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Measurements of the e + e~ cross section above BB threshold are reported. Structures are observed which could be the Y(5S) and Y(6S) resonances. The masses and widths are given and compared with various potential-model predictions. Average charged multiplicities and inclusive lepton yields are also presented.

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we obtain assistance in construction of the apparatus. a(H2) = 26.43(60) xlO-^ This is in good agreement with the theoretical value of Ramsey^'^^ for H2, a(Hj., = 26.2(4) xlO~^ 2 theor Our determination thus corroborates the presently accepted values of the shielding constant for protons in H2 gas and water. We anticipate that the precision of the experiment will be substantially improved and thus serve to check details of shielding calculations to which no experiment has so far been sensitive. Even at the present level of accuracy, however, the result eliminates the possibility of an error in the proton moment as an explanation for the discrepancy in the fine-structure constant as obtained from fine-and hyperfine-structure measurements.We wish to thank R. V. Pound and R. E. Richards for guidance in selection of the magnet used, which was designed and constructed by the Mullard Co., England, and D. A. Gray for Kusch, Phys. Rev. 95, 1474