B-factory interaction region design

High luminosity B-factories generally high current (1-3 A) e+e- storage ring accelerators that operate at a center-of-mass energy equal to the mass of the Upsilon (4S) resonance (10.58 GeV). The high beam currents are achieved by storing a large number of bunches (several hundred to several thousand) into each beam. Two designs, the ones located at SLAC and KEK, also have asymmetric beam energies. This imparts a boost to the nearly stationary B mesons formed from the decay of the 4S resonance and allows precision vertex tracking detectors to look for a difference between the decay profiles of the matter and anti-matter B mesons, thereby observing a violation of CP. Bringing the stored beams into collision is one of the major challenges of any B-factory design. In order to achieve high luminosity the beams must be tightly focused. This pushes the final focusing elements close enough to the interaction point to be inside the solenoidal field of the physics detector. In addition, beam-related detector backgrounds from synchrotron radiation and scattered beam particles must be kept below an acceptable level. The major B-factory designs at Cornell University, KEK, and SLAC have all addressed these problems in various ways that depend on specific accelerator design decisions. This paper discusses the accelerator parameters and detector constraints that influence an interaction region (IR) design, as well as how the various IR designs address the challenges posed by a high luminosity B-factory