
Another subsystem, the instrumented flux return (IFR), is a joint project of researchers at Livermore and in Italy. The IFR consists of a large number of specialized detectors called resistive plate chambers (RPCs). These chambers allow measurements of charged muons that traverse the outermost iron plates forming the magnetic field flux return for the BaBar detector. They also allow the detection of certain decay products that can be used to enhance the data sample from the detector. Livermore physicists have built a special receiving and assembly area at SLAC and are now beginning to receive the first RPCs from Italian manufacturers for testing and commissioning.
Lawrence Livermore computational physicists are simulating events in the interaction region that are telling the international B-Factory research community how the detectors will track the tens of thousands of daily collisions between electrons and positrons. Livermore physicist Craig Wuest notes that the simulations are becoming increasingly realistic as detector designs are finalized and the systems manufactured and installed.
Beyond the B-Factory
A Bright Future for the NLC
Key Words: accelerators, Accelerated Strategic Computing Initiative (ASCI), Accelerator Production of Tritium (APT), Advanced Hydrotest Facility (AHF), B-Factory, B meson, BaBar detector, Big Bang, Center for Accelerator Mass Spectrometry (CAMS), charge parity violation, LINAC, Main Injector Neutrino Oscillation Search (MINOS), Next Linear Collider (NLC), Photon-Electron New Heavy Ion Experiment (PHENIX), Stanford Linear Accelerator Center (SLAC), Stockpile Stewardship and Management Program, 100-terawatt laser.
For further information contact Karl van Bibber (510) 423-8949 (firstname.lastname@example.org).
MARSHALL MUGGE (left) joined the Laboratory as a physicist in 1985. He served as Assistant Division Leader in the Physics and Space Technology Directorate from 1990 to 1993. From 1977 to 1985, he was a physicist at Fermi National Accelerator Laboratory. He received his B.S. in physics and mathematics from Iowa State University and his Ph.D. in high-energy physics from the University of Colorado. He is currently deputy project leader of the Laboratory's B-Factory activities.
ROBERT YAMAMOTO is deputy division leader of the Applied Research Engineering Division of Lawrence Livermore's Engineering Directorate. He earned a B.S. in mechanical engineering from the University of California, Berkeley, and an M.B.A. from Golden Gate University in San Francisco. Yamamoto coordinates the Laboratory's engineering work for the B-Factory project.
KARL VAN BIBBER is a graduate of the Massachusetts Institute of Technology, with a B.S. in physics and mathematics and a Ph.D. in physics. He joined the Laboratory in 1985 as a senior physicist. Since July 1991, he has been group leader for High-Energy Physics and Accelerator Technology in the Physics and Space Technology Directorate. He is currently the project leader for Livermore's work on the B-Factory at the Stanford Linear Accelerator Center.