The BABAR Collaboration BABAR, the detector for the SLAC PEP-II asymmetric e + e − B Factory operating at the Υ (4S) resonance, was designed to allow comprehensive studies of CP -violation in B-meson decays. Charged particle tracks are measured in a multi-layer silicon vertex tracker surrounded by a cylindrical wire drift chamber. Electromagnetic showers from electrons and photons are detected in an array of CsI crystals located just inside the solenoidal coil of a superconducting magnet. Muons and neutral hadrons are identified by arrays of resistive plate chambers inserted into gaps in the steel flux return of the magnet. Charged hadrons are identified by dE/dx measurements in the tracking detectors and in a ring-imaging Cherenkov detector surrounding the drift chamber. The trigger, data acquisition and data-monitoring systems , VME-and network-based, are controlled by custom-designed online software. Details of the layout and performance of the detector components and their associated electronics and software are presented.

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We search for invisible decays of the Υ (1S) meson using a sample of 91.4 × 10 6 Υ (3S) mesons collected at the BABAR/PEP-II B Factory. We select events containing the decay Υ (3S) → π + π − Υ (1S) and search for evidence of an undetectable Υ (1S) decay recoiling against the dipion system. We set an upper limit on the branching fraction B(Υ (1S) → invisible) < 3.0 × 10 −4 at the 90% confidence level.

We report measurements of the inclusive electron momentum spectra in decays of charged and neutral B mesons, and of the ratio of semileptonic branching fractions BB ! Xe and BB 0 ! Xe. These were performed on a sample of 231 10 6 B B events recorded with the BABAR detector at the 4S resonance. Events are selected by fully reconstructing a hadronic decay of one B meson and identifying an electron among the decay products of the recoiling B meson. We obtain BB ! Xe=BB 0 ! Xe 1:074 0:041 stat 0:026 syst .