In a decades-long search for answers about the fundamental structure of matter and the forces between subatomic particles, SLAC scientists study the collisions of particles accelerated to nearly the speed of light.
BaBar and Antimatter
For each type of matter particle there exists a nearly identical anti-particle, but with opposite charge and other slight differences. Theories predict that the tremendous abundance of energy after the Big Bang should have created particles and anti-particles in equal amounts. Yet we live in a world of particles, not anti-particles. What happened to the antimatter?
The BaBar experiment explores this asymmetry in nature by looking at the decays of short-lived subatomic particles called B mesons. If B mesons disintegrate differently than their antiparticles, B mesons, this could help explain how the universe tipped in favor of matter over antimatter. Starting in 1998, scientists used SLAC's PEP-II accelerator to speed beams of electrons and their antimatter counterparts, positrons, to a collision point within the 1200-ton BaBar detector. The abundant electron–positron collisions of this "B Factory" produced particles called Υ(4S) mesons, which decay into equal numbers of B and anti-B mesons. These further disintegrate into other exotic states. True to predictions, the Bs outlasted the anti-Bs, confirming the role of particle decay in symmetry violation—but also demonstrating that it’s not enough to explain the amount of matter in the universe today.
The BaBar Collaboration - approximately 500 physicists and engineers from 10 nations - has delivered beyond expectations, providing not only a deeper understanding of asymmetric decay in B mesons, but new subatomic particles and decays. The results appear in more than 300 scientific publications so far. The BaBar detector finished collecting data and closed down in summer 2008. The collaboration continues to make new discoveries from the vast amount of data produced by this record-breaking experiment.
SLAC and the Large Hadron Collider
SLAC scientists contribute to the design, computing and scientific collaborations for the Large Hadron Collider, which recently began operations at CERN, the European particle physics center on the French/Swiss border. Since joining the collaboration in 2006, SLAC has helped to design and build one of the collider’s major detectors, called ATLAS, and continues to help plan upgraded components for the LHC accelerator systems.
SLAC is one of a few dozen ATLAS Tier 2 computing centers around the world, and one of only five in the United States. At the moment, the main job of a Tier 2 institution is to simulate collisions, to examine how best to approach the deluge of future LHC data.
Once the LHC turns on, the lab's role will expand to include data interpretation in search of a deeper understanding of physics at high energies. In addition, many SLAC theorists are at work modeling the behavior of physics that might—or might not—leave traces in the aftermath of LHC particle collisions. The theorists' job is to predict just what clues might be hidden in the debris, to make it easier to spot the traces of fascinating new physics.
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