Experimental Facilities : Positron Electron Project (PEP)
After SPEAR and several other machines around the world had demonstrated the great power of electron-positron colliding beam to produce important new physics results, the next logical step was to increase the energy of such colliders substantially. Both the American and European particle physics communities undertook such a step with the construction of the PETRA storage ring at the DESY Laboratory in Hamburg (Germany) and the PEP storage ring at SLAC. PEP, the Positron-Electron Project, was a collaborative effort of SLAC and Lawrence Berkeley Laboratory.
In 1980, PEP began operating and was used for a decade to support the research activities of several hundred physicists and graduate students. The PEP tunnel is about 800 meters in diameter and can produce electron-positron collisions up to center-of-mass energies of about 30 GeV. The PEP physics program did important work measuring the lifetimes of certain elementary particles, studying how the quarks that are initially produced in the collision then fragment or evolve into the various kinds of particles that are actually observed in the detection apparatus, and testing the theory (called Quantum Chromodynamics or QCD) that is presently believed to describe the strong force that binds quarks together. Of particular interest was the observation, made first at PEP, that the B quark had a much longer lifetime than had been anticipated.
As with all storage rings, the particles are kept in their circulating paths by magnets. In PEP, the electron and positron beams each could be stored with energies up to 10 GeV. Experiments at PEP did important work by measuring the lifetimes of B mesons, showing them to be unexpectedly long-lived. Experiments also tested the predictions of the Quantum Chromodynamics or QCD theory that describes the strong force that binds quarks together.
This facility was recently rebuilt as PEP-II, part of the B Factory. This facility produces large numbers of B mesons and their antiparticles (called B-Bar mesons) to study minute differences between their decay patterns. This will help to explore how matter and antimatter differ in an effort to understand why the universe contains lots of matter but very little antimatter.
PEP-II has two storage rings, because for the B factory we want to collide electrons and positrons with different energies. The original PEP ring has been reconditioned and rebuilt to store electrons a 9 GeV, while on top of it was added a lower energy storage ring stores positrons at 3.1 GeV.