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.

Using the CLEO II detector, we have measured the differential cross sections for exclusive two-photon production of light pseudoscalar mesons 0 , , and Ј. From our measurements we have obtained the form factors associated with the electromagnetic transitions ␥*␥→meson. We have measured these form factors in the momentum transfer ranges from 1.5 to 9, 20, and 30 GeV 2 for 0 , , and Ј, respectively, and have made comparisons to various theoretical predictions. ͓S0556-2821͑98͒01001-7͔

The spin polarization of current injected into GaAs from a CoFe/MgO(100) tunnel injector is inferred from the electroluminescence polarization from GaAs/AlGaAs quantum well detectors. The polarization reaches 57% at 100 K and 47% at 290 K in a 5 T perpendicular magnetic field. Taking into account the field dependence of the luminescence polarization, the spin injection efficiency is at least 52% at 100 K, and 32% at 290 K. We find a nonmonotonic temperature dependence of the polarization which can be attributed to spin relaxation in the quantum well detectors.

The spin polarization of electrons injected into GaAs from a CoFe/ MgO͑100͒ tunnel spin injector is inferred from the circular polarization of light emitted from a GaAs-based quantum well ͑QW͒ detector. The circular polarization strongly depends on the spin and electron hole recombination lifetimes in the QW. Using time-resolved optical techniques, we show that these lifetimes are highly temperature dependent. A peak in the charge lifetime versus temperature is likely responsible for the previously observed dip in the electroluminescence polarization. Evidence for a temperature-independent spin injection efficiency of ϳ70% from 10 K to room temperature is found.

Injection of spin-polarized hot-electron current from a magnetic tunnel transistor into GaAs is demonstrated by the observation of polarized light emission from a GaAs/In(0.2)Ga(0.8)As multiple quantum well light-emitting diode. Electroluminescence from the quantum wells shows a polarization of approximately 10% after subtraction of a linear background polarization. The polarization shows a strong dependence on the bias voltage across the diode, which may originate from changes in the electron spin relaxation rate in the quantum wells under varying bias conditions.

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Using the CLEO detector at the Cornell Electron Storage Ring, we have made a measurement of R ϵ (e ϩ e Ϫ →hadrons)/ (e ϩ e Ϫ → ϩ Ϫ )ϭ3.56Ϯ0.01Ϯ0.07 at ͱsϭ10.52 GeV. This implies a value for the strong coupling constant of ␣ s (10.52 GeV)ϭ0.20Ϯ0.01Ϯ0.06, or ␣ s (M Z )ϭ0.13Ϯ0.005Ϯ0.03.

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