B - Factories

The evolution of B-Factories is discussed, and comments are made about the common features of present-day asymmetric storage ring collider designs. * Work supported by Department of Energy contract DE-AC03-76SF00515. Presented at the 4th International Symposium on Heavy E’lavour Physics, Orsay, France, June 25-29, 1991 There was a symposium Progress on B-Factories the day before the Heavy Flavour Symposium. There the B-Factory designs being considered by six laboratories were presented and discussed. This paper is a brief summary of these designs and the evolution that led to them having many common features. A “B-Factory” was defined broadly a few years ago as an e+ecollider with sufficient luminosity that CP violation in B decays could be detected. Five different B-Factories were compared at Snowmass in 1988:l 1) An asymmetric collider (unequal beam energies) at the T(4S),2 2) A collider with equal beam energies (a symmetric collider) operating just above the T(4S), 3) A collider at 4 s = 16 GeV, 4) A collider at the Z with unpolarized beams, and 5) A collider at the Z with a polarized electron beam. This comparison together with the scale of colliders at the Z have led to everybody concentrating on the T(4S) region. Many independent studies came to the conclusion that storage rings could reach the luminosity needed, L 3x 1033cm-2s-1 , and there was no need to develop new types of colliders such as low-energy linear colliders or linac-storage ring colliders. The asymmetric collider has a significant luminosity advantage over the symmetric collider that comes primarily from the T(4S) resonance cross section. All laboratories are concentrating on asymmetric colliders with symmetric ones being backups only. The designs whose parameters are given in Table I have a great deal in common (there are interesting differences in the details, but these are outside the scope of and audience for this paper). Offbeat ideas such as round beams and isochronous rings have been rejected as have unequal circumference rings. The latter was discarded because of coherent beam-beam effects,3 the practical and political advantages of avoiding new civil construction, and preserving the options for a second detector and a symmetric collider. It was necessary to choose between a high and low collision frequency, fc. Both require the same total current; the