Accelerator Systems Division
PEP-II: Asymmetric B Factory

The PEP-II facility consists of two independent storage rings, one located atop the other in the PEP tunnel. The high-energy ring, which stores a 9-GeV electron beam, was an upgrade of the existing PEP collider; it reutilized all of the PEP magnets and incorporated a state-of-the-art copper vacuum chamber and a new radio-frequency system capable of supporting a stored beam of high current. The low-energy ring, which stores 3.1-GeV positrons, was newly constructed. Injection is achieved by extracting electrons and positrons at collision energies from the SLC and transporting them each in a dedicated bypass line. The low-emittance SLC beams are used for the injection process. The collider was completed in July 1998.

The term "asymmetric" refers to the fact that the e- and e+ energies are not equal. This results in a collision center of mass which is moving in the laboratory. The motion of the center of mass is crucial for the study of the matter versus anti-matter discrepancy discussed below.

The construction of PEP-II was a collaboration of SLAC, LBNL, and LLNL.

A new detector, BaBar was constructed to take advantage of the high-intensity collisions.

The B-Factory facility, PEP-II and BaBar, is pursuing a broad agenda of physics involving the heavy quark and heavy lepton sector. At design luminosity, PEP-II produces about 1E8 b and c quarks, and roughly the same number of tau's per year.

In particular, the B-Factory is pursuing the question of why we live in a matter-dominated Universe. At the beginning of the Big Bang, matter and anti-matter were produced in equal amounts -- what tipped the balance in favor of matter? Understanding the mechanism is crucial if we are to have a full understanding of the evolution of our Universe. A small imbalance in the matter to anti-matter ratio at primordial times, one part in about 1E9, would suffice. In such a situation, all the matter and anti-matter, except the tiny unpaired excess, would have annihilated to form photons (which we can detect now). The unpaired excess, all matter, evolved to form the Universe.

To substantiate this paradigm, we need experimental evidence that conclusively establishes the mechanism responsible for creating the tiny matter excess. There are several competing theoretical models for this mechanism. The main role for the B-Factory is to make a broad set of measurements capable of confronting the crucial question of what happened to all the anti-matter. It is fascinating to realize that with the B-Factory, we are able to confront crucial physics issues which took place less than 1E-34 seconds after the start of the Big Bang.

LBNL | LLNL