The B-Factory

The B-Factory was dedicated on October 26th 1998 by the US Energy Secretary Bill Richardson. It has two main parts - the PEP-II storage rings and the BaBar detector.

The 3-km long linear accelerator in Stanford, California uses electromagnetic fields to accelerate electrons and positrons to close to the speed of light:

Click here for more information on the accelerator components.

The electrons and positrons are then guided into the two PEP-II storage rings (PEP stands for Positron Electron Project). The rings are located one on top of the other. Electrons go clockwise round the lower ring (which is an upgrade of the older PEP storage ring, which came into operation in 1980). Positrons go anticlockwise round the newly built upper ring.

The electrons and positrons are then brought together in the BaBar detector. They annihilate to produce gamma rays, which in turn transform back to particles. The energies of the electrons and positrons are precisely tuned to produce and mesons. The electrons have more energy than the positrons, so the B mesons cannot be produced at rest (because of conservation of momentum).

The B mesons live for about a billionth of a second, in which time they travel less than a millimetre. The BaBar detector observes the particles to which the Bs decay. From the decay products, the physicists can deduce which was the and which was the . They can also measure how far and fast the Bs travel before decaying, and hence they can calculate their lifetimes.

The physicists are interested in the difference in the decay times of the and the . By observing millions of decays, they can build up a distribution of the differences in the decay times. It is predicted that the actual distribution will be different from that which you would get if there were complete symmetry between matter and antimatter.

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