$\boldmathe^+e^-$ Colliders

B meson production via e⁺e^- colliders is being pursued at laboratories in the United States (Cornell and SLAC) and Japan (KEK). The luminosity at these machines is of the ${\cal O}(10^{33}){\rm \ cm^{-2}s^{-1}}$ which is equivalent to approximately 4 $b\bar b$ pairs produced every second. Because the B mesons produced from the decay of $\Upsilon(4S)$ are coherent it is necessary to be able to measure the time separation between the two B's in order to measure the CP asymmetry through $B^0 - \bar{B^0}$ mixing. To give the $\Upsilon(4S)$ sufficient boost to allow the two B⁰ decay vertices to be reconstructed and thus the distance between the two B mesons to be measured, the beam energies at SLAC and KEK are asymmetric. The Cornell B-facility has symmetric beam energies and will be unable to measure CP asymmetry through $B^0 - \bar{B^0}$ mixing though there are possibilities to measure CP violation through the decays of charged B's. The KEK and SLAC facilities are asymmetric with e^-(e⁺) energies of 3.5 GeV (8 GeV) and 3.1 GeV (9 GeV) respectively. The need for the large luminosity and the asymmetric beam energies poses great challenges on the machine design. The advantages of this approach is the very clean production environment of the 2 B mesons, with no underlying event from which to extract the signal. By running at the mass of $\Upsilon(4S)$ it is not possible, simply by kinematic contraints, to study the B_s system. In order to study the B_s system the machine can be operated with energies at the mass of $\Upsilon(5S)$ but the cross sections are much smaller.